materials (FIG. 47A). Notably, performance toward arsenate was not simply additive, with Fh3-CTAB3 exhibiting a higher capacity for arsenate uptake (11.8–0.4 mg As/g from Langmuir-type isotherm model fit) than would be expected from the sum ofFh3 (7.7–0.1 mgAs/g) and CTAB3 (<1 mg As/g). We attribute the greater arsenate uptake in these ternary systems to the influence of positively charged qua› ternary ammonium groups on the surface charge of the embedded Fh, making the Fh surface more electrostatically favorable for arsenate uptake at circunmeutral pH. Notably, loss of CTAB with extensive rinsing (and thus the positive influence of adjacent quaternary ammonium groups) reduced the arsenate capacity to 8.4–0.2 mg As/g, on par with the Fh3 composite (FIG. 47B). 

[0359] Further evaluation of ternary PAN-QAS-Fh com› posites focused on materials containing 1 wt % TBAB, based on the sustained performance ofTBABl after wash› ing. As with CTAB, performance ofTBAB ternary compos› ites for chromate and arsenate uptake was predicted by the performance of its binary analogs (FIGS. 48A-48C). Chro› mate uptake on Fh3-TBAB1 composites was comparable to that of TBABl, with capacity largely unaffected by rinsing (FIG. 48A and Table 5). In bromide release experiments identical to those performed with the TBABl material (see FIGS. 43A-43D), an equivalent degree of bromide release occurred concurrent with chromate uptake on Fh3-TBAB1, confirming uptake via ion exchange at QAS sites. Further, negligible bromide release in chromate-free controls indi› cates that co-inclusion of Fh did not adversely affect TBAB retention. 

[0360] In a similar evaluation of the Fh3-TBAB1 material for arsenate removal, Fh3-TBAB1 exhibited a capacity of 9.3–0.2 mg As/g (equivalent to 34.6–0.7 mg As/g Fh), and no loss in capacity was observed after rinsing (FIG. 48B). Thus, per mass of Fh, Fh3-TBAB1 not only outperformed the Fh3 composite, but exhibited arsenate uptake at least equal to that observed in an aqueous suspension of unsup› ported Fh nanoparticles (FIG. 48C). Importantly, bromide release was not measureable concurrent with arsenate removal, thereby implying that Fh nanoparticles are the primary sites in the composite for arsenate uptake, presum› ably via inner-sphere bidentate complexation, as proposed in the literature for Fh. Given the comparable specific surface areas of Fh3 and Fh3-TBAB1 materials, the improved arsenate removal by the ternary composite is primarily attributable to the positive influence of adjacent quaternary ammonium groups on the surface charge of Fh nanopar› ticles, analogous to the phenomenon observed for CTAB composites (see FIGS. 47A-47B). We note, however, that surface enrichment of Fh in Fh3-TBAB1, relative to the binary Fh3 material, may also contribute to the improved performance of the ternary composite (see FIG. 41). 

[0361] pH Edge Experiments 

[0362] The results of pH edge experiments further support the selective binding of chromate to quaternary ammonium sites and arsenate to Fh sites in ternary composites. FIGS. 49A-49B shows pH-dependent uptake of chromate and arsenate on ternary Fh3-TBAB 1, while also providing sorp› tion data on binary TBABl and a suspension of Fh nano› particles for comparison. As expected for oxyanion uptake on an iron oxide, sorbed chromate concentrations decreased with increasing pH in suspensions of freely dispersed Fh nanoparticles. Relative to TBAB- and Ph-containing com› posites, unsupported Fh nanoparticles exhibited much 

higher sorbed chromate concentrations at low pH values, but chromate uptake was essentially equivalent to that observed with composites at higher pH (pH>7). 

[0363] These pH edge results match expectations for chro› mate uptake being governed primarily by electrostatic inter› actions. At higher pH (i.e., above pH 7 or 8, the typical zero point of charge (zpc) of Fh), the Fh nanoparticle surface becomes increasingly anionic, thus limiting uptake of nega› tively charged chromate ions. In contrast, the charged qua› ternary ammonium head on TBAB remains positively charged across the entire range of pH values investigated. Thus, the similar pH dependence of Fh3-TBAB1 and TBABl further implicates the quaternary ammonium sites as being primarily responsible for chromate binding via a mechanism analogous to anion exchange. As the quaternary ammonium group is not affected by pH, the slight decrease in chromate uptake on Fh3-TBAB1 and TBABl composites at higher pH (relative to limited pH dependence below pH 7) can likely be attributed to the speciation of chromate. At pH values above the pKa$_{2 }$ value (i.e., pH>6.5), Cro/- is the dominant species relative to HCrO$_{4}$ -, and it has been suggested that cr0/- utilizes two quaternary ammonium sites for uptake, rather than one, thereby limiting the total number of exchange sites available at higher pH. 

[0364] For arsenate uptake, Fh3-TBAB1 and the freely dispersed Fh nanoparticles exhibited nearly identical pH dependent performance, with sorbed arsenate concentrations decreasing linearly with increasing pH as the Fh nanopar› ticle surface becomes more anionic. This is yet another piece of evidence that the embedded Fh nanoparticles are the primary sites for arsenate removal by the composite. We note that the polymer composite slightly, but reproducibly, outperforms the Fh nanoparticles at pH 8 and 9. As noted previously, the positive charge of the adjacent quaternary ammonium groups likely makes the Fh surfaces more elec› trostatically favorable for arsenate uptake, particularly at pH values above the zpc. 

[0365] Influence of Co-Solutes on Oxyanion Uptake 

[0366] The potential for competitive inhibition between arsenate and chromate in dual-target systems (i.e., simulta› neous uptake) was examined. Time-dependent uptake experiments were conducted with Fh3-TBAB1 composite at both excess ( e.g., 20 mg/L Cr and As) and environmentally relevant oxyanion concentrations (e.g., 5 times the MCL for each target, or 500 µg/L Cr and 50 µg/L As; FIGS. SOA- 50B). At high concentrations, removal of chromate in the presence of arsenate was slightly inhibited, indicating that saturation of available Fh sites with arsenate may block accessibility of quaternary ammonium sites for chromate uptake (FIG. SOA). However, this effect was not observed at lower concentrations, where arsenate and chromate were reduced below their respective MCLs (100 µg/L Cr, 10 µg/L As) within 1 h in both single- and multi-target systems (FIG. SOB). Thus, under conditions most representative of point› of-use application (i.e., low oxyanion concentration and relatively short contact times), the specific binding sites for arsenate and chromate work independently of and do not adversely influence one another, highlighting their potential as multi-target sorbents. 

[0367] As a final practical consideration, batch studies with chromate were also performed in the presence of competitive ionic co-solutes ( e.g., chloride, sulfate, bicar› bonate) at relatively high concentrations (250 mg/L) to assess their impact on the ion exchange capacity of TBAB- 

