US 2019/0002309 Al  37  Jan. 3, 2019  [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- [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.  [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  [0361]  pH Edge Experiments  [0365]  Influence of Co-Solutes on Oxyanion Uptake  [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).  [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.  [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.  [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.  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).  higher sorbed chromate concentrations at low pH values, but  chromate uptake was essentially equivalent to that observed  with composites at higher pH (pH>7). 