Comment on "Polymerization of silicate on hematite surfaces and its influence on arsenic sorption"

Enhanced Arsenite Removal from Silicate-containing Water by Using Redox Polymer-based Fe(III) Oxides Nanocomposite

The efficient removal of arsenite [As(III)] from groundwater remains a great challenge. Nanoscale oxides of Fe(III), Zr(IV), and Al(III) can selectively remove arsenic from groundwater through inner-sphere complexation. However, owing to polysilicate coatings formation on nanoparticles surface, the ubiquitous silicate exerts remarkably adverse effects on As(III) removal. Herein, we propose a new strategy to enhance silicate resistance of nanoscale oxides by embedding them inside the redox polymer host. As a proof-of-concept, the nanocomposite HFO@PS-Cl was employed to remove As(III) from silicate-containing water. The polymer host (PS-Cl) contains active chlorine to oxidize As(III) into arsenate [As(V)], and the embedded Fe(III) oxides enabling specific adsorption toward arsenic. Silicate exerts negligible effects on As(III) removal by HFO@PS-Cl in pH 3-7, but increasing the residual arsenic concentration from 49 µg/L to 166 µg/L for the solutions treated by HFO@PS-N, i.e., the nanoscale Fe(III) oxides embedded inside the polymer host without active chlorine. During the six cyclic decontamination-regeneration assays, HFO@PS-Cl steadily reduces As(III) below 10 µg/L. As for HFO@PS-N, however, the residual arsenic increases to ~57 µg/L in the sixth run. In column mode, HFO@PS-Cl column generates >3200-bed volume (BV) clean water ([As]<10 µg/L) from the simulated As(III)-contaminated groundwater. In contrast, the values for As(V)-contaminated water and HFO@PS-N column are only ~650 BV and ~608 BV, respectively. The stoichiometric assays, XPS, and in-situ ATR-FTIR analysis demonstrate that silicate polymerization is intensively suppressed by the protons produced during As(III) oxidation, thus rendering HFO@PS-Cl with excellent silicate resistant properties.

Quantification and isotherm modelling of competitive phosphate and silicate adsorption onto micro-sized granular ferric hydroxide

Adsorption onto ferric hydroxide is a known method to reach very low residual phosphate concentrations. Silicate is omnipresent in surface and industrial waters and reduces the adsorption capacity of ferric hydroxides. The present article focusses on the influences of silicate concentration and contact time on the adsorption of phosphate to a micro-sized iron hydroxide adsorbent (μGFH) and fits adsorption data to multi-component adsorption isotherms. In Berlin drinking water (DOC of approx. 4 mg L^-1) at pH 7.0, loadings of 24 mg g^-1 P (with 3 mg L^-1 initial PO₄ ^3--P) and 17 mg L^-1 Si (with 9 mg L^-1 initial Si) were reached. In deionized water, phosphate shows a high percentage of reversible bonds to μGFH while silicate adsorption is not reversible probably due to polymerization. Depending on the initial silicate concentration, phosphate loadings are reduced by 27, 33 and 47% (for equilibrium concentrations of 1.5 mg L^-1) for 9, 14 and 22 mg L^-1 Si respectively. Out of eight tested multi-component adsorption models, the Extended Freundlich Model Isotherm (EFMI) describes the simultaneous adsorption of phosphate and silicate best. Thus, providing the means to predict and control phosphate removal. Longer contact times of the adsorbent with silicate prior to addition of phosphate reduce phosphate adsorption significantly. Compared to 7 days of contact with silicate (c ₀ = 10 mg L^-1) prior to phosphate (c ₀ = 3 mg L^-1) addition, 28 and 56 days reduce the μGFH capacity for phosphate by 21 and 43%, respectively.