Polymerization of Silicate on Hematite Surfaces and Its Influence on Arsenic Sorption

Abstract

Iron oxides and oxyhydroxides are important sorbents for arsenic in soils, sediments, and water treatment systems, but their long-term potential for arsenic retention may be diminished by the formation of polymeric silicate on their surfaces. To study these interactions, we first investigated the sorption of silicate to colloidal hematite (α-Fe(2)O(3)) in short-term (48 h) and long-term (210 days) batch experiments. The polymerization of silicate on the hematite surface was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The pH dependence of silicate sorption exhibited a maximum between pH 9.0 and 9.5. The condensation of silicate on hematite surfaces adsorbed from monomeric silicate solutions steadily continued over the 210 day period, whereby surface polymerization was slower at pH 3 than at pH 6. The effect of silicate surface polymerization on arsenate and arsenite sorption was studied by use of hematite pre-equilibrated with silicate for different time periods of up to 210 days. The competitive effect of silicate on arsenate and arsenite sorption increased with increasing silicate pre-equilibration time. Only under strongly acidic conditions (pH 3), where silicate sorption was weakest and surface polymerization was slowest, was arsenate and arsenite sorption not affected by the presence of silicate. We conclude that the long-term exposure to dissolved silicate can decrease the potential of natural iron (oxyhydr)oxides for adsorbing inorganic arsenic.

Recent progress of arsenic adsorption on TiO 2 in the presence of coexisting ions: A review

“…For TiO 2 that adsorbs groundwater containing As and coexisting Si, the formation of Si polymers can occupy the surface sites and consequently inhibit As adsorption and TiO 2 regeneration (Christl et al, 2012;Hu et al, 2015c). Hu et al (2015a) studied granular TiO 2 regeneration using NaOH and HCl after adsorbing real groundwater containing the following analytes of interest at pH 8.2: 454 μg/L As(III), 88 μg/L As(V), 35.3 mg/L Ca, and 5.10 mg/L Si.…”

Polymerization of silicate on TiO2 and its influence on arsenate adsorption: An ATR-FTIR study

“…The bands at 902 and 952 cm −1 were attributed to monomeric silicate surface complexes [16,25,33]. IR bands at 922 and 1020 cm −1 were due to oligomeric silicate surface complexes [16,18,25,33]. The main oligomeric silicate anions found in minerals are the dimer (Si 2 O 7 6− ) and cyclic anions such as (SiO 3 2− ) n with n = 3, 4 or 6.…”

“…For example, at a total silicate concentration of 0.1 mM, the adsorption of As(V) was decreased only at pH above 8, but when silicate concentrations were increased to 1.8 mM, As(V) adsorption was already decreased at pH 6 due to the polymerization of silicate [20]. Only under strongly acidic conditions (pH < 3), could As(V) adsorption not be interfered with by silicate [18]. Due to their different dissociation properties, adsorption of As(V) is strong under acidic conditions and gradually decreases with increasing pH, while silicate has the opposite adsorption behavior owing to its pK 1 at 9.8.…”

“…Furthermore, depending on pH, concentration, and other factors, silicate can be combined as silicate polymers both in the aqueous phase and on metal oxide surfaces [11,16,17]. These silicate polymers on metal oxides could greatly increase silicate adsorption capacities and affect the adsorption/desorption behavior of As(V) in groundwater [11,18,19].…”

“…Due to their different dissociation properties, adsorption of As(V) is strong under acidic conditions and gradually decreases with increasing pH, while silicate has the opposite adsorption behavior owing to its pK 1 at 9.8. Thus, at the pH of the majority of As contaminated groundwater (pH = 7.5-8.5) [21], As(V) has low affinity for metal oxides while the adsorption of Si is relatively high [18]. Therefore, the influence of silicate on As(V) adsorption in groundwater should be seriously considered.…”