water Article Agro-Industrial Waste as Potential Heavy Metal Adsorbents and Subsequent Safe Disposal of Spent Adsorbents Daiana Simón 1 , Cristina Palet 2 , Agustín Costas 3 and Adrián Cristóbal 1, * 1 Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata (UNMdP)–Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mar del Plata 7600, Argentina 2 GTS–UAB Research Group, Department of Chemistry, Faculty of Science, Edifici C-Nord, Universitat Autònoma de Barcelona, Carrer dels Til·lers, 08193 Bellaterra, Spain 3 Centro Biotecnológico Fares Taie, Mar del Plata 7600, Argentina * Correspondence: acristobal@fi.mdp.edu.ar Abstract: Water pollution is an environmental problem that affects the ecosystem and living beings. Adsorption is one of the best technologies for the removal of heavy metals. Since waste recovery is the basis of the Circular Economy, agro-industrial waste is emerging as low-cost adsorbents for these pollutants from wastewater. Residues of pine sawdust, sunflower seed hulls and corn residues mix were evaluated as adsorbents of synthetic aqueous solutions of Ni(II), Zn(II) and Cd(II). These residues were characterized to determine their structure and composition, and to understand the adsorption mechanism. Adsorption efficiencies and capacities for the adsorbents and adsorbates were determined and compared. From the obtained results, it is possible to affirm that all biomasses used are good alternatives to the synthetic materials, with adsorption efficiencies greater than 50%. The order of adsorption was Cd > Zn > Ni. At the concentration range checked, adsorption efficiencies decreased in sawdust when a mixture of all metals together was considered (as present in real sewage). Finally, the heavy metals were immobilized, with efficiencies over 88.5%, in clay ceramics (as brick’s Citation: Simón, D.; Palet, C.; Costas, precursors). This procedure would help to minimize the contamination that could be generated by A.; Cristóbal, A. Agro-Industrial the disposal of spent adsorbents, rarely explored in the literature. Waste as Potential Heavy Metal Adsorbents and Subsequent Safe Keywords: heavy metals; agro-industrial waste; adsorption; wastewater treatment; contaminant Disposal of Spent Adsorbents. Water immobilization 2022, 14, 3298. https://doi.org/ 10.3390/w14203298 Academic Editor: Laura Bulgariu 1. Introduction Received: 23 September 2022 Water pollution is a serious world environmental problem mainly caused by the Accepted: 14 October 2022 climate change, rapid urbanization and advance of industrialization [1,2]. Heavy metals are Published: 19 October 2022 among most released pollutants or contaminants into the water and are not biodegradable; Publisher’s Note: MDPI stays neutral therefore, they accumulate in living organisms entering in the food chain, also through with regard to jurisdictional claims in the consumption of water and other contaminated products, producing corresponding published maps and institutional affil- pollution biomagnification [3,4]. These metals are an environmental and public health iations. concern, not only because of their persistence and concentration that influence exposure, but also because of their toxicity, and their mobility in the environment that determines their bioavailability, which is given by the type of compound or metabolite that each metal can form, and also by the characteristics of each the specific environment [5,6]. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. Nickel, zinc and cadmium are common and relevant heavy metals in the environ- This article is an open access article ment [4,7]. Electroplating, metallurgical and batteries industries are some of the anthro- distributed under the terms and pogenic sources of nickel, zinc and cadmium contamination [8,9]. Also, nickel and zinc can conditions of the Creative Commons easily leach due to mineral weathering [10]. Ni(II) and Zn(II) are essential elements and, in Attribution (CC BY) license (https:// low concentrations, they are necessary for the metabolic development of humans, plants creativecommons.org/licenses/by/ or animals. However, these elements can be toxic and harmful to health effects when ex- 4.0/). posure/assimilation exceeds the upper limit of the physiologically required range [11–13]. Water 2022, 14, 3298. https://doi.org/10.3390/w14203298 https://www.mdpi.com/journal/water Water 2022, 14, 3298 2 of 19 For example, high exposure to nickel can cause cancer, dry cough and lung problems, dermatitis, nausea, gastrointestinal and kidney problems in humans, and high exposure to zinc can cause fever, vomiting, anemia, and skin problems in humans [7,8]. Cd(II) is a highly toxic metal, even at very low concentrations, and is a non-essential element because it has no known benefit to human health or other living beings [6,10]. Cadmium is a human carcinogen as established by The International Agency for Research on Cancer (IARC) and can cause kidney problems, hypertension, stomach irritation, among others, and its chronic exposure can lead to the development of “Itai-Itai” disease [11,14]. In the province of Buenos Aires, Argentina, the permitted maximum discharge limits of these heavy metals in sewers, surface water or stormwater conduits and the open sea vary from 2 to 3 mg/L for Ni, 2 to 5 mg/L for Zn and 0.1 to 0.5 mg/L for Cd [15]. Industrial effluents generally can contain concentrations of heavy metals above the maximum per- missible limits; therefore, industries must treat their effluents before discharging them into the environment [13,14]. There are conventional technologies for treating wastewater and minimizing heavy metal pollution (e.g., chemical precipitation, coagulation/flocculation, membrane filtration, electrochemical technologies, ion exchange), however they can be ex- pensive, and can generate by-products or sludge, involving complicated procedures [8,16]. Adsorption is considered one of the best options for heavy metal removal due to its flex- ibility in operation and design, low energy consumption, minimization of sludge and by-products, possibility of regenerating adsorbents, and high removal efficiency even at a very low metal concentrations [17,18]. Activated carbon (AC) is the most used and recognized heavy metal adsorbent but it is expensive due to its preparation process and the impossibility of its regeneration, which limits its use at large-scale application [19,20]. For developing countries, the application and development of heavy metal removal technologies represents a challenge [20]. In recent years, agro-industrial residues have emerged as low-cost adsorbents, also for heavy metals, due to its availability and abun- dance, allowing to apply processes under the bases of the Circular Economy (which corre- sponds to the recovery and reuse of wastes) [7,10]. Every year, worldwide, tons of waste are produced from the agro-industrial sector that are stored in the open air and disposal in landfills, causing negative environmental impacts due to leachates and gases, following with the CO2 generation with their burning [21]. Literature examples of agro-industrial residues used as heavy metal adsorbents are: cow dung [5], potato peel [22], cucumber peel [23], groundnut husk [24], eggshells [25], pine and modified pine [26], rice and rape- seed [27], coffee husk and lignin [28], among others. All the plant-based wastes are made up of hemicellulose, cellulose and lignin, and has a wide variety of functional groups (e.g., aldehydes and ketones, carboxyl groups, phenolics, hydroxyls, methyls, ethers, amides, aminos, etc.) that can interact with pollutants through various mechanisms [7,8]. As reported by the national government in 2020, in Argentina agro-industrial is really important and constitutes the 25% of the manufacturing industry and represents the 40% of exports. Among the agro-industrial residues, sawdust constitutes from 9 to 15% of the forest biomass discarded by sawmills and comes mainly from pine and eucalyptus plantations [29]. Sunflower crop is one of the most important in Argentina with a production of 3.5 million tons of seeds per year, obtaining 50% by weight of discarded hulls per seed [30]. On another hand, the corn production extends over a large area of the country and generates a great volume of biomass when compared to others such as wheat or barley [31]. This paper focuses the study of adsorption processes of Ni(II), Zn(II) and Cd(II) by using pine sawdust, sunflower seeds hulls and corn residues mix as adsorbents. A comparison of these three agro-industrial wastes as adsorbents is here presented, by checking firstly each individual heavy metal adsorption process, and secondly the influence of the mixture of these three heavy metals on their adsorption (as they are present in real sewage all together), by corresponding adsorption experiments. Later, such biomass residues containing heavy metals are immobilized in clay ceramics (as brick’s precursors), to here propose an environmentally safe way to dispose the spent adsorbents together Water 2022, 14, x FOR PEER REVIEW 3 of 20 Water 2022, 14, 3298 3 of 19 propose an environmentally safe way to dispose the spent adsorbents together with ad- sorbates (heavy metals). with adsorbates This procedure (heavy metals). would help This procedure to minimize would the secondary help to minimize contami- the secondary nation that could be generated by the disposal of spent adsorbents, which is rarely contamination that could be generated by the disposal of spent adsorbents, which is rarely ex- plored in the adsorption literature and is fundamental for the real application of the explored in the adsorption literature and is fundamental for the real application of the ad- sorption from adsorption fromlow-cost materials. low-cost materials. 2.2.Materials Materialsand and Methods Methods 2.1. 2.1.Chemicals Chemicalsand and Reagents Reagents All Allreagents reagentsused used were were of analytical of analyticalgrade. grade. Stock solutions Stock of concentration solutions of concentration 1000 1000 mg/L ofmg/L individual heavyheavy of individual metalsmetals ions, Ni(II), Zn(II)Zn(II) ions, Ni(II), and Cd(II), werewere and Cd(II), prepared prepareddissolving ade- dissolving adequate quate amounts amounts of Ni(NO of Ni(NO ) 3 2 .6H3) 2 2 .6H O, 2O, Zn(NOZn(NO ) 3 2 .6H 3 ) 2.6H 2 O 2O and and Cd(NO Cd(NO ) 3 2 3) .4H2.4H 2 O2O (all (all from from Pan- Panreac, reac, Spain), Castellar respectively. del Vallès, Spain),From these solutions respectively. From these the corresponding dilutions useddilutions solutions the corresponding in the adsorption experiments were prepared and the pH (Omega 300 used in the adsorption experiments were prepared and the pH (Omega 300 pH meter, pH meter, Crison Instru- ments,Instruments, Crison S.A) was adjusted with HNOSpain) S.A, Barcelona, 3 70% (Panreac, was adjusted Spain). Solutions with HNO3 were prepared Spain). 70% (Panreac, with Milli-Q water. Solutions were prepared with Milli-Q water. 2.2. 2.2.Biomass Biomass The Thebiomasses biomassesof of sawdust, sawdust, sunflower and and corn cornwere wereselected selectedaccording accordingtotothethereasons reasons already mentioned. already mentioned. The The pine (Pinus elliottii) sawdust residues were provided elliottii) sawdust residues were provided by a sawmillby a sawmill ininthe theprovince provinceof of Corrientes, Corrientes, Argentina, and and corresponded correspondedto tothe themain maincutting cuttingprocess processofof the thewood, wood,before before any any addition. addition. Sunflower Sunflower seed seed hulls hulls(Helianthus annuus)were (Helianthusannuus) wereprovided provided bybyaacompany company located located inin the the province province of SantaSanta Fe,Fe, Argentina, Argentina,dedicated dedicatedtotothe theoilseed oilseed market, market,and andwere wereobtained obtainedfromfromthe processing the processing of of sunflower sunflower grains. The grains. Thecorn cornresidues residuesmix mixmays (Zea var. saccharata) (Zea mays werewere var. saccharata) kindly provided kindly providedby the National by the National Institute of Agricultural Institute of Agricul- Technology (INTA), tural Technology and corresponded (INTA), and corresponded to theto harvest stage.stage. the harvest The Thedevelopment development of of the the adsorbents includedthe adsorbents included thecollection collectionofofthe thebiomass, biomass,grinding grinding with a knife mill (IKA with a knife mill (IKA A10) A10) and sieving to a particle size of less than 1 mm sieving to a particle size of less than 1 mm to promote to promote adsorption.The adsorption. Thewaste wastedid didnotnot receive receive additional additional processing processing (chemical (chemical or thermal or thermal treat- treatment) toment) maketoit make it as friendly as friendly as possibleas possible to the environment to the environment and reduce andcosts. reduce costs.1 shows Figure Figure the 1 shows the macroscopic macroscopic appearanceappearance of the usedofwaste, the used afterwaste, afterand grinding grinding and sieving. sieving. Figure 1. Ground and sieved residues (A) pine sawdust, (B) sunflower seed hulls and (C) corn res- Figure 1. Ground and sieved residues (A) pine sawdust, (B) sunflower seed hulls and (C) corn idues mix. residues mix. 2.3.Biomass 2.3. BiomassCharacterization Characterization Physicochemical properties Physicochemical properties of of adsorbents adsorbentscontribute contributetotothe theprocess processofofadsorption adsorptionofof contaminants. Characteristics contaminants. Characteristicsofofpotential potentialadsorbents adsorbentswere weredetermined determinedfrom a number from of a number techniques, that included the Brunauer-Emmett-Teller (BET) (Micromeritics of techniques, that included the Brunauer-Emmett-Teller (BET) (Micromeritics Accusorb, Accusorb, model2100), model 2100), Scanning Scanning Electron Electron Microscopy Microscopy (SEM) (SEM)(ZEISS (ZEISSEVOEVO®MA ® MA1010atatthe theUAB UAB Mi- Mi- croscopy Service and FEI ESEM Quanta 200), Energy-Dispersive X-ray Spectroscopy croscopy Service and FEI ESEM Quanta 200), Energy-Dispersive X-ray Spectroscopy (EDS) (EDS) (Oxford (Oxford SDD software: SDD X-Act, X-Act, software: AZTecOne), AZTecOne), Attenuated Attenuated Total Reflectance–Fourier Total Reflectance–Fourier Trans- Transform Infrared Spectroscopy (ATR-FTIR) (Nicolet 6700, form Infrared Spectroscopy (ATR-FTIR) (Nicolet 6700, Thermo Electron Thermo Corp. Electron Corp. equipment, equipment), Waltham, MA,X-ray USA),Fluorescence (XRF) (PW4024 X-ray Fluorescence Minipal2 Minipal2 (XRF) (PW4024 PANalytical X-ray spectrometer PANalytical X-ray spec- trometer with copper anode and operating conditions nitrogen flow, voltage 20 kV, current 5 mA and time 100 s), and Differential Thermal Analysis (DTA) and Thermogravimetric Water 2022, 14, 3298 4 of 19 Analysis (TGA) (Shimadzu TGA-50 and Shimadzu DTA-50 instruments, with TA-50 WSI analyzer and operating conditions air, heating rate of 10 ◦ C/min to 1000 ◦ C and approxi- mately 20 mg of mass). The mineral content of biomass it was determined following the guidelines of the ASTM E1755-01 standard [32]. In addition, the possible changes produced in the biomasses after the adsorption experiments were analyzed. 2.4. Batch Adsorption Experiments The removal of heavy metals by means of the aforementioned agro-industrial residues was carried out under batch adsorption experiments at room temperature. A volume of 10 mL of a mono-metal solution, of Ni(II), Zn(II) or Cd(II) of 0.18 mmol/L, or a multi-metal solution of all together with a concentration of 0.18 mmol/L for each heavy metal was placed in contact with 0.1 g of adsorbent in tubes. The pH of the solutions was initially adjusted to 4–5 following previous literature [26,33]. The system was stirred at 40 rpm in a rotary mixer (CE 2000 ABT-4, SBS Instruments SA) for 24 h to ensure that equilibrium was reached. The liquid phase was filtered through 0.22 µm filters (Millex-GS, Millipore, Burlington, MA, USA). The metal concentration in the aqueous solution (not adsorbed) was determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (XSERIES 2 ICP-MS, Thermo Scientific, Waltham, MA, USA) from the Autonomous University of Barcelona. The adsorption of each heavy metal was expressed as adsorption percentage (A%), calculated from Equation (1), and the adsorption capacity of each adsorbent (qe ) was calculated from Equation (2): (C0 − Ce ) A% (%) = × 100 (1) C0 (C0 − Ce ) × V   mmol qe = (2) g m where C0 and Ce (mmol/L) are the initial and equilibrium concentrations of heavy metal in solution, respectively, V (L) is the volume of the heavy metal solution, and m (g) is the mass of the adsorbent [34,35]. Adsorption experiments are prepared by duplicate and the average results are reported. 2.5. Spent Adsorbents Disposal in Clay Ceramics Safe disposal of spent adsorbents is necessary to minimize secondary contamination, especially if large-scale adsorption technology implementation is considered. Pine sawdust, sunflower seed hulls and corn residues mix, after being used as adsorbents, were added to the clay, and clay ceramics were prepared with the aim of immobilizing adsorbed heavy metals. The amount of residue contaminated with Ni(II), Zn(II) and Cd(II) that was used in the clay ceramics corresponded to 20% in volume with respect to the volume of clay, in accordance with what was observed by the authors in previous studies [36]. For this reason, adsorption experiments were previously carried out scaling 20 times the amount of adsorbent (2 g) and 20 times the moles of metal ion (3.6 × 10− 5 ). As a consequence of the increase in adsorbent mass, the solution volume had to be increased to 40 mL so that the liquid covers the entire surface of the biomass. The methodology used in the preparation process of the clay ceramics is detailed in Figure 2 and it was designed considering the characteristics of the raw materials, the pressure and firing conditions used in the brick factory. The clay was provided by a local brick factory and is the same that the manufacturer uses in the hollow brick production process. Firing of clay ceramic was carried out in an electric oven INDEF 332. filtered through filter paper washed with 1 mol/L nitric acid and rinsed with distilled wa- ter. The TCLP extracts of the solid phases were acidified with concentrated nitric acid until pH < 2 and stored refrigerated at 4 °C. Finally, the extracts were analyzed by Atomic Ab- sorption Spectrophotometry (AAS) (Shimadzu 6800 with flame) from the Fares Taie Bio- technological Center. The results of the leaching tests were compared with the estimated Water 2022, 14, 3298 5 of 19 initial mass of heavy metals in the clay ceramics. In this way, the retention efficiency was calculated according to Yilmaz et al. [38]. Figure 2. Clay ceramics preparation process. Figure 2. Clay ceramics preparation process. 3. Results and Discussion It is essential to evaluate the mobility of heavy metals present in manufactured clay 3.1. Biomass Characterization ceramics to determine the feasibility of immobilize these contaminants in the ceramic structure. The phenomena that Leaching occur in an tests were carried adsorbent out based are related onspecific to its EPA method surface1311 and,[37], which is a therefore, to themethod accepted total volume by Argentine of pores and theirlaws for hazardous dimensions, waste. the interaction that influence with the adsorbate and Claytheceramics obtainedprepared adsorptionfromefficiency. spent adsorbents Table 1 were shows crushed and sieved the results ob- to a particle size of less than 9.5 mm. In beakers, crushed clay tained from the BET analysis for pine sawdust, sunflower seed hulls and corn residues ceramics were mixed with leaching solution in a 1:20 solid-liquid ratio. According to the mix. All biomasses showed the presence of mesopores and the surface area values are inalkalinity of the ceramic, an extraction agreement withfluid thoseofreported pH 4.93 in ± the 0.05literature was prepared from 5.7 of for adsorbents mLlignocellulosic of acetic acid,origin64.3 mL of sodium hydroxide 1 mol/L and completing with distilled water [34,39]. Corn biomass presented a higher surface area, total pore volume, and mean pore up to 1 L. The covered beakers size comparingwere with shaken the other at 100 two rpm biomassin anresidues orbital shaker (SK-0330-Pro) (pine sawdust for 22 h. seed and sunflower The mixtures were hulls). Bilal et al. [7] reported that the adsorption of contaminants increases with the in- with distilled then filtered through filter paper washed with 1 mol/L nitric acid and rinsed water.area crease in the surface Theof TCLP extracts of since the adsorbent, the solid phases wereprocess the adsorption acidified is awith concentrated surface phe- nitric acid until pH < 2 and stored refrigerated at 4 ◦ C. Finally, the extracts were analyzed by Atomic nomenon. Absorption Spectrophotometry (AAS) (Shimadzu 6800 with flame) from the Fares Taie Biotechnological Table 1. BET analysis Center. results of surface The area, results total poreofvolume the leaching and meantestspore were compared size for biomasswith the estimated residues. initial mass of heavy metals in the clay ceramics. In this way, the retention efficiency was calculated according to Yilmaz et al. [38]. Total Pore Volume Biomass Surface Area (m2/g) Mean Pore Size (nm) 3. Results and Discussion (cm3/g) Pine sawdust 1.1 3.1. Biomass Characterization 0.003 9.4 Sunflower seed hulls 0.7 0.0009 4.9 The phenomena that occur in an adsorbent are related to its specific surface and, there- Corn residues mix 1.5 0.006 14.9 fore, to the total volume of pores and their dimensions, that influence the interaction with the adsorbate and the obtained adsorption efficiency. Table 1 shows the results obtained from the BET analysis for pine sawdust, sunflower seed hulls and corn residues mix. All biomasses showed the presence of mesopores and the surface area values are in agreement with those reported in the literature for adsorbents of lignocellulosic origin [34,39]. Corn biomass presented a higher surface area, total pore volume, and mean pore size comparing with the other two biomass residues (pine sawdust and sunflower seed hulls). Bilal et al. [7] reported that the adsorption of contaminants increases with the increase in the surface area of the adsorbent, since the adsorption process is a surface phenomenon. Water 2022, 14, 3298 6 of 19 Table 1. BET analysis results of surface area, total pore volume and mean pore size for biomass residues. Biomass Surface Area (m2 /g) Total Pore Volume (cm3 /g) Mean Pore Size (nm) 14, x FOR PEER REVIEW Pine sawdust 1.1 0.003 6 of9.420 Sunflower seed hulls 0.7 0.0009 4.9 Corn residues mix 1.5 0.006 14.9 SEM images allow to obtain information about the morphological characteristics (tex- ture, topography andSEM images surface allow to obtain characteristics) of theinformation adsorbents.about theSEM So, the morphological images of the characteristics three studied agro-industrial wastes are presented in Figure 3. Biomass analyzed particlesSEM images (texture, topography and surface characteristics) of the adsorbents. So, the of the shape showed an elongated three studied agro-industrial and a fibrous wastes An microstructure. are irregular presentedand in Figure rough3.surface Biomass analyzed with cavities can be observed in all cases, which forms a network of holes and fibers. These and rough particles showed an elongated shape and a fibrous microstructure. An irregular surface with cavities can be observed in all cases, which forms a network of holes and fibers. characteristics can facilitate the adsorption of heavy metals [40]. No appreciable changes These characteristics can facilitate the adsorption of heavy metals [40]. No appreciable were detected in the morphology of the adsorbents related to the interaction with metal changes were detected in the morphology of the adsorbents related to the interaction ions, as reportedwith by Zhang et al. as metal ions, [33], after the reported byadsorption Zhang et al. of[33], heavy metals. after The combina- the adsorption of heavy metals. tion of SEM withThe EDS combination of SEM with EDS detector and analyzer system allowedofto obtain the detector and analyzer system allowed to obtain the distribution heavy metals ondistribution the biomasses aftermetals of heavy adsorption on theby mapping. biomasses Asadsorption after shown in by Figure 4, theAs shown in mapping. distribution, andFigure therefore adsorption, 4, the of nickel, distribution, zinc and and therefore cadmium of adsorption, in nickel, the adsorbents zinc and wascadmium in the across the entireadsorbents surface. was across the entire surface. Figure 3. SEM images of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. Figure 3. SEM images of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. Water x FOR PEER 2022, 14, 3298 REVIEW 7 of 20 7 of 19 Figure 4. Elemental distribution obtained by SEM-EDS by mapping analysis for (A) pine sawdust, Figure 4. Elemental distribution obtained by SEM-EDS by mapping analysis for (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix, after adsorption of Cd(II), Zn(II) and Ni(II), respec- (B) sunflower seed hulls, (C) corn residues mix, after adsorption of Cd(II), Zn(II) and Ni(II), respectively. tively. ATR-FTIR made it possible to determine the presence of functional groups in biomass ATR-FTIR made it possible responsible for theto determine metal adsorptionthemechanism, presence of forfunctional example, eithergroups in bio- by electrostatic forces mass responsible for the metal adsorption mechanism, for example, either by electrostatic or complexation. ATR-FTIR spectra for the biomass residues studied here are shown in Figure 5. ATR-FTIR forces or complexation. The large numberspectra of IR forbands was associated the biomass residueswith the typicalhere studied complexare nature of shown in Figure 5. The large number of IR bands was associated with the typical complexrespective agro-industrial biomasses [41–43]. The assignment of the main IR bands at the approximate wavelengths are summarized in Table 2. The great similarity between the nature of agro-industrial biomasses [41–43]. The assignment of the main IR bands at the ATR-FTIR spectra of sawdust, sunflower and corn was due to the fact that the composition respective approximate wavelengths are summarized in Table 2. The great similarity be- of these three biomasses is based on cellulose and lignin. tween the ATR-FTIR spectra of sawdust, The presence sunflower of numerous andgroups functional corn was due to facilitates in biomass the fact that the the adsorption of composition of these heavythree biomasses metals is based of [7]. A comparison onthecellulose ATR-FTIR andspectra lignin.of the biomasses before and after The presencetheofadsorption numerousoffunctional groupsisin the heavy metals biomass also shown in facilitates Figure 5, the being adsorption both spectra of were very heavy metals [7]. similar A comparison of the ATR-FTIR in each biomass case. However,spectra of differences slight the biomasses werebefore andsuch observed, afteras a shift of the adsorption of the theband heavyat 1603–1624 metals is alsocm− 1shown in the three biomasses, in Figure 5, beingand both shift of the band spectra at 1224–1238 were very cm− 1 in the pine similar in each biomass case.sawdust However, and slight corn residues, afterwere differences the contact observed,with such heavyasmetals. a shiftThese of results may be indicative that carboxyl, alcohol, phenol, amide, and other functional groups could the band at 1603–1624 cm−1 in the three biomasses, and shift of the band at 1224–1238 cm−1 provide possible adsorption sites for the retention of the studied heavy metals, and were in the pine sawdust and corn residues, after the contact with heavy metals. These results similar to those found in previous works in the literature [44,45]. may be indicative that carboxyl, alcohol, phenol, amide, and other functional groups could provide possible adsorption sites for the retention of the studied heavy metals, and were similar to those found in previous works in the literature [44,45]. Water 2022, 14, 3298 8 of 19 Water 2022, 14, x FOR PEER REVIEW 8 of 20 Figure 5. ATR-FTIR spectra of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix, Figure 5. ATR-FTIR spectra of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix, before and after the adsorption process. before and after the adsorption process. Water 2022, 14, 3298 9 of 19 Table 2. Assignment of the main bands obtained by ATR-FTIR for biomass residues. Wavenumber (cm−1 ) Assignment 3332–3336 O-H stretching of carboxylic acids and alcohols/phenols, and N-H of amino and amide groups 2918–2922 Asymmetric C-H stretching of CH3 2846–2850 Symmetric C-H stretching of CH2 and stretching of methoxy groups 1722–1726 C-O stretching of carbonyl, C=O of acetyl, carboxyl, aldehydes and aromatic/conjugated esters COO- stretching of carboxyl groups, C=C of the aromatic ring, and C=O, C-N, C-N-H stretching of 1603–1624 amides, and O-H bending 1508–1510 C=C stretching of aromatic ring, N-H bending 1458 C=C aromatic, C=O stretching and symmetric bending of C-H, O-H O-H bending of acids, vibrations of aromatic rings and bending of CH2 and aromatic functional 1419–1437 groups such as C=C and C=O 1371 Asymmetric C-H bending of CH3 , CH2 1321–1325 O-H bending of phenol group, C–N groups 1224–1240 C-O stretching of phenols and carboxylic acids, and alkyl aryl ether bonds 1153–1157 Asymmetric stretching of the C-O-C pyranose backbone 1095–1109 C-OH and C-H stretching 1034–1036 C-O stretching in carboxyl group, C-O-C, dialkyl ether, C-H of aromatics, and C=C and C-C-O 874–895 Changes in aromatic structures such as C–H stretching of aromatics The XRF analysis for the biomasses before and after the adsorption of heavy metals is presented in Figure 6. The Cr peaks come from the tube used as the source of the equipment (anode). Signals corresponding to Cl, K, Ca, Mn and Fe were observed, although the intensities varied according to the residue. The presence in biomasses of Ni, Zn and Cd Water 2022, 14, x FOR PEER REVIEW 10 of 20 was observed after adsorption (Cd signals were detected in the Ca and K energy zone), associated with the decrease in the intensities of elements such as Ca and K, mainly. Figure 6. XRF of: (A) biomasses before the adsorption process, and after adsorption process with Figure 6. XRF of: (A) biomasses before the adsorption process, and after adsorption process with (B) pine sawdust, (C) sunflower seed hulls, (D) corn with adsorbed heavy metals. (B) pine sawdust, (C) sunflower seed hulls, (D) corn with adsorbed heavy metals. Water 2022, 14, 3298 10 of 19 The percentage of ash obtained was 0.2%, 2.0% and 10.2% for pine sawdust, sun- flower seed hulls and corn residues mix, respectively. The XRD patterns of the biomasses (Figure S1 of the supplementary material) evidenced the presence of a significant amount of amorphous phase, in agreement with the mentioned results. The XRF equipment used does not allow the measurement of elements lighter than Na, and H, C and O are part of hemicellulose, cellulose and lignin as the main components of the biomasses. For that reason, the composition of the corresponding ashes was also analyzed by XRF (results collected in Figure 7). Differences in mineral content and composition were observed. Probably Figure 6. potassium, calcium, magnesium XRF of: (A) biomasses and phosphorus before the adsorption areafter process, and involved in theprocess adsorption adsorption with of(B) pine sawdust, heavy (C) sunflower metals through seed hulls,mechanism, ion exchange (D) corn withas adsorbed reportedheavy metals. [46]. previously Figure 7. XRF of pine ashes, sunflower ashes and corn ashes. Figure 7. XRF of pine ashes, sunflower ashes and corn ashes. Biomassesofofplant Biomasses plantorigin originare are mainly mainly composed composed of of lignin lignin and andcellulose, cellulose,asasmentioned mentioned above, together with hemicellulose, low molecular weight compounds, lipids,proteins, above, together with hemicellulose, low molecular weight compounds, lipids, proteins, starch,water, starch, water,etc. etc.[7]. [7].The TheDTA-TGA DTA-TGA profiles profiles were were obtained obtained for for the the three threebiomasses biomasseshere here selected,and selected, andprovided providedaadescription description of of the the thermal thermal behavior behavior and andan anestimated estimatedpercentage percentage compositionofofbiomasses, composition biomasses,which which was was in in agreement agreement with with the the literature literature[47,48]. [47,48].AsAsseen seeninin Figure8,8,TGA Figure TGAanalysis, analysis,the thetotal totalweight weight loss loss was was 96% 96% for for pine pinesawdust, sawdust,97% 97%for forsunflower sunflower seed hulls, and 86% for corn residues mix, and was divided into three stages. The first weight loss stage (up to 230 ◦ C) was 7% for pine sawdust, 12% for sunflower seed shells and 10% for corn residues mix. This stage was related to the loss of moisture, which was characterized by an endothermic peak at 52 ◦ C in pine sawdust and at 60 ◦ C in sunflower seed hulls. The degradation of low molecular weight compounds was identified with an exothermic peak at 263 ◦ C, and was observed in sawdust. The second weight loss stage was 48% for sawdust (up to 311 ◦ C), 56% for sunflower (up to 297 ◦ C) and 50% for corn (up to 338 ◦ C). This stage could be related to the degradation of hemicellulose and pectin into volatile compounds of lower molecular weight. Finally, the third weight loss stage was 41% (up to 500 ◦ C), 29% (up to 460 ◦ C) and 26% (up to 508 ◦ C) for biomasses of pine sawdust, sunflower seed hulls and corn, respectively, and it could be related to the degradation of cellulose and lignin into CO2 , H2 O and ashes. From the TGA analysis, the final residue corresponds to the mineral content and was higher for corn than for sawdust and sunflower, in accordance with the results obtained following the guidelines of the ASTM E1755-01 standard. The two large exothermic peaks in the three DTA curves were assigned to the decomposition of the biopolymers present in the biomass: such as hemicellulose, cellulose and lignin. Since these biopolymers are closely related in the biomass structure, the thermal degradation of each biopolymer cannot be clearly defined independently, probably being Water 2022, 14, 3298 11 of 19 Water 2022, 14, x FOR PEER REVIEW 12 of 20 hemicellulose responsible of the first peak and cellulose and lignin together of the second peak [30]. Figure 8. DTA-TGA of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. Figure 8. DTA-TGA of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. turing wastes [49,50]. This behavior is related to the different characteristics and affinity of the metal ions for adsorbent adsorption sites [51]. Comparing the values of the hydra- tion energies (Ni(II): −2106 kJ/mol, Zn(II): −2044 kJ/mol and Cd(II): −1806 kJ/mol), related to hydrolysis of metal ions, the nickel ion has a higher hydration energy than the zinc and cadmium ions and, therefore, it is less easy to lose its water molecules from its coordina- Water 2022, 14, 3298 12 of 19 tion sphere, which would prevent it from being adsorbed by the adsorbent through com- plexation or ion-exchange mechanisms. According to Mahmood-ul-Hassan et al. [52] and Qu et al. [53], smaller ions are more hydrated than larger ones, which could hinder ad- 3.2. Adsorption Process Characterization sorption. The higher Biomasses of surface area, pore pine sawdust, volume sunflower andhulls seed mineral fraction and corn in corn residues mixresidues could were evaluated ashave positively adsorbents of contributed mono-metaltoaqueous the adsorption of heavy solutions metals. of Ni(II), However, Zn(II) the differences and Cd(II). The results in the adsorption obtained are shownresults are not in Figure 9. too significant between the three biomasses. Figure 9. Comparison of the adsorption of mono-metal solutions of Ni(II), Zn(II) and Cd(II) on pine Figure 9. Comparison of the adsorption of mono-metal solutions of Ni(II), Zn(II) and Cd(II) on pine sawdust, sunflower seed hulls and corn residues mix. sawdust, sunflower seed hulls and corn residues mix. The obtained According results to the are of results of the same order adsorption as results(A%) percentage reported and in the literature adsorption for capacity batch adsorption experiments of Ni(II), Zn(II) and Cd(II) using sawdust, sunflower (qe ) obtained, Ni(II) presented a lower adsorption, compared to Zn(II) and Cd(II), in all and corn residues (Table S1, supplementary material). However, the differences between the the biomasses studied. The adsorption follows the order: Ni(II) < Zn(II) < Cd(II), as results of adsorption percentage (A%) are not only due to the characteristics of the bio- shown in Figure 9, for sawdust, sunflower and corn, as it was also reported for other mass adsorbent but were also due to the concentration of contaminant, dosage of the ad- waste of lignocellulosic origin, such as coffee residues, rice husks, cocoa husks, and pa- sorbent, pH of the solution, temperature, contact time, among others, that are factors that per manufacturing wastes [49,50]. This behavior is related to the different characteristics can affect the adsorption process [7]. Some of these factors were evaluated by the authors and affinity of the metal ions for adsorbent adsorption sites [51]. Comparing the val- in previous work on the adsorption of heavy metals on adsorbent materials of plant origin ues of the hydration energies (Ni(II): −2106 kJ/mol, Zn(II): −2044 kJ/mol and Cd(II): [26,46,54,55]. −1806 kJ/mol), related to hydrolysis of metal ions, the nickel ion has a higher hydration The determination of the adsorption of heavy metals of a mixing metal solutions rep- energy than the zinc and cadmium ions and, therefore, it is less easy to lose its water resents a closer situation to a real effluent. The competition between the metal ions for the molecules from its coordination sphere, which would prevent it from being adsorbed by the adsorption sites occurs due to the saturation of the adsorption sites of the adsorbent whose adsorbent through complexation or ion-exchange mechanisms. According to Mahmood- dosage remains fixed to that of the individual systems [7]. Figure 10 compares the results ul-Hassan et al. [52] and Qu et al. [53], smaller ions are more hydrated than larger ones, of A% for the adsorption of mono-metal aqueous solutions of Ni(II), Zn(II) and Cd(II), and which could hinder adsorption. the multi-metal aqueous solution made up of heavy metals mentioned above with a con- The higher surface area, pore volume and mineral fraction in corn residues could have centration of 0.18 mmo/L of each of them. At this initial concentration, the results of A% positively contributed to the adsorption of heavy metals. However, the differences in the adsorption results are not too significant between the three biomasses. The obtained results are of the same order as results reported in the literature for batch adsorption experiments of Ni(II), Zn(II) and Cd(II) using sawdust, sunflower and corn residues (Table S1, supplementary material). However, the differences between the results of adsorption percentage (A%) are not only due to the characteristics of the biomass adsorbent but were also due to the concentration of contaminant, dosage of the adsorbent, pH of the solution, temperature, contact time, among others, that are factors that can affect the adsorption process [7]. Some of these factors were evaluated by the authors in previous work on the adsorption of heavy metals on adsorbent materials of plant origin [26,46,54,55]. The determination of the adsorption of heavy metals of a mixing metal solutions represents a closer situation to a real effluent. The competition between the metal ions for the adsorption sites occurs due to the saturation of the adsorption sites of the adsorbent whose dosage remains fixed to that of the individual systems [7]. Figure 10 compares the results of A% for the adsorption of mono-metal aqueous solutions of Ni(II), Zn(II) and Cd(II), and the multi-metal aqueous solution made up of heavy metals mentioned above with a concentration of 0.18 mmo/L of each of them. At this initial concentration, the results of A% for each of the heavy metals were similar when the adsorption was carried out separately and when it was carried out within the mixture, on sunflower seed Water 2022, 14, x FOR PEER REVIEW 14 of 20 Water 2022, 14, 3298 13 of 19 for each of the heavy metals were similar when the adsorption was carried out separately and when it was carried out within the mixture, on sunflower seed hulls and corn residues mix.and hulls However, the adsorption corn residues of heavythe mix. However, metals in pineof adsorption sawdust was lower heavy metals for the in pine multi- sawdust metal aqueous solution than for each metal separately, being the nickel was lower for the multi-metal aqueous solution than for each metal separately, being the ion the most af- fected by the competition with the other two heavy metals for the adsorption nickel ion the most affected by the competition with the other two heavy metals for the sites of the adsorbent,sites adsorption as expected (as Ni wasas of the adsorbent, theexpected less adsorbed, (as Ni as wasseen theinless Figure 9). Zhao adsorbed, asetseen al. [46], in also reported Figure 9). Zhaoaetlower performance al. [46], also reported of sawdust as an adsorbent a lower performance when comparing of sawdust the re- as an adsorbent sultscomparing when of A% obtained for the the results of adsorption A% obtained of for a mixture consisting the adsorption of aofmixture Cr(III),consisting Cd(II), Cu(II) of and Pb(II), on poplar sawdust and two other agricultural residues. Cr(III), Cd(II), Cu(II) and Pb(II), on poplar sawdust and two other agricultural residues. Figure 10. Comparison of the adsorption percentage (A%) of Ni(II), Zn(II) and Cd(II) in mono-metal Figure 10. Comparison of the adsorption percentage (A%) of Ni(II), Zn(II) and Cd(II) in mono-metal and multi-metal solutions on: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. and multi-metal solutions on: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. 3.3.Spent 3.3. SpentAdsorbent AdsorbentDisposal Disposal Thereisislimited There limitedinformation informationavailable availableininthe theliterature literatureon onthe thetoxic toxiceffects effectsofofspent spent adsorbents, and their regeneration decreases their performance and adsorbents, and their regeneration decreases their performance and generate new contam- generate new con- taminant inant materials. materials. The safe The safe disposal disposal of used of used and/or and/or spent spent adsorbents adsorbents is is nowadaysraising nowadays raising asasa aneed needtotoconsider considerititfor fora amore moresustainable sustainableprocesses processesthatthatcancanhelp helptotopreserve preservethe the environment environment[56]. [56].Based Basedon onthe theprevious previousexperience experienceofofthe theauthors authors[54,55], [54,55],the thelocal localpro- pro- duction ductionofofceramics ceramicsfor forbricks bricksconstruction constructionisispresented presentedasasa apossible possiblealternative alternativeforforthe the safe safedisposal disposalofofspent adsorbents spent adsorbents that would that wouldcontribute contributeto the realreal to the applicability of them applicability as of them metal adsorbents. as metal adsorbents. In Figure 11, the adsorption percentages obtained from the experiments using 0.1 g of adsorbent and 1.8 × 10− 6 moles of each of the heavy metals in the multi-metal system Water 2022, 14, x FOR PEER REVIEW 15 of 20 Water 2022, 14, 3298 14 of 19 In Figure 11, the adsorption percentages obtained from the experiments using 0.1 g of adsorbent and 1.8 × 10−6 moles of each of the heavy metals in the multi-metal system arecompared are comparedwith withthose those results results obtained by by increasing increasingthetheresidue residuemass massbyby 2020 times to 2 times tog2and moles g and of adsorbate moles to 3.6 of adsorbate to×3.6 10 ×moles. −5 10 − 5 As can be moles. Asseen, by seen, can be maintaining the adsor- by maintaining bent/adsorbate the ratio constant, adsorbent/adsorbate ratio the adsorption constant, percentage percentage the adsorption remained constant. remained These results constant. wereresults These considered were in the preparation considered of clay ceramics in the preparation with of clay 20% bywith ceramics volume 20%ofbycontaminated volume of biomass withbiomass contaminated respect with to therespect volume toof theclay. volume of clay. Figure 11. Comparison of the percentage of adsorption (A%) when increasing 20 times the mass of Figure 11. Comparison of the percentage of adsorption (A%) when increasing 20 times the mass of adsorbent and moles of adsorbate in the multi-metal system of heavy metals for: (A) pine sawdust, adsorbent and moles of adsorbate in the multi-metal system of heavy metals for: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix. (B) sunflower seed hulls, (C) corn residues mix. Themacroscopic The macroscopicappearance appearance of of the the clay clay ceramics ceramicsisisshown showninin Figure Figure12.12. All All the the sam- ples presented a reddish color due to the Fe content of the natural clay and samples presented a reddish color due to the Fe content of the natural clay and a porous a porous sur- face according surface accordingtotothe theTGA TGAand andDTA DTAresults resultsofofthe theincluded included lignocellulosic lignocellulosic residues. As residues. can be seen in Figure 8, at the ceramic firing temperature (950 ◦ °C), the As can be seen in Figure 8, at the ceramic firing temperature (950 C), the added biomass added biomass burned burned out out creating creating pores pores and and releasing releasing gases gases inin thethe clay clay ceramics ceramics matrix. matrix. Leaching tests based on EPA Method 1311 were performed to determine the possible leaching levels of Ni(II), Zn(II) and Cd(II) from the clay ceramics prepared with the addition of each of the spent adsorbents. Heavy metal concentrations were not detected in the TCLP extracts of the ceramic matrices because they were below the detection limits of the equipment used (Ni(II) < 0.05 mg/L, Zn(II) < 0.02 mg/L and Cd(II) < 0.05 mg/L, by AAS). For this reason, they were lower than the permissible limits of Argentina (nickel 5 mg/L and cadmium 1 mg/L, zinc not reported) [57]. Water 2022, 14, 3298 15 of 19 Water 2022, 14, x FOR PEER REVIEW 16 of 20 Figure 12. Macroscopic appearance of clay ceramics including spent biomass of: (A) pine sawdust, Figure 12. Macroscopic appearance of clay ceramics including spent biomass of: (A) pine sawdust, (B) sunflower seed hulls, (C) corn residues mix, and comparison with a sample without residue (B) sunflower seed hulls, (C) corn residues mix, and comparison with a sample without residue addition. addition. Table 3 shows the retention efficiency calculated for each one clay ceramics matrices. Leaching tests based on EPA Method 1311 were performed to determine the possible It was calculated from the mass of each of the heavy metals added in the ceramic (included leaching levels of Ni(II), Zn(II) and Cd(II) from the clay ceramics prepared with the addi- in the spent adsorbent) and in the TCLP extract obtained for each tested ceramic. Based on tion of each of the spent adsorbents. Heavy metal concentrations were not detected in the these results, with heavy metal retentions above 88.5% in all cases, we can propose such TCLP extracts of the ceramic matrices because they were below the detection limits of the clay ceramics prepared with added spent adsorbents with potential use in construction, equipment used (Ni(II) < 0.05 mg/L, Zn(II) < 0.02 mg/L and Cd(II) < 0.05 mg/L, by AAS). and useful for the For this reason, stabilization they were lower and thanimmobilization of heavy the permissible limits metals together of Argentina (nickel 5with mg/Lthe corresponding spent adsorbents. At firing and cadmium 1 mg/L, zinc not reported) [57]. temperatures in the followed leaching tests (EPA Method Table 3 shows the retention efficiency calculated for each one clay ceramics matrices. to 1311), the organic residues can burn out and the heavy metals would be able form stable phasesfrom It was calculated withthe the clayofminerals, mass which each of the would heavy metalsdecrease added intheir bioavailability the ceramic (included[58]. in the spent adsorbent) and in the TCLP extract obtained for each tested ceramic. Based Table 3. Heavy on these metal results, retention with heavyefficiency for clay ceramics metal retentions prepared above 88.5% in from spentwe all cases, adsorbent with the can propose mixture of heavy metals. such clay ceramics prepared with added spent adsorbents with potential use in construc- tion, and useful for the stabilization and immobilization of heavy metals together with the Retention Efficiency (%) corresponding Clay Ceramics spent adsorbents. At firing temperatures in the followed leaching tests (EPA Method 1311), the organic Ni(II) residues can burn out Zn(II) and the heavy metals Cd(II) would be ablePine to form stable phases with>88.5 sawdust the clay minerals, which would decrease their >98.2 bioavaila- >97.1 Sunflower bility [58]. seed hulls >95.9 >99.1 >98.5 Corn residues mix According to Mohajeran et>93.4al. [59], the particle size>98.7 of the sample determines >97.9 the con- tact surface with the leaching solution and therefore influences the concentration of heavy metals detected.toThis According fact is important Mohajeran because et al. [59], in the leaching the particle size oftests the the clay ceramics sample werethe determines used crushed, but in practice the clay bricks will be used whole, so contact surface with the leaching solution and therefore influences the concentration the leaching may be of even lower, even there is still no legislation that imposes a test and limits on heavy metals detected. This fact is important because in the leaching tests the clay ceramics the leaching of heavy were usedmetals crushed, in construction but in practicematerials the clay[60]. bricks will be used whole, so the leaching may be even lower, even there is still no legislation that imposes a test and limits on the leaching Table 3. Heavy metal retention efficiency for clay ceramics prepared from spent adsorbent with the ofmixture heavyofmetals in construction materials [60]. heavy metals. 4. Conclusions Retention Efficiency (%) Clay Ceramics Ni(II) Zn(II) of waste generated One of the challenges of these times is the minimization Cd(II) by agro- Pineactivities industrial sawdust and/or the reuse >88.5of this waste in applications >98.2 >97.1the quality that improve Sunflower of life. In thisseed hullsconsidering>95.9 sense, >99.1 that there is still no universal process to >98.5 remove heavy Corn residues mix >93.4 >98.7 >97.9 metals from wastewater and effluents, adsorption from agroindustry residues is emerging as a simple, low-cost and efficient alternative. Agro-industrial residues such as pine sawdust, sunflower seed hulls and corn residues mix, without any additional treatment, are characterized. These results are correlated with the performance of these materials as adsorbents of heavy metals such as Ni(II), Zn(II) and Water 2022, 14, 3298 16 of 19 Cd(II). In addition, SEM-EDS and XRF confirmed the presence of these heavy metals in the residues after the adsorption process. Batch adsorption experiments from aqueous mono-metal solutions of nickel, zinc and cadmium ions, with concentrations of 0.18 mmol/L, with all three biomass residues selected showed promising results, with adsorption percentages greater than 50%. Ni(II) presented the lowest adsorption percentages and adsorption capacities compared to Zn(II) and Cd(II), possibly due to the higher hydration energy that could hinder its accessibility to the adsorbent. At this concentration, in a multi-metal solution, the decrease in adsorption due to the competition of heavy metals for the limited adsorption sites of pine sawdust determined that it is necessary to study multi-component systems to evaluate the actual performance of the adsorption process in practice. Sawdust, sunflower and corn residues could be used as an alternative to traditional synthetic materials to remove heavy metals from wastewater given their properties, low cost and availability. However, more research is needed for the scale-up and possible commercial application of these agro-industrial residues as adsorbents of toxic metals from industrial wastewater. Furthermore, a solution to the safe disposal of such biomass adsorbents after the adsorption process (containing heavy metals as pollutants) was proposed in this research work. The stabilization of these spent adsorbents in clay ceramics with possible use in construction is presented as an alternative for the immobilization of Ni(II), Zn(II), Cd(II) and their mixture. The heavy metal leaching tests of the ceramic matrices prepared with added spent adsorbents confirmed an effective immobilization for all heavy metals, whose concentrations were found to be below the permissible limits. The clay ceramics showed retention efficiencies over 88.5%. Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/w14203298/s1, Figure S1: XRD patterns of pine sawdust, sunflower seed hulls, and corn residues mix; Table S1: Main results of literature studies about the adsorption of Ni(II), Zn(II) and Cd(II) on sawdust, sunflower and corn. References [44,52,61–67] are cited in the “supplementary materials”. Author Contributions: Conceptualization, C.P. and A.C. (Adrián Cristóbal); methodology, D.S., C.P. and A.C. (Adrián Cristóbal); validation, D.S., A.C. (Agustín Costas) and A.C. (Adrián Cristóbal); investigation, D.S. and A.C. (Adrián Cristóbal); resources, C.P., A.C. (Agustín Costas) and A.C. (Adrián Cristóbal); writing—original draft preparation, D.S., C.P. and A.C. (Adrián Cristóbal); writing—review and editing, D.S., C.P. and A.C. (Adrián Cristóbal); project administration, C.P. and A.C. (Adrián Cristóbal); funding acquisition, C.P. and A.C. (Adrián Cristóbal). All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by grants from Spanish research projects (CTM2015-65414- C2-1-R and AGL2015-70393-R), National Scientific and Technical Research Council (PIP 2021-2023 GI 11220200100739CO), National University of Mar del Plata (UNMdP 15/G577, 2020-2021) and H2020-MSCA-RISE-2014 Research Executive Agency (UE) (GA645024 (NANOREMOVAS)). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: All the authors are grateful to the UAB Microscopy Service (Servei de Microscòpia Electrònica from UAB, Catalunya, Spain) for the SEM analysis and the Centro Biotecnológico Fares Taie (Mar del Plata, Argentina) for the AAS analysis of the TCLP extracts. Conflicts of Interest: The authors declare no conflict of interest. Water 2022, 14, 3298 17 of 19 References 1. Uddin, M.K. A Review on the Adsorption of Heavy Metals by Clay Minerals, with Special Focus on the Past Decade. Chem. Eng. J. 2017, 308, 438–462. [CrossRef] 2. Hokkanen, S.; Bhatnagar, A.; Sillanpää, M. A Review on Modification Methods to Cellulose-Based Adsorbents to Improve Adsorption Capacity. Water Res. 2016, 91, 156–173. [CrossRef] [PubMed] 3. Tounsadi, H.; Khalidi, A.; Machrouhi, A.; Farnane, M.; Elmoubarki, R.; Elhalil, A.; Sadiq, M.; Barka, N. Highly Efficient Activated Carbon from Glebionis coronaria L. 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