Enhanced removal of arsenic from water by using sub-10 nm hydrated zirconium oxides confined inside gel-type anion exchanger

Effective oxidation and adsorption of As(III) in water by nanoconfined Ce-Mn binary oxides with excellent reusability

Herein, a highly efficient As(III) purifier Ce-Mn@N201 with excellent reusability was developed by stepwise precipitating hydrated cerium(IV) oxides (HCO) and hydrated manganese(IV) oxides (HMO) inside N201, a widely-used gel-type anion exchange resin. Owing to confinement of unique nanopores in N201, the in-situ generated nanoparticles (NPs) inside Ce-Mn@N201 were highly dispersed with ultra-small sizes of around 2.6 nm. Results demonstrated that HMO NPs effectively oxidized As(III) to As(V) with the conversion of Mn(IV) to Mn(II), while the generated Mn2+ was mostly re-adsorbed onto the negatively-charged surface of HMO NPs. During the regeneration process by simple alkaline treatment, the re-adsorbed Mn2+ was firstly precipitated as (hydr)oxides of Mn(II) and then oxidized to HMO NPs by dissolved oxygen to fully refresh its oxidation capacity. Though HCO NPs mainly served as adsorbent for arsenic, they could partially oxidize As(III) to As(V) at the beginning, while the oxidation capacities continuously diminished with the irreversible conversion of Ce(IV) to Ce(III). In 10 consecutive adsorption-regeneration cycle, Ce-Mn@N201 efficiently decontaminated As(III) from 500 μg/L to below 5 μg/L with Mn2+ leaching less than 0.3% per batch. During 3 cyclic fixed-bed adsorptions, Ce-Mn@N201 steadily produced 8500-9150 bed volume (BV) and 3150-3350 BV drinkable water from the synthesized and real groundwater, respectively, with Mn leaching in effluent constantly < 100 μg/L.

Nanoconfinement boosts affinity of hydrated zirconium oxides to arsenate: Surface complexation modeling study

Nanoscale hydrated zirconium oxide (HZO) holds great potential in groundwater purification due to its ability to form inner-sphere coordination with arsenate. Despite being frequently used, especially as encapsulations in host materials for practical application in water treatment, the adsorption mechanisms of solutes on HZO are not appropriately explored, in particular for arsenate adsorption. In this study, we investigated the Zr-As coordination configuration and identified the most credible Zr-As configuration using surface complexation modeling (SCM), XPS and FT-IR analysis. The corresponding intrinsic coordination constants (Kintr) values was calculated by SCM, and the nanoconfinement effects were distinguished by comparing bare HZO with the HZO nanoparticles (NPs) encapsulated inside the strongly basic anion exchanger D201. Potentiometric titration suggests that the surface Zirconium hydroxyl groups (≡ZrOH) mainly exist in protonated form (≡ZrOH2+). Batch adsorption experiments demonstrate that the D201 hosts could adsorb As(V) through ion exchange by the quaternary ammonium groups under the low ionic strength (≤0.01 M NaNO3) and at pH > 6. The nanocomposite (HZO@D201) exhibits a higher adsorption capacity in a wide range of pH (3-10) and ionic strength (0.001-0.1 M NaNO3) than bare HZO. SCM simulations reveal that the coordination configuration of diprotonated monodentate mononuclear (MM-H2) dominates at pH 3-6, while deprotonated bidentate binuclear (BB-H0) dominates at pH > 7. For each configuration, the intrinsic coordination constants (Kintr) of HZO@D201 (10-0.66 and 10-16.10, respectively) are significantly higher than those of bare HZO (10-12.24 and 10-44.42, respectively), indicating a superior chemical bonding affinity caused by nanoconfinement. The obtained Kintr values are used to predict arsenate adsorption isotherms in pH 3 and 9, and the results align with the SCM simulation outcomes. This study may offer a feasible method for investigating the nanoconfinement effect of emerging nanocomposite adsorbents from a thermodynamic perspective, and provide reference coordination equilibrium constants of HZO for research and practical application.

Robust polystyrene resin-supported nano-CoFe2O4 mediated peroxymonosulfate activation for efficient oxidation of 1-hydroxyethane 1,1-diphosphonic acid

Nanosized spinel cobalt ferrite (CoFe₂O₄) shows high performance in peroxymonosulfate (PMS) activation for decontamination in water, but is yet challenged by the easily leached Co(II) with high toxicity. Herein, macroporous polystyrene resin is used as the support to improve the stability of CoFe₂O₄ nanoparticles during PMS activation. CoFe₂O₄@S201 exerted high catalytic activity toward PMS activation for oxidation of 1-hydroxyethane 1,1-diphosphonic acid (HEDP), with the apparent rate normalized by Co content 38.2 times higher than that of the unsupported CoFe₂O₄. Meanwhile, one order of magnitude lower Co leaching (< 2.1 μg L^-1) was detected during the catalytic oxidation. The Co(II)-PMS complex was the primary oxidant responsible for the oxidation of HEDP. The catalytic durability and stability of CoFe₂O₄@S201 for degradation of HEDP in actual wastewater were systematically evaluated in both batch and continuous-flow mode. It is found that the organic resin, which is often considered to be intolerant to oxidation, is rather stable during the non-radical process. The total cobalt leaching of the fresh CoFe₂O₄@S201 cannot be ignored in the 100-h continuous-flow run. In contrast, much lower cobalt leaching and slightly higher oxidation efficiency were observed for the regenerated CoFe₂O₄@S201, which might be due to the removal of unreactive and unstable Co sites on the surface in the first trial. The findings shed light on the potential of organic supports for improving the stability and activity of nanosized CoFe₂O₄ and other nano-catalysts toward practical application.

Highly efficient removal of arsenic (III/V) from groundwater using nZVI functionalized cellulose nanocrystals fabricated via a bioinspired strategy

Utilizing nanoscale zero valent iron (nZVI) to purify groundwater contaminated by arsenic species [As(III/V)] is an efficient technology, but the fast and severe aggregation of nZVI limits its practical applications. Herein, nZVI was anchored onto the mussel-inspired polydopamine-coated cellulose nanocrystals (CNCs-PDA-nZVI) as an efficient material for As groundwater remediation. In this set, the introduction of nZVI was expected to significantly enhance the arsenic removal property, while cellulose nanocrystals (CNCs) endowed nZVI with ultrahigh dispersibility. The batch results showed that the maximum As adsorption capacities of CNCs-PDA-nZVI (i.e., 333.3 mg g^-1 and 250.0 mg g^-1 for As(III) and As(V), respectively) were ten times higher compared with those of pristine CNCs. The kinetics results revealed that chemical adsorption was dominant for As adsorption. The isotherms indicated that a homogeneous adsorption for As(III) and heterogenous adsorption for As(V) on the surface of CNCs-PDA-nZVI. The removal mechanisms for As by CNCs-PDA-nZVI included adsorption-oxidation, coprecipitation and inner-sphere complexation. Overall, the excellent arsenic removal efficiency makes CNCs-PDA-nZVI a promising material for the remediation of As polluted groundwater, and this in-situ anchoring strategy can be extended to overcome the aggregation bottleneck of other nanoparticles for various applications.

The ZrO2 NPs enhanced the risk of arsenate by promoting its accumulation and reducing its detoxification during food chain transfer from Daphnia magna to zebrafish

Arsenic (As) can co-occur with zirconium dioxide nanoparticles (ZrO₂ NPs) in aquatic environments, but their combined influence along the aquatic food chain is barely explored. This study constructed water flea Daphnia magna - zebrafish Danio rerio to evaluate the impact of ZrO₂ NPs on the accumulation, trophic transfer, transformation, and detoxification of arsenate (As(V)). The zebrafish were fed D. magna exposed to As(V), ZrO₂ NPs, or As(V) + ZrO₂ NPs for 20 d. Results demonstrated that ZrO₂ NPs significantly facilitated total As and As(V) sorption in D. magna and in tissues of zebrafish. ZrO₂ NPs enhanced the transformation of inorganic arsenic (iAs) to monomethylated acid (MMA), while decreased synthesis of arsenobetaine (AsB) in tissues, leading to iAs increased. Co-exposed As(V) and ZrO₂ NPs facilitated upregulation of absorption-related genes (aqp7) and As biotransformation-related genes (gst, gss), and detoxification and oxidative stress-related genes (mt2, cat, sod1 and sod2). Therefore, genetic expression coupling with biotransformation for the first time demonstrated that As(V) combined with ZrO₂ NPs led to increased harm to D. magna and zebrafish and amplified the ecological risks of As(V) along the aquatic food chain. Attention should be paid to the combined toxicity of As(V) and ZrO₂ NPs in aquatic environment.

Metastable nano-zirconium phosphate inside gel-type ion exchanger for enhanced removal of heavy metals

Nanotechnology has provided a new opportunity for water decontamination from trace heavy metals, yet the relatively poor acidic stability remains a major obstacle for the nano-adsorbents, given that acidic treatment is frequently used to regenerate the heavy metal-saturated adsorbents. Zirconium phosphate (ZrP) is very promising for water treatment due to its absolute insoluble nature, though it interacts with heavy metals mainly through the non-specific electrostatic attraction. Herein, we prepared the ultrafine ZrP (~3.9 nm) inside the commercially available gel-type cation exchanger (N001), i.e., the sulfonated poly(styrene-co-divinylbenzene) bead. The resultant nanocomposite ZrP@N001 contained the amorphous nanoparticles (NPs) with metastable γ-ZrP structure as the main phase, unlike the layered α-ZrP formed inside the macroporous cation exchanger D001 (referred to as ZrP@D001). As a result, ZrP@N001 could selectively adsorb heavy metals through inner-sphere coordination, possessing a much stronger adsorption affinity than ZrP@D001, as confirmed by XPS analysis. In both batch and column assays on the Pb(II)-polluted water, ZrP@N001 exhibited superior adsorption performance over ZrP@D001. After adsorption, the exhausted ZrP@N001 was fully refreshed by acidic treatment for a 5-cyclic adsorption-regeneration run with constant removal efficiencies. This study may open a door for the rational design of highly efficient water purifiers for heavy metal control.

Zirconium hydroxide nanoparticle encapsulated magnetic biochar composite derived from rice residue: Application for As(III) and As(V) polluted water purification

Finding a low-cost and suitable adsorbent is still in urgent need for efficient decontamination of As(III) and As(V) elements from the polluted waters. A novel zirconium hydroxide nanoparticle encapsulated magnetic biochar composite (ZBC) derived from rice residue was synthesized for the adsorptive capture of As(III) and As(V) from aqueous solutions. The results revealed that ZBC showed an acceptable magnet separation ability and its surface was encapsulated with lots of hydrous zirconium oxide nanoparticles. Compared to As(III), the adsorption of As(V) onto ZBC was mainly dependent on the pH of the solution. The intraparticle diffusion model described the adsorption process. ZBC showed satisfactory adsorption performances to As(III) and As(V) with the highest adsorption quantity of 107.6 mg/g and 40.8 mg/g at pH 6.5 and 8.5, respectively. The adsorption of As(III) and As(V) on ZBC was almost impervious with the ionic strength while the presence of coexisting ions, especially phosphate, significantly affected the adsorption process. The processes of complexation reaction and electrostatic attraction contributed to the adsorption of As(III) and As(V) onto ZBC. ZBC prepared from kitchen rice residue was found to be a low cost environmentally friendly promising adsorbent with high removal capacity for As(III) and As(V) and could be recycled easily from contaminated waters.

Adsorption-Desorption Behavior of Arsenate Using Single and Binary Iron-Modified Biochars: Thermodynamics and Redox Transformation

Arsenic (As) is a dangerous contaminant in drinking water which displays cogent health risks to humans. Effective clean-up approaches must be developed. However, the knowledge of adsorption-desorption behavior of As on modified biochars is limited. In this study, the adsorption-desorption behavior of arsenate (As^V) by single iron (Fe) and binary zirconium-iron (Zr-Fe)-modified biosolid biochars (BSBC) was investigated. For this purpose, BSBC was modified using Fe-chips (FeBSBC), Fe-salt (FeCl₃BSBC), and Zr-Fe-salt (Zr-FeCl₃BSBC) to determine the adsorption-desorption behavior of As^V using a range of techniques. X-ray photoelectron spectroscopy results revealed the partial reduction of pentavalent As^V to the more toxic trivalent As^III form by FeCl₃BSBC and Zr-FeCl₃BSBC, which was not observed with FeBSBC. The Langmuir maximum As^V adsorption capacities were achieved as 27.4, 29.77, and 67.28 mg/g when treated with FeBSBC (at pH 5), FeCl₃BSBC (at pH 5), and Zr-FeCl₃BSBC (at pH 6), respectively, using 2 g/L biochar density and 22 ± 0.5 °C. Co-existing anions reduced the As^V removal efficiency in the order PO₄ ^3- > CO₃ ^2- > SO₄ ^2- > Cl^- > NO₃ ^-, although no significant inhibitory effects were observed with cations like Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺. The positive correlation of As^V adsorption capacity with temperature demonstrated that the endothermic process and the negative value of Gibbs free energy increased (-14.95 to -12.47 kJ/mol) with increasing temperature (277 to 313 K), indicating spontaneous reactions. Desorption and regeneration showed that recycled Fe-chips, Fe-salt, and Zr-Fe-salt-coated biochars can be utilized for the effective removal of As^V up to six-repeated cycles.