Influence of phosphate on tungstate sorption on hematite: A macroscopic and spectroscopic evaluation of the mechanism

Abstract

Environmental fate of tungstate (VI) oxyanion [i.e. mono tungstate and several polytungstate, generally expressed by W (VI)] is largely controlled by sorption on soil minerals, especially on iron oxide minerals. Molecular scale evaluation of W (VI) retention on iron oxides in the presence of competing oxyanions is scarce in the literature. Here we report surface interaction mechanisms of W (VI) on hematite in the presence of phosphorus (P) using macroscopic and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic experiments. Batch sorption experiments were conducted using 2g L-1 hematite and 100 µM W (VI) and P, in single ion system and in binary mixtures as a function of pH (4-11). In situ ATR-FTIR spectroscopic evaluation of P and W (VI) sorption on hematite was also carried out. The results from macroscopic experiments indicated that W (VI) sorption on hematite was not affected when W (VI) was added first. The influence of P on W (VI) sorption was noticed when W (VI) & P were added simultaneously or P was added first. The in situ ATR-FTIR spectroscopic data corroborated this finding. In addition, the spectroscopic data revealed that in the presence of P, surface complexation mode of W (VI) differed as noted from either the absence of W-O antisymmetric infrared (IR) band or the W-O-W stretching band. This study provides useful information on molecular level understanding of W (VI) surface complexation on hematite in the presence of competing ions such as P.

Antimony (V) Adsorption at the Hematite–Water Interface: A Macroscopic and In Situ ATR-FTIR Study

“…Hematite (α-Fe 2 O 3 ) was synthesized using the methods described in Sugimoto et al (1993) [29], by Elzinga and Kretzschmar (2013) [30], and in our earlier studies [31,32]. To summarize, a gel was formed by the slow addition of 500 mL of 2 M FeCl 3 solution over a period of 5 min to a continuously stirred 500 mL solution of 5.4 M NaOH.…”

“…To obtain molecular mechanisms of Sb (V) adsorption on hematite, in situ ATR-FTIR experiments were carried out using 45 • ZnSe horizontal ATR (HATR) flow cell attachment as described in our earlier studies and in the other studies in the literature [22][23][24][25]31,32,[34][35][36]. Extensive discussions on the flow cell set up and experimental system can be found in these cited references.…”

Natural organic matter decreases uptake of W(VI), and reduces W(VI) to W(V), during adsorption to ferrihydrite

“…When W is present as dissolved tungstate oxyanions (e.g., WO4 2− ), strong uptake may occur at mineral-water interfaces, influencing its mobility and fate in soils, sediments and water systems (Johannesson et al, 2013). Recently, iron-bearing minerals such as ferrihydrite, hematite, goethite, iowaite and pyrite have been reported as an important sink for tungstate oxyanionic species (Gustafsson, 2003;Cui and Johannesson, 2017;Iwai and Hashimoto, 2017;Rakshit et al, 2017;Sallman et al, 2018;Cao et al, 2019). For example, iowaite, an iron-bearing layered double hydroxide [Mg6Fe2(OH)16Cl2•4H2O], can sorb up to 71.9 mg/g tungstate from aqueous solution (Cao et al, 2019).…”

“…For example, iowaite, an iron-bearing layered double hydroxide [Mg6Fe2(OH)16Cl2•4H2O], can sorb up to 71.9 mg/g tungstate from aqueous solution (Cao et al, 2019). In general, tungstate adsorption increases with decreasing pH (Gustafsson, 2003;Cui and Johannesson, 2017;Sallman et al, 2018), and spectroscopic measurements, such as those made using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray absorption spectroscopy (XAS), demonstrate the formation of inner-sphere type adsorption complexes during tungstate binding to iron (hydr)oxides including ferrihydrite and hematite (Sun and Bostick, 2015;Rakshit et al, 2017;Sallman et al, 2018), and the strength of this interaction is greater with decreasing pH. Tungsten oxyanions can also polymerize to form poly tungstate species (Strigul et al, 2009;Strigul, 2010), which can decrease tungstate adsorption onto ferrihydrite (Gustaffson, 2003), thereby potentially mobilizing W in the environment (Bostick et al, 2018).…”

“…Tungsten oxyanions can also polymerize to form poly tungstate species (Strigul et al, 2009;Strigul, 2010), which can decrease tungstate adsorption onto ferrihydrite (Gustaffson, 2003), thereby potentially mobilizing W in the environment (Bostick et al, 2018). Of particular relevance to natural systems, competing oxyanions such phosphate (P) can inhibit the retention of tungstate on hematite (Sallman et al, 2018), and the surface complexation of tungstate in the competitive system differs from that in the absence of PO4 3− . This indicates that moieties that compete with tungstate for available sorption sites on iron (hydr)oxides are able to significantly modify W mobility and fate in soils and sediments.…”