Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
Environ Sci Technol. 2012 Aug 7;46(15):8044-51. doi: 10.1021/es300988p. Epub 2012 Jul 11.
The photochemistry of an aqueous suspension of goethite in the presence of arsenite (As(III)) was investigated with X-ray absorption near edge structure (XANES) spectroscopy and solution-phase analysis. Irradiation of the arsenite/goethite under conditions where dissolved oxygen was present in solution led to the presence of arsenate (As(V)) product adsorbed on goethite and in solution. Under anoxic conditions (absence of dissolved oxygen), As(III) oxidation occurred, but the As(V) product was largely restricted to the goethite surface. In this circumstance, however, there was a significant amount of ferrous iron release, in stark contrast to the As(III) oxidation reaction in the presence of dissolved oxygen. Results suggested that in the oxic environment ferrous iron, which formed via the photoinduced oxidation of As(III) in the presence of goethite, was heterogeneously oxidized to ferric iron by dissolved oxygen. It is likely that aqueous reactive oxygen species formed during this process led to the further oxidation of As(III) in solution. Results from the current study for As(III)/goethite also were compared to results from a prior study of the photochemistry of As(III) in the presence of another iron oxyhydroxide, ferrihydrite. The comparison showed that at pH 5 and 2 h of light exposure the instantaneous rate of aqueous-phase As(V) formation in the presence of goethite (12.4 × 10(-5) M s(-1) m(-2)) was significantly faster than in the presence of ferrihydrite (6.73 × 10(-6) M s(-1) m(-2)). It was proposed that this increased rate of ferrous iron oxidation in the presence of goethite and dissolved oxygen was the primary reason for the higher As(III) oxidation rate when compared to the As(III)/ferrihydrite system. The surface area-normalized pseudo-first-order rate constant, for example, associated with the heterogeneous oxidation of Fe(II) by dissolved oxygen in the presence of goethite (1.9 × 10(-6) L s(-1) m(-2)) was experimentally determined to be considerably higher than if ferrihydrite was present (2.0 × 10(-7) L s(-1) m(-2)) at a solution pH of 5.
使用 X 射线吸收近边结构（XANES）光谱和溶液相分析研究了针铁矿存在亚砷酸盐（As(III)）的水悬浮液的光化学。在溶液中存在溶解氧的条件下辐照砷酸盐/针铁矿导致砷酸盐（As(V)）产物吸附在针铁矿上和溶液中。在缺氧条件下（不存在溶解氧），发生了 As(III)氧化，但 As(V)产物主要局限于针铁矿表面。然而，在这种情况下，释放了大量的二价铁，与溶解氧存在下的 As(III)氧化反应形成鲜明对比。结果表明，在有氧环境中，通过针铁矿存在下的光诱导氧化形成的二价铁被溶解氧异相氧化为三价铁。很可能是在这个过程中形成的水相活性氧物种导致了溶液中 As(III)的进一步氧化。当前研究中 As(III)/针铁矿的结果也与先前研究中三价砷在另一种铁氢氧化物水铁矿存在下的光化学进行了比较。比较表明，在 pH 5 和光照 2 小时的条件下，针铁矿存在下水中 As(V)形成的瞬时速率（12.4×10(-5)M s(-1)m(-2)）明显快于水铁矿存在下（6.73×10(-6)M s(-1)m(-2)）。提出在针铁矿和溶解氧存在下，二价铁氧化速率的增加是与 As(III)/水铁矿系统相比，As(III)氧化速率更高的主要原因。例如，与针铁矿存在时溶解氧异相氧化 Fe(II)相关的表面积归一化准一级速率常数（1.9×10(-6)L s(-1)m(-2)）实验确定明显高于水铁矿存在时（2.0×10(-7)L s(-1)m(-2)）在溶液 pH 为 5 的情况下。
