Molecular Scale Assessment of Methylarsenic Sorption on Aluminum Oxide

Abstract

Methylated forms of arsenic (As), monomethylarsenate (MMA) and dimethylarsenate (DMA), have historically been used as herbicides and pesticides. Because of their large application to agriculture fields and the toxicity of MMA and DMA, the sorption of methylated As to soil constituents requires investigation. MMA and DMA sorption on amorphous aluminum oxide (AAO) was investigated using both macroscopic batch sorption kinetics and molecular scale extended X-ray absorption fine structure (EXAFS) and Fourier transform infrared (FTIR) spectroscopic techniques. Sorption isotherm studies revealed sorption maxima of 0.183, 0.145, and 0.056 mmol As/mmol Al for arsenate (As(V)), MMA, and DMA, respectively. In the sorption kinetics studies, 100% of added As(V) was sorbed within 5 min, while 78% and 15% of added MMA and DMA were sorbed, respectively. Desorption experiments, using phosphate as a desorbing agent, resulted in 30% release of absorbed As(V), while 48% and 62% of absorbed MMA and DMA, respectively, were released. FTIR and EXAFS studies revealed that MMA and DMA formed mainly bidentate binuclear complexes with AAO. On the basis of these results, it is proposed that increasing methyl group substitution results in decreased As sorption and increased As desorption on AAO.

Density Functional Theory Study of Arsenate Adsorption onto Alumina Surfaces

“…The small aluminum hydroxide cluster models are composed of two Al octahedra, which contain bound hydroxyl and bound water groups so that the clusters remain neutral. This structure was based on cluster model figures in previously-reported literature, and is also shown in Figure 1 [16, 42,67]. We modelled arsenate adsorption in multiple configurations to compare with the surface complex models, and in all cases the cluster models are overall charge neutral.…”

Coagulation of methylated arsenic from drinking water: Influence of methyl substitution

“…As(V) can form three different surface complexes on Fe/Al oxide, including monodentate complex, bidentate binuclear complex and bidentate mononuclear complex [41,43], while both MMA and DMA can form bidentate binuclear complexes on Fe/Al oxide [15,42]. As(V) has more possible edge sites than MMA and DMA when forming complexes with Fe/Al oxide due to more hydroxyl groups when compared with methylated As [15]. Fewer reactive sites might result in lower adsorption affinity on Fe/Al hydroxide, leading to the decrease in coagulation removal efficiency of As with the increase in methyl substitution degree.…”

“…Fewer reactive sites might result in lower adsorption affinity on Fe/Al hydroxide, leading to the decrease in coagulation removal efficiency of As with the increase in methyl substitution degree. In addition, the formation of a bidentate binuclear complex between DMA and Fe/Al oxide requires that both As O and the As OH group are involved in the complexation, while the formation of the complex between MMA or As(V) and Fe/Al oxide only requires the involvement of two As OH groups [15,42]. Thus, the complex structure between DMA and Fe/Al oxide can be less stable than the complex structure for MMA or As(V) [15].…”

“…In addition, the formation of a bidentate binuclear complex between DMA and Fe/Al oxide requires that both As O and the As OH group are involved in the complexation, while the formation of the complex between MMA or As(V) and Fe/Al oxide only requires the involvement of two As OH groups [15,42]. Thus, the complex structure between DMA and Fe/Al oxide can be less stable than the complex structure for MMA or As(V) [15]. This can also contribute to the lower adsorption affinity on Fe/Al hydroxide and lower removal by Fe/Al salt coagulation for DMA than that for MMA or As(V).…”

“…As a result of the widespread distribution and toxicity of methylated As in the environment, research on methylated As removal from water has grown in importance. The adsorption of MMA and DMA on several minerals has been studied previously [9,[12][13][14][15][16]. Results found that adsorption of DMA on goethite and ferrihydrite was reportedly low compared with As(V) and MMA [13], adsorption of As(V) on aluminum oxide reached 100% within 5 min, while 78% and 15% of MMA and DMA were adsorbed, respectively [15], and adsorption of MMA on nanocrystalline TiO 2 achieved 100% at pH 7.5, with DMA removal reaching 65% at pH 5.5 [17].…”