polymers Article Boron Removal from Aqueous Solutions by Using a Novel Alginate-Based Sorbent: Comparison with Al2O3 Particles Hary Demey 1, * , Jesus Barron-Zambrano 2 , Takoua Mhadhbi 3 , Hafida Miloudi 4 , Zhen Yang 5 , Montserrat Ruiz 1 and Ana Maria Sastre 1 1 Department of Chemical Engineering, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain; montserrat.ruiz@upc.edu (M.R.); ana.maria.sastre@upc.edu (A.M.S.) 2 Faculty of Chemical Engineering, Universidad Autonoma del Yucatan, Periférico Norte kilómetro 33.5, Tablaje Catastral 13615, Chuburna de Hidalgo Inn, Mérida, Yucatán, C.P. 97203, Mexico; jesus.barron@correo.uady.mx 3 Laboratory of Environmental Biomonitoring (LBE), University of Carthage, Faculty of Sciences of Bizerte, Avenue de la République, 7021 Jarzouna, Tunisia; mhadhbitakoua@gmail.com 4 Laboratory of Chemistry of Materials, University of Oran 1, B.P 1524 El M’naouer-Oran, Algeria; mmiloudi@yahoo.com 5 School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory of Material Cycling and Pollution Control, Nanjing Normal University, Nanjing 210023, China; yangzhen@njnu.edu.cn * Correspondence: hary.demey@upc.edu; Tel.: +34-938-937-778  Received: 8 August 2019; Accepted: 13 September 2019; Published: 16 September 2019  Abstract: Boron removal was evaluated in the present work by using calcium alginate beads (CA) and a novel composite based on alginate–alumina (CAAl) as sorbents in a batch system. The effects of different parameters such as pH, temperature, contact time, and composition of alginate (at different concentrations of guluronic and mannuronic acids) on boron sorption were investigated. The results confirm that calcium alginate beads (CA) exhibited a better adsorption capacity in a slightly basic medium, and the composite alginate–alumina (CAAl) exhibited improved boron removal at neutral pH. Sorption isotherm studies were performed and the Langmuir isotherm model was found to fit the experimental data. The maximum sorption capacities were 4.5 mmol g−1 and 5.2 mmol g−1 , using CA and CAAl, respectively. Thermodynamic parameters such as change in free energy (∆G0 ), enthalpy (∆H0 ), and entropy (∆S0 ) were also determined. The pseudo-first-order and pseudo-second-order rate equations (PFORE and PSORE, respectively) were tested to fit the kinetic data; the experimental results can be better described with PSORE. The regeneration of the loaded sorbents was demonstrated by using dilute HCl solution (distilled water at pH 3) as eluent for metal recovery. Keywords: alginate; alumina; boron; composite; novel sorbent; sorption 1. Introduction Boron compounds are extensively used in the chemical industry for manufacturing of cosmetics, heat-resistant glasses, ceramics, and fire retardants materials. Boric acid is also widely applied as a disinfectant due to its antibacterial and antifungal properties [1]. World boron demand has increased over the last decade and is expected to increase over the following years; this fact has accelerated the introduction of this element into environmental ecosystems, and in the pollution of rivers and soils of European countries. Water containing boron may affect agricultural harvests, causing a negative impact on local economies. A typical example of this contamination, is the case of the Valencia region in Spain (where the densest area of ceramic industries in the country is located); wastewaters are treated Polymers 2019, 11, 1509; doi:10.3390/polym11091509 www.mdpi.com/journal/polymers Polymers 2019, 11, 1509 2 of 17 with conventional methods before discharging into the public network, but specifically designed techniques for the removal of boron from the effluents are not available. Although the growth of many plants needs trace amounts of boron, this element at high concentrations becomes harmful to both plants and animals [1]. According to the statements of the World Health Organization (WHO), clear influences on the male reproductive system after either short- or long-term exposure have been found: dogs, mice, and rats developed testicular lesions when fed with boric acid or borax. The upper limit of the concentration of boron in drinking water has been set to 2.4 mg·L−1 by the WHO [2]. Boron could be toxic in harvests depending on the strength of the plants in resisting high levels of this element [3]. Valencia is the region in Spain characterized by the highest production of citric fruits (such as orange and lemon), and also is the focus of the major porcelain production of the country. However, these fruits are among the most sensitive to boron compounds. Important losses in agricultural fields have been reported by government authorities due to the high concentration of boron in waters. Innovative technologies must be developed to provide solutions to this environmental concern. Besides boron, ceramic industries also use metals oxides during the manufacturing process of frits glaze (typically ZnO); zinc oxide is utilized as a fluxing agent in the preparation of frits and enamels, improving the elasticity by reducing the change in viscosity (as a function of temperature). Therefore, Zn(II) is largely present with boron in the ceramic effluents (frits production). Several methods for boron and zinc removal from aqueous solutions have been previously reported. These methods include electrodialysis [4] precipitation [5], chemical coagulation and electrocoagulation [6,7], complexation/nanofiltration [8], phytoremediation [9], ion-exchange [10,11], reverse osmosis [12–14], and sorption [15–20]. Biopolymer-based materials could be an economical alternative technology for the removal of metalloids and heavy metals from aqueous solutions. The use of biopolymers for boron sorption has received little attention over the last decade. Published papers report the possibility of using chitosan modified with N-methyl-D-glucamine [21], cotton cellulose [22], and sugars [23]; the potential abilities of alginates for boron removal and its application as an encapsulating material have been demonstrated as well [24–27]. Alginate is a polysaccharide existing in brown algae, which are abundant in different marine ecosystems of the earth. Alginate consists of β-D-mannuronic and α-L-guluronic acids; these contain hydroxyl groups on their molecular backbones, making the biopolymer reactive towards boron compounds (esters formation). In turn, when divalent ions coexist, a three-dimensional hydrogel network could be formed, which typically follows the egg-box model [28]. The addition of functional groups into the biopolymer matrix may increase the reactivity and/or the strength of the resulting sorbent; low operating costs and the availability of the feedstock are the main criteria for the selection of competitive materials. Alumina is one of the most common sorbents used in recent years, since aluminum sources are present throughout the world; however, alternative sustainable materials have to be developed for reducing and recycling for the utilization of mineral compounds. Bouguerra et al. [29] developed sorption of boron using alumina particles (particle size <200 µm) and the results were compared with the conventional reverse osmosis systems. Although alumina is able to remove boron, Bouguerra et al. [29] did not study the elution step and it is not clear whether the particles can be reused. A good sorbent conceived for large-scale applications, needs to be recyclable and have ease of manipulation in order to be competitive. The aim of the present work was to employ calcium alginate (CA)-based materials for boron removal from waters; the possibility of using a new alginate/alumina-based composite is reported to facilitate the handling of alumina on sorption processes. Sorption capacities of commercial samples, namely alumina and CA were tested for comparison. The equilibrium, as well as kinetic, sorption data were evaluated in detail. Polymers 2019, 11, 1509 3 of 17 2. Materials and Methods 2.1. Materials Boron solutions were prepared by dissolving boric acid (H3 BO3 , provided by Merck KGaA, Darmstadt, Germany) in Milli-Q water. The alumina (Al2 O3 ) particles, and the salts of sodium and calcium were supplied by Panreac (Barcelona, Spain). The content in mannuronic and guluronic acids (M and G blocks, respectively) was determined by an NMR spectroscopy technique. The molar ratio was estimated as 64% M-blocks and 36% G-blocks [25]. 2.2. Preparation of Sorbents The sorbent manufacturing can be summarized in three steps [25]: i) Two liters of sodium alginate solution (2% w/w) were obtained by dissolving 40 grams of sodium alginate in 1960 g of pure water; the mixture was mechanically stirred at 300 rpm for 4 h to achieve full homogenization. Then, the stirring speed was changed to 50 rpm for 2 h, to dissipate the air and to avoid bubble formation. Alginate solution was divided into two different recipients (of one liter) and these were labeled as A and B. ii) Twenty grams of alumina were carefully introduced into the B recipient and it was kept under agitation (at 300 rpm) overnight. iii) Four liters of ionotropic gelation solution of calcium nitrate (0.05 M) were prepared and divided into two recipients of two liters each one (C and D solutions). Then, the A and B solutions were systematically dosed under continuous agitation into C and D solutions, respectively. The produced beads were identified as CA (calcium alginate) and CAAl (composite alginate–alumina). The scheme of the manufacturing process is represented in Figure S1 (in the Supplementary Materials Section). A volume of 500 mL of each sorbent material was air-dried at 20 ◦ C for 72 h. The average diameter of the beads was determined by using a Malvern-Mastersizer 3000TM instrument; it was 2.3 mm and 0.8 mm for hydrogel and air-dried beads, respectively. 2.3. Characterization of Sorbents 2.3.1. Scanning Electron Microscopy Surface morphology of the beads was analyzed using a JEOL JSM 7100F field emission scanning electron microscope (JEOL Ltd., Peabody, MA, USA), equipped with an Energy Dispersive X-ray (EDX) spectrometer (INCA 250, Oxford instruments, Oxford, UK). The sorbent samples were analyzed before and after sorption of boron from aqueous solutions through EDX-technique; although the detection of boron is difficult due to the low photon energy of this element (which emits low energy peaks close to the electronic noise of the detection system), the main elements of the natural alginates were identified. 2.3.2. Thermal-Gravimetric Analyses Thermal-gravimetric analyses were performed under N2 atmosphere (with a flow rate of 50 mL min−1 ) at a heating rate of 10 ◦ C min−1 using a Perkin-Elmer TGA 6 instrument (Waltham, MA, USA). The samples of 5 mg were degraded within a temperature range of 30–300 ◦ C. 2.4. Batch Sorption Experiments The pH values were adjusted to be within the range of 3 to 12 by using 0.1 M NaOH and/or 0.1 M HCl aqueous solution (as required). An amount of 0.7 g of sorbent was added to 100 mL of boron solution (50 mg L−1 ) at room temperature (20 ± 1 ◦ C). After 72 h of agitation (at 150 rpm) using an orbital shaker (Rotabit J.P. Selecta, Barcelona, Spain), the solutions were forced through 1.2 µm filtration membranes and the filtrates were analyzed using a microwave plasma-atomic emission spectrometer 4100 MP-AES equipment (from Agilent technologies, Melbourne, Australia) at a wavelength of 249.7 nm. The sorption capacity was calculated by means of Equation (1). Equilibrium experiments (sorption isotherms) were carried out by mixing 0.7 g of adsorbent with 100 mL of boron solutions at different concentrations (10–500 mg L−1 ) and agitated for 72 h. The initial Polymers 2019, 11, 1509 4 of 17 pH of the boron solutions was adjusted to 11. The residual boron concentration was determined using 4100 MP-AES equipment. The uptake kinetics experiments were performed by adding (under continuous stirring) a known amount of adsorbent (i.e., 1.75 g) to 250 mL of boron solution (50 mg L−1 ) at a fixed initial pH 11; aliquots of adsorbate were withdrawn for analysis at different times over 72 h of contact (at 150 rpm). The collected samples were then filtered through a 1.2 µm filtration membrane. The kinetic profiles were compared for two different configurations of the beads: • Hydrogel beads (Ø 2.3 mm) • Air-dried beads (Ø 0.8 mm) The sorption uptake and the sorption efficiency of the materials were obtained through Equations (1) and (2):   V C0 − Ceq q= (1) m   C0 − Ceq % SE = ·100 (2) Ceq where q is the sorption capacity of sorbents (mg g−1 ), % SE is the boron sorption efficiency, C0 is the initial boron concentration (mg L−1 ), Ceq is the boron concentration at equilibrium (mg L−1 ), m is the mass of adsorbent (g), and V is the volume of the boron solution (L). All experiments were carried out at room temperature. 3. Fitting of Sorption Isotherms and Kinetics 3.1. Sorption Isotherms Equilibrium data are commonly described by the Langmuir and Freundlich models (Equations (3) and (4), respectively). These models allow the distribution of the boron species between the adsorbent and the solution interface to be described [30]: qmax bCeq q= (3) 1 + bCeq 1 q = KF Ceq n (4) where qmax is the maximum adsorption capacity of sorbents (mg g−1 ), and Ceq is the equilibrium concentration in the solution. In the Langmuir model, b is related to the energy of adsorption (L mg−1 ). The parameters n and KF are Freundlich adsorption constants, indicative of adsorption intensity and relative capacity, respectively. 3.2. Kinetic Studies The effect of the contact time is typically described by using the pseudo-first-order (PFORE) [31] and pseudo-second-order (PSORE) rate equations [32,33]. PFORE: dqt   = K1 q1 − qt (5) dt Integrating for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt :   K1 log qeq − qt = log(qeq ) − t (6) 2.303 Polymers 2019, 11, 1509 5 of 17 PSORE: dqt  2 = K2 dt (7) qeq − qt Integrating for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt : 1 1 1 = + t (8) qt K2 q2eq qeq where qeq is the equilibrium sorption capacity (mg g−1 ), qt is the sorption capacity (mg g−1 ) at any time t (min), k1 and k2 are the pseudo-constants of the models (g mg−1 min−1 ). The intraparticle diffusion was obtained with Equation (9) (Weber and Morris equation, W&M) [34]: qt = Kp t1/2 + C (9) where C is the intercept, and Kp is the intraparticle diffusion rate constant. 3.3. Effect of Temperature Two temperatures (20 ◦ C and 35 ◦ C) were selected to assess the temperature effect on the removal performance using CA and CAAl as sorbents. Van’t Hoff Equation (10) was used to determinate the thermodynamic parameters of the sorption process: standard entropy (∆S◦ ), standard enthalpy change (∆H◦ ) and free energy change [19]: ∆H◦ ∆S◦ lnKC = − + (10) RT R Gibbs free energy changes (∆G◦ ) were determined according to ref. [19]: ∆G◦ = −RTlnKC (11) CS KC = (12) Ceq where T is the solution temperature (K), Kc is the equilibrium constant, R is the gas constant (8.314 J mol−1 K−1 ), Cs is the equilibrium concentration of boron onto the sorbent (mmol L−1 ), and Ceq is the equilibrium boron concentration (mmol L−1 ). 3.4. Towards an Application with Ceramic Wastewater In order to evaluate further real effluents from the ceramic industries in the column system (which will be the scope of a future work), the assessment of the CAAl sorbent was first performed with Zn(II) ions in a batch system. The material was evaluated with Zn(II), since it is one of the most representative metals used in the frits production, and it is commonly found in coexistence with boron species [35]. The maximum sorption capacity and kinetic profiles for zinc removal were determined. 4. Results and Discussion 4.1. Characterization of the Materials 4.1.1. SEM–EDX Analyses The examination of the topography and surface structure of the sorbents was possible through SEM micrographs (Figure 1). The roughness of the material surface is reported in Figure 1a,b, the SEM images indicate the presence of some fissures that were formed during the drying process. Figure 1c shows the alumina particles which are attached to the calcium alginate network (onto the surface and in the whole volume of the beads). Polymers 2019, 11, 1509 6 of 17 Some beads were cross-sectioned to observe the distribution of alumina in the volume of the sorbent (Figure 1b). The alumina was not homogenously distributed into the beads and this may be due to the low agitation speed used during the manufacturing process; nevertheless, it was found Polymers that the2019, 11, 1509content in the composite was constant (2% w/w approximately). The spectrum alumina 6 of in 18 the cross-sectioned surface of the material (Figure 1c) demonstrates the presence of calcium, carbon, cross-sectioned oxygen, alumina, surface of the chloride andmaterial sodium (Figure ions, as 1c) demonstrates expected the are since these presence of calcium, the main elementscarbon, of the oxygen, alumina, chloride and sodium ions, as expected since these composite. Some alumina particles can be observed embedded in the external areaare the main elements of the of the beads composite. (Figure 1d); Some alumina could their presence particles can the modify be observed embedded surface charge in the external of the sorbent, areathe enhancing ofremoval the beadsof (Figure 1d); their presence different metal ions [29]. could modify the surface charge of the sorbent, enhancing the removal of different metal ions [29]. Figure 1. Scanning Electron Microscopy (SEM) images of the CAAl material: (a,b) Topography of the Figure 1. Scanning Electron Microscopy (SEM) images of the CAAl material: (a,b) Topography of the sorbent; (c) Energy Dispersive X-ray (EDX) analysis on the surface area of the sorbent, (d) Alumina sorbent; (c) Energy Dispersive X-ray (EDX) analysis on the surface area of the sorbent, (d) Alumina particles immobilized in the alginate support. particles immobilized in the alginate support. 4.1.2. TGA Thermal-gravimetric analyses of CA and CAAl were performed to examine the degradation of the materials with temperature. The TGA and derivative curves are presented in Figures S2a and S2b (in the Supplementary Materials Section). The thermal decomposition of polysaccharides include Polymers 2019, 11, 1509 7 of 17 4.1.2. TGA Thermal-gravimetric analyses of CA and CAAl were performed to examine the degradation Polymers 2019, 11, 1509 7 of 18 of the materials with temperature. The TGA and derivative curves are presented in Figures S2a,b (in the C–O andSupplementary C–C bonds in the Materials Section). ring units, The thermal as a result, decomposition the production of CO, of COpolysaccharides include three 2, and H2O molecules is increased), (iii) the development of poly-nuclear aromatic and graphitic carbon structures [36].ofBoth different phases: (i) dehydration, (ii) depolymerisation (which is an adjunct to the rupture C–O and C–C bonds in the ring units, as a result, the production of CO, materials have similar degradation profiles with an initial weight loss 2(4% for CA, CO , and H 2 O molecules is and 7% for CAAl) increased), (iii) to due thethe development of poly-nuclear residual moistures aromatic of the samples. and graphitic Thermograms carbon show that structures [36]. Both the decomposition of materials the adsorbents begins at 195.4 °C for CA, with a residual mass of 33% and at 210.2 °C for CAAl with ato the have similar degradation profiles with an initial weight loss (4% for CA, and 7% for CAAl) due residualmass residual moistures of 57%. ofThese the samples. Thermograms results show show that that these sorbents the decomposition could even be used atof thetemperatures high adsorbents begins at 195.4 ◦ ◦ with CAAlChaving for CA,a with bettera thermal residualstability; mass of 33% and atof210.2 this being C for scientific CAAl for interest with a residual further mass of 57%. applications with These CAAl material. results show that these sorbents could even be used at high temperatures with CAAl having a better thermal stability; this being of scientific interest for further applications with CAAl material. 4.2. Effect of pH 4.2. Effect of pH The effect of pH is a key factor on the sorption process. The pH may affect the speciation of the metalsThe through formation effect of pH is a keyof complex factor onmolecules the sorption[37],process. and theThe protonation pH may affect of carboxylic groupsof the the speciation present in the alginate chains (the mannuronic and guluronic blocks; metals through formation of complex molecules [37], and the protonation of carboxylic groupsi.e., M and G-blocks) [38]. The present experiments were carried out over a pH range from 3 to 12; the results in Figure in the alginate chains (the mannuronic and guluronic blocks; i.e., M and G-blocks) [38]. The experiments 2a demonstrated that were higher carriedsorption efficiency out over a pH wasrangeobtained from 3ontodecreasing the proton 12; the results concentration in Figure of the solutions. 2a demonstrated that higher Initially (over pH 3–9), the best sorption efficiency was obtained with sorption efficiency was obtained on decreasing the proton concentration of the solutions. CAAl material, which is the Initially result of the individual contribution of alumina and alginate in this interval. (over pH 3–9), the best sorption efficiency was obtained with CAAl material, which is the result However, from pH > 9of the the best boron uptake is obtained with CA material (the sorption with alumina decreases with an individual contribution of alumina and alginate in this interval. However, from pH > 9 the best boron increase in the pH). uptake is obtained with CA material (the sorption with alumina decreases with an increase in the pH). Figure 2. Influence of pH on boron removal. (a) Removal performances using alumina, CA and CAAl. Figure 2. Influence of pH on boron removal. (a) Removal performances using alumina, CA and CAAl. materials (b) variation in pH (pH0 : 3–12, [B]initial = 50 mg L−1 , V = 100 mL, T room, 1 atm). materials (b) variation in pH (pH0:3–12, [B]initial= 50 mg L−1, V = 100 mL, T room, 1 atm). In our previous studies, it was suggested that the binding reaction of boron species [B(OH)3 and B(OH)In our − previous studies, it was suggested that the binding reaction of boron species [B(OH)3 and 4 ] occurs with the OH- groups in the cis-position on the reagent sorbent [26]. Thus, boron B(OH)4-] occurs with the OH- groups in the cis-position on the reagent sorbent [26]. Thus, boron sorption onto CA, CAAl, and alumina is the result of the ester formation between the OH− groups sorption onto CA, CAAl, and alumina is the result of the ester formation between the OH- groups of of the boron species and those present in the sorbent material. The removal of boron by alumina the boron species and those present in the sorbent material. The removal of boron by alumina (pH 3– (pH 3–9) is attributed to both chemical and physical processes: i) interaction with hydroxyl ions, and 9) is attributed to both chemical and physical processes: i) interaction with hydroxyl ions, and ii) ii) interaction with electrostatic forces between the sorbent and boron species. The results obtained interaction with electrostatic forces between the sorbent and boron species. The results obtained with with alumina agree with the trend presented by Seki et al. [16] and Bouguerra et al. [29], who reported a alumina agree with the trend presented by Seki et al. [16] and Bouguerra et al. [29], who reported a negative influence on boron removal with increasing pH (using alumina and alumina-based materials as sorbents). Polymers 2019, 11, 1509 8 of 17 Polymers 2019, 11, 1509 8 of 18 Polymers 2019, 11, 1509 8 of 18 negative influence Theon pHboron and concentration removal with increasing determinepH the(using chemical species alumina and ofalumina-based boron present materials in the solution. As as sorbents). Thethe pH pKaand for boric concentration Polymers 2019, 11, 1509 acid is 9.2 and determine the the point of chemical zero charge species of (pH boron PZC) present for different in the types solution. of alumina As 8 ofis18 The the pH pKa Polymersaround and11, for 2019, 8.7–9.0 boric1509acid[29], concentration is 9.2 the andsurface determine the point of alumina the of zeroisspecies chemical negatively charge of (pH charged boron PZC ) present at pH for different >pH in the PZC. Thus, solution. types Asanionic of alumina the 8 of is18 boron pKaaround species for boric acid 8.7–9.0 [B(OH) is [29],9.2 and the -] would the surface pointhave of The pH and concentration determine thePZC 4 of lower zero alumina interactions charge is (pH negatively with ) for charged alumina different at pH (and types >pH derivatives of alumina . Thus, chemical species of boron present in the solution. As PZC materials), is anionicaround boron giving a 8.7–9.0 lower [29], speciesThe the [B(OH) the pH boron surface pKaand4 ]for - uptake of would alumina borichave concentration at pH acid loweris >9; hence negatively 9.2 interactions is determineand thethe alginate charged present with chemical point at pH alumina of zero > in speciesthe pH charge composite (and of (pH PZC . Thus, derivatives boron becomes anionic PZC)present the boron materials), for differentin the types active species giving solution. material Asa of alumina foris [B(OH) − ]boron would sorption have (Figure lower 2a). interactions with alumina (and derivatives materials), giving aalumina lower lower boron the 4pKa for uptake around boric8.7–9.0 at acid pH is >9; [29], 9.2 hence theand the surface alginate point of alumina present of zeroisin chargethe composite negatively (pHPZC ) for charged becomes at pHthe different >pH active types PZC. of material Thus, foris boron anionic boron uptake boron around sorptionat pH(Figure 8.7–9.0 species >9; hence [29], [B(OH) the 42a). alginate -]surface would of have present alumina B(HO) lower 3+ inis H the ↔ B(HO) O composite 2negatively interactions 4- becomes charged with +H + at pH alumina thepKaactive >pH (and =9.2 material . Thus, anionic derivatives PZC for boron materials), boron (13)a giving sorption (Figure species [B(OH) lower 2a). 4 ] would boron Figure - 2buptake presentshaveB(HO) at pHlower the >9; 3impact 2O ↔ interactions + hence H B(HO) ofalginate with +H 4present - the variation alumina + ofinboron the(and pKa derivatives =9.2 composite solutions pHmaterials), becomes on the the sorptiongiving active (13)a material process.for lower boron boron Between Figure uptake sorption at pH 3 andthe 2b presents pH (Figure>9; hence 2a). 5, carboxylic impact of the alginate groups present in theof variation in the alginate composite boron formedsolutions becomes a weak pH onbase the the thatactive sorption material neutralized process. for the acid − + boronsolution; Between sorption pH 3 and (Figure this5, was B(HO) 2a). carboxylic 3 +groups verified Hby 2B(HO)↔inB(HO) O comparing3the 4 ↔ + Halginate 2O +H the initial formed B(HO) pKa 4- and +H += a weak 9.2 base final pH pKathat of =9.2boron neutralizedsolutions. (13)acid the Haug [39] (13) solution; Figure reported 2b presentsthisFigure waspKa the impact values verified 2b presents of ofbyB(HO) COO- thecomparing the 3 + H groups O impactofthe variation 2 ↔ in of B(HO) mannuronic the boron initial4 - + H+ and guluronic and final variation solutions of pKa pHboronpHtheof on =9.2 acids as boron solutions. solutions sorption pH 3.38 process. and on the 3.65 Haug sorption Between (13) respectively; [39]process. pH 3 and 5, carboxylic groups in the alginate formed a weak base that neutralized the acid solution;negligible. reported thus, Figure pKa Between at 2b pH values below presents pH of 3 COO- and 3.5 the5, the groups impact carboxylic carboxylic in of the groups groups mannuronic variationin are and the of protonated guluronic boron alginate and acids solutions formed a the as pH weak sorption 3.38on and the base of 3.65 boron respectively; sorption that is process. neutralized the acid thus, at Between At pH pH solution; this was verified > below 3 3.5 and the 3.5 5, protons the carboxylic this was verified by comparing of carboxylic the groups the initial solution groups by and in the comparing compete are protonated alginate final pHthe with formed boric and a acid the weak initial solutions. of boron and final Haug for sorption base the active of that pH of[39] boron sites neutralized boron is and a negligible. solutions. reported the “buffering acid pKa Haug [39] At pH effect” solution;> 3.5 the this reported iswas produced. protons pKa verified values of Upon theby of increasing solution comparing COO- compete groups the the in pH, with initial mannuronic values of COO- groups in mannuronic and guluronic acids as 3.38 and 3.65 respectively; thus, at pH the boric and proton acid final and forconcentration pH the guluronic of active boron acids sitesis lower and solutions. as 3.38 a and and the “buffering Haug 3.65 sorption [39] respectively; effect” belowreportedbecomes is 3.5 the thus,produced. pKa carboxylicmore atvalues pH below efficient Upon of groupsCOO- 3.5 (aprotonated similar increasing aregroups the inthe carboxylic trendpH, mannuronic and was the also groups the arereported proton and sorption guluronic protonated of boronbyacids concentration Fiol and et asisal. the 3.38 is negligible. [27]). lower and sorption The Atand ofmaximum 3.65 pH the sorption respectively; > boron3.5 the removal is negligible. becomes thus, of at protons of the boron more pH below At solution is pH > 3.5competeobtained efficient 3.5 (a the the protons at similar equilibrium carboxylic trend withofboric was groups the solution pHalso acid for the are9.5–9.8. reported protonated compete The active with by interaction Fiol and sites and et the borica acidal. of [27]). sorptionboron The of for theeffect” “buffering species maximum boron activeissites is with alginate removal negligible. and a “buffering produced. and Uponof pHalumina Atboron >effect” increasing 3.5 the can is obtained is the beat protons summarized produced. pH, the equilibrium of the Upon proton solutionbypH the concentration following 9.5–9.8. compete increasing isThe the withScheme pH, lower interaction boric the (1)–(4): acid andproton the of boron forconcentration sorption species the active becomes sites iswith andalginate lower more aefficient “buffering and the andsorption alumina effect” (a similar trend iscan becomespHbe produced.