Lanthanum-based materials are effective for sequestering phosphate in water, however, their removal mechanisms remain unclear, and the effects of environmentally relevant factors have not yet been studied. Hereby, this study explored the mechanisms of phosphate removal using La(OH) by employing extended X-ray absorption spectroscopy (EXAFS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), density functional theory (DFT) and chemical equilibrium modeling. The results showed that surface complexation was the primary mechanism for phosphate removal and in binary phosphate configurations, namely diprotonated bidentate mononuclear (BM-H2) and bidentate binuclear (BB-H2), coexisting on La(OH) in acidic conditions. By increasing the pH to 7, BM-H1 and BB-H2 were the two major configurations governing phosphate adsorption on La(OH), whereas BB-H1 was the dominant configuration of phosphate adsorption at pH 9. With increasing phosphate loading, the phosphate configuration of on La(OH) transforms from binary BM-H1 and BB-H2 to BB-H1. Amorphous Ca(PO) forms in the presence of Ca, leading to enhanced phosphate removal at alkaline conditions. The contributions of different mechanisms to the overall phosphate removal were successfully simulated by a chemical equilibrium model that was consistent with the spectroscopic results. This study provides new insights into the molecular-level mechanism of phosphate removal by La(OH).

Rare earth-modified adsorbents (REMAs) have been widely used to remove oxyanion pollutants from water, including arsenic (As). However, the molecular-level structural information and reactions at the liquid/solid interface are still murky, which limits the design of applicable REMAs. Herein, a lanthanum-impregnated activated alumina (LAA) was synthesized as a representative REMA, and its As uptake mechanisms were explored using multiple complementary characterization techniques. Our adsorption experiments showed that LAA exhibited 2−3 times higher As adsorption capacity than AA. In contrast to the bidentate configuration formed on most metal oxide surfaces, our EXAFS and DFT results suggest that As(III) and As(V) form monodentate surface complexes on LAA through As-O-La coordinative bonding. In situ flow cell ATR-FTIR observed a strong dependence of As-O peak positions on pH, which could be interpreted as the change in the fractions of As(V) surface complexes with zero-to double-protonation on LAA, AA, and LaOOH. As(V) on LAA existed as singly and doubly protonated surface species, and the pK a of transition from double to single protonation (∼5.8) was lower than that for its soluble counterpart (6.97). The surface reaction and structural configuration were incorporated in a CD-MUSIC model to satisfactorily predict macroscopic As adsorption behaviors. The insights gained from the molecular-level reactions shed light on the design and application of REMAs in environmental remediation for As and its structural analogues.

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Arsenic- and iron-reducing bacteria play an important role in regulating As redox transformation and mobility. The motivation of this study was to compare the contributions of different As- and Fe-reducing bacteria to As biotransformation. In this work, three bacteria strains with different functional genes were employed including Pantoea sp. IMH with the arsC gene, Alkaliphilus oremlandii OhILAs possessing the arrA gene, and Shewanella oneidensis MR-1, an iron reducer. The incubation results showed that Pantoea sp. IMH aerobically reduced 100% of As(V) released from waste residues, though total As release was not enhanced. Similarly, strain OhILAs anaerobically reduced dissolved As(V) but could not enhance As release. In contrast, strain MR-1 substantially enhanced As mobilization because of iron reduction, but without changing the As speciation. The formation of the secondary iron mineral pyrite in the MR-1 incubation experiments, as evidenced by the X-ray absorption near-edge spectroscopy (XANES) analysis, contributed little to the uptake of the freed As. Our results suggest that the arsC gene carriers mainly control the As speciation in the aqueous phase in aerobic environments, whereas in anaerobic conditions, the As speciation should be regulated by arrA gene carriers, and As mobility is greatly enhanced by iron reduction.

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A pressing challenge in arsenic (As) adsorptive filtration is to decipher how the As atomic surface structure obtained in the laboratory can be used to accurately predict the field filtration cycle. The motivation of this study was therefore to integrate molecular level As adsorption mechanisms and capacities to predict effluent As from granular TiO 2 columns in the field as well as its health impacts. Approximately 2,955 bed volumes of groundwater with an average of 542 μg/L As were filtered before the effluent As concentration exceeded 10 μg/L, corresponding to an adsorption capacity of 1.53 mg As/g TiO 2 .After regeneration, the TiO 2 column could treat 2,563 bed volumes of groundwater, resulting in an As load of 1.36 mg/g TiO 2 . Column filtration and EXAFS results showed that among coexisting ions present in groundwater, only Ca 2+ , Si(OH) 4 , and HCO 3 − would interfere with As adsorption. The compound effects of coexisting ions and molecular level structural information were incorporated in the PHREEQC program to satisfactorily predict the As breakthrough curves. The total urinary As concentration from four volunteers of local residences, ranging from 972 to 2,080 μg/L before groundwater treatment, decreased to the range 31.7−73.3 μg/L at the end of the experimental cycle (15− 33 days). ■ INTRODUCTIONAdsorptive filtration is one of the most cost-effective and userfriendly techniques to provide arsenic (As) safe drinking water in geogenic As contaminated areas.1,2 Integration of mounting molecular-level evidence over the past decade has shown that the mechanism of As adsorption mainly involves the formation of bidentate binuclear inner sphere surface complexes. 3−5Although substantial progress has improved our understanding of batch and field filtration experiments, a pressing challenge is to decipher how the molecular-level mechanism obtained in the laboratory can be used to accurately predict the field filtration cycle.Rethinking the failure in prediction of field filtration breakthrough curves using parameters obtained with batch experiments, we realized that the adsorption kinetics might not be the critical contributor, as initially thought. The difference in reaction kinetics between batch complete-mixed and column flow-through conditions has long been recognized and solved by incorporating a nonequilibrium kinetics block in simulation code such as PHREEQC. 6,7 On the other hand, even without the kinetics block, the adsorptive filtration of As(III) in synthetic water matrices through iron-oxide-coated rock can be successfully modeled. 8 This success may be attributed to instantaneous uptake of As by most metal oxides. However, a real difficulty surfaces when predicting the groundwater As breakthrough curves in field columns, even with the kinetics block and additional As adsorption reactions. 8,9 The compound effects of groundwater matrices on As adsorption cannot be overstated.Groundwater As adsorption on metal oxides can be suppressed or enhanced in the presence of coexisting ions. Previo...

Elevated fluoride (F) in groundwater presents an obvious environmental concern. Adsorption using activated alumina (AA) is currently the best available technology for F removal, despite its low efficiency, pH dependence, and aluminium (Al) release. The motivation for our study is to synthesise a new F adsorbent by impregnating commercially available granulated AA with lanthanum oxide (LAA), and to explore its F adsorption mechanism on the molecular scale. Five cycles of lanthanum impregnation on AA followed by calcination at 573 K increased the La content up to 19.1% and the F removal from 18.1% by pristine AA to 92.4% by LAA. The SEM, TEM, XRD, TGA, and EXAFS results demonstrated that the 5-20 nm thin flakes of LaOOH on LAA were in an amorphous form, with 7.6 oxygen atoms around each La. This LaOOH layer was uniformly distributed inside the micropores of the 1-3 mm AA granules.LAA exhibited four fold higher F adsorption capacity than AA in the pH range 3.9 to 9.6, with substantially reduced Al release. The ability to regenerate and reuse LAA makes it an attractive sustainable material. Multiple complementary spectroscopic analyses demonstrated that ligand exchange between F and surface hydroxyl groups is the mechanism for F adsorption on LAA. Our work improves the understanding of F interaction with metal oxides on the molecular scale and presents an alternative solution for elevated F water treatment.

The exposure of millions of people to unsafe levels of arsenite (As III ) and arsenate (As V ) in drinking waters calls for the development of low-cost methods for on-site monitoring these two arsenic species in waters. Herein, for the first time, tetradecyl (trihexyl) phosphonium chloride ionic liquid was found to selectively bind with As III via extended Xray absorption fine structure (EXAFS) analysis. Based on the finding, an As III -specific probe was developed by modifying gold nanoparticles with the ionic liquid. Futhermore, Hofmeister effect was primarily observed to significantly affect the sensitivity of gold nanoparticle probe. With the colorimetric probe, we developed a protocol for naked eye speciation test of As III and As V at levels below the World Health Organization (WHO) guideline of 10 μg L −1 . This method featured with high tolerance to common coexisting ions such as 10 mM PO 4 3− , and was validated by assaying certified reference and environmental water samples. KEYWORDS: arsenic, ionic liquid, interaction, gold nanoparticle, Hofmeister effect, visual test ■ INTRODUCTIONArsenic shows various adverse health effects including carcinogenic disease relative to skin, lung, and kidney. 1−3 More than 140 million people worldwide are exposed to superfluous arsenic, mainly by drinking ground waters polluted by inorganic arsenic that are more toxic than the organic ones. 4,5 Although As III and As V are coexisting and undergo mutual transformation in groundwater, As III is usually the main form and more toxic than As V . 6 Consequently, monitoring of inorganic arsenic species in waters is of prior interest in arsenic detection. Numerous instrumental methods have been developed to quantify trace arsenic, 7−10 but the requirement of bulk equipment and strict laboratory conditions restricts their application in field analysis. On the contrary, the colorimetric or visual methods are quite suitable for field determination. Because of their poor sensitivity, however, traditional visual probes are incapable of detecting As III and As V at 10 μg L −1 level, which is the arsenic guideline value in drinking water required by the World Health Organization (WHO) and U.S. Environmental Protection Agency (EPA). Additionally, the fairly poor tolerance to interferences of these probes restricts their application in real sample analysis. 11−13 Several arsenic testing field kits are able to semiquantify 10 or 50 μg L −1 As, but most of them fail to meet the requirements of an ideal test kit for As. 14−16 Gold nanoparticles (AuNPs) are widely used as optical probes in colorimetric detection in recent years, 17−19 because of their higher extinction coefficient in the visible region and thus higher sensitivity in comparison to the organic probes. Since the pioneering work of the Mirkin group, 20 AuNPs have served as optical probes for sensing cations, anions, small organic compounds, and biological molecules, 21−23 with Hg 2+ as one of the most attractive target analytes. 24−31 Surprisingly, studies on the u...