Extremely fast and high Pb removal capacity using a nanostructured hybrid material

Ultra-fast toxic metal removal using a hybrid and nanostructured vaterite–poly(ethyleneimine) (NV–PEI) is reported. Especially for Pb2+, an extremely high and fast adsorption capacity without precedents is observed. Within a 3 min contact time (CT), 97–100% of Pb2+ contaminant in water samples at pH 6, with a large concentration range (from 2 to 1000 ppm) and using a dosage of 0.5 g NV–PEI per L, has been removed. The maximum adsorption capacity (qmax) calculated for a 1500 ppm Pb2+ initial concentration was 2762 mg of Pb2+ per g NV–PEI. In addition to the excellent Pb2+ removal, this material is cheap, easy to synthesize, biocompatible, and biodegradable, which makes it superior to others reported to date and an attractive alternative for the treatment of industrial wastewaters.


Introduction
Pure water progressively becomes a rare resource on Earth, because human activities release increasing amounts of soluble 15 chemical species that are not appropriately removed before returning water to the environment. Among the number of polluting species heavy metals ions, especially mercury, cadmium and lead, are of special concern because of their high toxicity. Other less toxic ions such as copper or zinc may 20 nevertheless prove harmful at slightly elevated concentrations. Wastewaters containing copper, lead, nickel, cadmium and zinc are produced by a variety of industries such as metal finishing, batteries manufacturing, non-ferrous metallurgical manufacturing, electrical industry, paper industry, printed wiring 25 board industry and mining industry. Currently, water treatment technologies for the removal of toxic metals ions occurs through the use of several unit operations, such as chemical precipitation, sedimentation, filtration and sludge treatment. 1,2 In a wastewater treatment plant in the basin of filtration ionic interchanger 30 membranes or adsorption membranes, or mixture of both types are introduced to improve fundamentally the toxic metals removal at trace levels. [1][2][3] For the sake of sustainable development, the new environmental regulations will request wastewater treatment systems of increasing efficiency. In this 35 aspect nanotechnology has been a useful tool in the fabrication of materials to remove toxic metals during the water treatment process. 4 In this way many different nanoscale materials such as oxides, hydroxides or salts of metals such as iron 5-9 and titanium, 10,11 carbon nanotubes, [12][13][14][15][16] various noble metals (mainly 40 as nanoparticles), 17,18 inorganic-organic hybrid materials, 19,20 biomaterial as nanoporous silica, 21 calcium carbonates 22-25 and calcium phosphathes, [26][27][28] nanoscale networks (such as zeolites, metal-organic framework (MOF)) 29 and others [30][31][32] have been explored as adsorbents for waters remediation. 45 Here a hybrid material composed of nanostructured vaterite-(poly)ethyleneimine (NV-PEI) microparticles has been evaluated for toxic metals removal from model contaminated waters. New characterizations of the NV-PEI material and metal removal studies are reported here. For the first time, the results of metal 50 adsorption onto the NV-PEI with the analysis of the N2 adsorption-desorption isotherms, determination of the Pb 2+ adsorption model and proposals of metal removing mechanisms are shown. An ultra fast removal capacity achieved within 6 min, never reached before, has been found for several metals at 100 55 ppm of initial concentration. A time of 3 min is completely sufficient to remove almost the 100% Pb 2+ in a wide concentrations range from 2-1000 ppm using a dosage of 0.5 g NV-PEI/L at pH 6. Although there is a large number of studies using multiples nanostructure materials, this extremely rapid and 60 high removal capacity to Pb 2+ has never been achieved so far. 5, 10-12, 14- 16,19,[21][22][23]32 Generally, the toxic metal removal processes using the nanostructured microparticles (like fibres, sheet, tubes, rods, spheres, etc) are generally described by Langmuir isotherms 9,10,21 that describes a superficial adsorption. In contrast, in the 65 case of our material for the removal process by exchange of the Pb 2+ with the Ca 2+ of the vaterite structure only the Freundlich adsorption isotherm is the model that better fits with the experimental data. In addition to the excellent Pb 2+ removal, the material here evaluated is cheap, easy to be synthesized, 70 biocompatible, biodegradable, which makes it superior to others reported so far and an attractive alternative for treatment of industrial wastewaters.

Experimental Section
Synthesis of the adsorbent material 75 The NV-PEI adsorbent material was synthesized mixing under 45 min sonication equal volumes (5 mL) and concentrations (0.33 M) of CaCl2 and Na2CO3 solutions that contain PEI (4 mg mL -1 ). To prepare the CaCl2/PEI solution a mixed solvent of water/ethanol 1:1 (v/v) was used. The precipitate product was 80 washed three times by centrifugation, air dried and collected. 33

Metal removal experiments
In a typical metal removal experiment the used Pb 2+ , Cu 2+ , Hg 2+ , Zn 2+ , Cd 2+ and Ni 2+ solutions at different concentrations were prepared by dissolving a weighed quantity of Pb(NO3)2, CuCl2·2H2O, HgCl2, ZnCl2, CdCl2 and NiCl2·6H2O, respectively, 5 in Milli-Q water (resistivity 18.2 Ωcm at 25 °C) and pH adjusted to 6. As-prepared hybrid NV-PEI microparticles (25 mg) were added to 50 mL of the metal solutions under stirring. To follow the experiments aqueous samples (500 µL) were taken during 6 h using a homemade filter-collector sample at several fixed time 10 intervals. The metal ions concentrations were analyzed by using an inductively coupled plasma mass spectrometry (ICPMS) Agilent 7500ce model system. The amount qt (mg g -1 ) of metal adsorption onto microparticles surface at the time interval t was calculated by: 15 where C0 and Ct (mg L -1 ) are the liquid phase concentrations of Pb 2+ ions at initial and any time interval t, respectively, V is the volume of the solution (L) and W is the mass of NV-PEI used as 20 adsorbent (g).

Results and Discussion
Synthesis and characterization of the adsorbent material. Metal removal assays.
Our previous experimental results demonstrated the high loading 25 capacity of the NV-PEI for a large variety of (bio)molecules. 33 Based on these preliminary results we decided to test the NV-PEI particles in model assays for water treatment purposes to evaluate for the first time the behaviour of this material versus toxic metals and carry out removal studies. Figure 1A-D shows SEM 30 and TEM images of the NV-PEI particles used as water treatment material in the tested removing experiments. The CaCO3 product is 100% of vaterite polymorph ( The weight loss of about 4% that appears in the TGA diagram at range of 150-350 o C corresponds to the PEI delivery from CaCO3 structure. Simultaneously, the PEI presence in this 40 material was also observed by the nitrogen appearing at the EDX pattern. Taking into account these evidences, the nebulous zones (indicated by the red arrows) in the Fig. 1D could be related with the presence of the PEI organic phase. These results indicate that the NV-PEI hybrid material was synthesized. 45 The first removing probes were carried out in multi-metal competitive assays. The evaluated metals have been chosen based on their toxicity and the fact that these are reported as usual contaminants in waters. The NV-PEI adsorbent material exhibited the highest selectivity and removal capacity for Pb 2+ 50 and Cu 2+ , followed for Hg 2+ , Zn 2+ and Cd 2+ with a similar removal preference between them. The NV-PEI showed the lowest selectivity and removal for Ni 2+ between all the assayed metals (Fig. 1E). The change of colour from white to blue in the adsorbent material after 6 min of CT with the metal solution is 55 related to the adsorption of Cu 2+ and Ni 2+ onto this material (Fig.  1F). The qmax for the six assayed metals using both 20 and 100 ppm as initial concentration is reached at 6 min of sample collection time or CT of the material with the metal dissolution (lower times of sample collection or CT were not assayed). It 60 indicates that the whole removal process is very fast (Fig. 1E). The qmax for the total metal concentration of 120 ppm was around 152 mg/g which corresponds to 63% of total metal removal, meanwhile it was around 430 mg/g for the total initial metal concentration of 600 ppm which corresponds to 31% of removal.
When 20 ppm of each metal is used as initial concentration, almost 100% of Pb 2+ and Cu 2+ are removed followed by around 50-60% of removal for Hg 2+ , Zn 2+ and Cd 2+ and 20% for Ni 2+ . While using 100 ppm of each metal as initial concentration, around 60% of Pb 2+ and Cu 2+ , 20% of Hg 2+ , Zn 2+ , Cd 2+ and 10% same total initial metal concentration of 120 or 600 ppm (results not shown here) could be related with the fact that especially at 110 high concentrations the different removing mechanisms for these metals start blocking/interfering each other and some removal mechanisms could to reach the saturation onto the NV-PEI surface (as for example the removal by surface adsorption), allowing a major removal for each metal in comparison to the case when only one metal should be present. As NV-PEI material in the multi-metal competitive removal assay showed the highest removal capacity for Pb 2+ , the following adsorption studies for the Pb 2+ removal as shown below 25 were performed. The equilibrium adsorption capacity increases dramatically by increasing of Pb 2+ initial concentration, until it reaches the qmax of 2762 mg Pb 2+ /g NV-PEI at 1500 ppm of Pb 2+ initial concentration ( Fig. 2A-i). For concentrations from 2 to 1000 30 ppm, 3 min of CT is enough to remove around 97-100% of the total Pb 2+ initial mass using a dose of 0.5 g NV-PEI/L ( Fig. 2Aii). CTs longer than 3 min don't change significantly the adsorption capacity (see Fig. 2B), because all the Pb 2+ contaminant from 2 to 1000 ppm is completly removed in the 35 first 3 min of reaction reaching the qmax. This fast and high Pb 2+ removal capacity has not been reported so far as shown in table S1.

The N2 adsorption-desorption isotherms
The nitrogen adsorption and desorption isotherms of the NV-PEI 40 adsorbent material exhibit type II isotherm with a type III hysteresis loop in the relative pressure range of 0.45-1 P/P0 (Fig.  3 A), suggesting that the adsorbing material structure is composed in majority by macropores with good pore connectivity associated with slit-like pores. 34, 35 In addition, as shown in Fig. 3 45 A and B, some mesoporous part in the size range of 2-50 nm for low relative pressures (P/P0 ˂ 0.4) is also present. The nitrogen amount adsorbed rises very steeply at high relative pressure (P/P0 > 0.85), which also suggests the presence of an appreciable amount of very large pores in the material. 34, 35 The distribution 50 curve of BJH pore size derived from the adsorption branch of the isotherm shows one very narrow distribution with a peak at 2.1 nm and one broad distribution in the range of 2.8-191 nm with a peak at 40 nm. It indicates that few microporous parts make up the material structure and a high percentage of the structure 55 porosity are mesoporous and macroporous morphologies (see Fig.  3 B). It is interesting to highlight that the final part of the adsorption branch reaches large macropores diameter (50-190 nm). Moreover the material has a BET surface area of 20 m 2 /g and a BJH pore volume of around 0.07 mL/g. XRD patterns and SEM images in Fig. 4 A and B, respectively, indicate that a new precipitation process is taking place during the treatment process of contaminated water with Pb 2+ . Fig. 4 A-1 shows the pattern of vaterite before its contact with Pb 2+ solutions 5 while its corresponding SEM images of spherical/ellipsoidal microparticles are shown in Fig. 4 B-1. After the contact of the NV-PEI material with Pb 2+ solutions of increasing initial concentration a transformation of part of vaterite in cerussite (orthorhombic PbCO3) (Fig. 4 A-2 and B-2) occurs, until that 10 using 1500 ppm of Pb 2+ solution a complete vaterite transformation into cerussite takes place (Fig. 4 A-3 and B-3). The recrystallization process is mediated via ionic exchange, where the NV-PEI behaves like an ionic exchanger network of Ca 2+ by Pb 2+ cations. The exchanging process takes place both at 15 superficial level as well as at the interior of NV-PEI material. The structure of NV-PEI collapses during the ion exchange, resulting in a recrystallization process by an irreversible ion exchange. The SEM images in Fig. 4B  of use of this material in model assays for waters treatment with around 2 ppm of Pb 2+ initial concentration, the residual quantity 5 of Pb 2+ in the solution is about 50 to 10 ppb, which adittionaly sugests that this material could be adequate to treat waters for uses as irrigation water, groundwater and agricultural and livestock considering the fact that 50 ppb is the acceptable maximum limit of Pb 2+ for these waters (Table S2). 36-38 10 The obtained results show that the NV-PEI hybrid material has powerful capability for fast and high removal of Pb 2+ toxic ions (nearly 100% of Pb 2+ removal after 3 min of CT). This phenomenon is related with three factors: the large pore size in the NV-PEI structure, the PEI presence in the material surface 15 that entraps metallic cations through the adsorption via electrophilic-nucleophilic interactions and the recrystallization process mediated by the ionic exchange of Pb 2+ contaminant through the entire calcite structure. Fig. 6 is a representative scheme of the proposed principle of the Pb 2+ removing 20 mechanism. The large size of the macropores that dominates in the structure 50 (typical SEM images are displayed in Fig. 1A, B and D where large pore sizes of this material can be observed) as found by BJH method (see section The N2 adsorption-desorption isotherms), and the high affinity power of the PEI with metallic cations as part of this hybrid material can explain the ultra rapid 55 metal adsorption process that takes place in this system. The large size of the pores of this material permits large accessibility, as well as a fast and simultaneous penetration of a high quantity of ions toward the inner parts of the NV-PEI microparticles. In addition to the pore size, the high affinity of PEI for metal cations 60 acting as ionic entrapment network of chemical adsorption (Fig. 6  A and B) is another important and determinant factor in the fast Pb 2+ removal process. The fast uptake of toxic metal ions indicates a high affinity rate between the Pb 2+ ions and the amine groups of PEI that cover the material, as well as a rapid mass 65 transference to the inner parts of the structure propitiated by the large size of the pores. On the other hand, the recrystallization process mediated by the ionic exchange between the Pb 2+ and the Ca 2+ in the particle structure (see Fig. 6  which is far a way of a simple superficial adsorption. This recrystallization via ionic exchange that happens here could be considered as a multilayer adsorption through the whole NV-PEI material, which consequently produces a superior removal capacity. The Pb 2+ removal process onto the as-obtained NV-PEI 85 microparticles, as explained in the section of Adsorption isotherm of Pb 2+ onto the NV-PEI material at SI, obeys well the Freundlich isotherm model rather than the Langmuir one. 39,40 Additional discussion about the mechanisms of removing for other assayed metals appears in the corresponding section at SI.

Conclusions
In summary, a very fast toxic metals removal by using the hybrid NV-PEI is here reported. Metal removal studies such as the results of metal adsorption onto the NV-PEI, the analysis of the N2 adsorption-desorption isotherms, the determination of the Pb 2+ 95 adsorption model and the proposals of removing mechanisms are shown for the first time for this material. Many results related to metal adsorbent materials reported in the literature for water treatment are obtained using an optimized pH value (usually pH 3-4) with long times of adsorption duration that end up to 2 h. 100 Therefore, we believe that the exceptionally fast and high removal capacities obtained by using the NV-PEI under pH 6 are more indicative for a promising use of this material in real water treatment applications. Although the obtained results are in the laboratory scale, we expect satisfactory removal efficiencies at 105 industrial scale especially for Pb 2+ . This material could be useful to remove efficiently Pb 2+ (almost 100% of removal) between 2 and 1000 ppm (using the discrete dosage of 0.5 g NV-PEI/L and just 3 min of CT) from industrial wastewater and transforming this residual water to an adequate one (with Pb 2+ content under 110 500 ppb) able to be discharged into rivers, seas and sewers, or even (depending of the Pb 2+ initial contamination) to use the treated water for the reuse in agriculture or livestock, taking into account the maximum acceptable limit of Pb 2+ for these waters (table S2). Moreover, this material could be easily produced at industrial amounts because it is facile to synthesize, cheap, biodegradable and environmentally friendly. In addition, it has a 5 high loading capacity for organic molecules (dyes such as bromocresol green and methyl orange) through the hydrogen bond interaction of acid hydrogens in the contaminants molecules and the nitrogen´s PEI onto the vaterite microparticles surface as reported previously. 33 This would increase the NV-PEI interest as 10 potential adsorbent for simultaneous removing of many coexisting pollutants that usually appear in industrial effluents. The development of such multipurpose adsorbent which can remove both organic and toxic metal pollution would improve the cost / efficiency of water treatment process and might have impact on 15 both the wastewater treatment technology as well as the science behind the phenomena occurring during the operation of such integrated and hybrid nanostructured materials.

Notes and references
a Nanobioelectronics & Biosensors Group, Catalan Institute of Nanoscience and Nanotechnology, Campus de la UAB, 08193, Bellaterra, Barcelona, Spain