Violante Antonio, Ricciardella Mariarosaria, Del Gaudio Stefania, Pigna Massimo
Dipartimento di Scienze del Suolo, della Pianta e dell'Ambiente, Università di Napoli Federico II, Portici, Italy.
Environ Sci Technol. 2006 Aug 15;40(16):4961-7. doi: 10.1021/es052321m.
Arsenic mobilization in soils is mainly controlled by sorption/desorption processes, but arsenic also may be coprecipitated with aluminum and/or iron in natural environments. Although coprecipitation of arsenic with aluminum and iron oxides is an effective treatment process for arsenic removal from drinking water, the nature and reactivity of aluminum- or iron-arsenic coprecipitates has received little attention. We studied the mineralogy, chemical composition, and surface properties of aluminum-arsenate coprecipitates, as well as the sorption of phosphate on and the loss of arsenate from these precipitates. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0, or 10.0 and As/Al molar ratio (R) of 0, 0.01, or 0.1 and were aged 30 or 210 d at 50 degrees C. In the absence of arsenate, gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate, very poorly crystalline precipitates formed. Short-range ordered materials (mainly poorly crystalline boehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0, and 10.0 (R= 0.1) and did not transform into Al(OH)3 polymorphs even after prolonged aging. The surface properties and chemical composition of the aluminum precipitates were affected by the initial pH, R, and aging. Chemical dissolution of the samples by 6 mol L(-1) HCl and 0.2 mol L(-1) oxalic acid/ oxalate solution indicated that arsenate was present mainly in the short-range ordered precipitates. The sorption of phosphate onto the precipitates was influenced by the nature of the samples and the amounts of arsenate present in the precipitates. Large amounts of phosphate partially replaced arsenate only from the samples formed at R = 0.1. The quantities of arsenate desorbed from these coprecipitates by phosphate increased with increasing phosphate concentration, reaction time, and precipitate age butwere always lessthan 30% of the amounts of arsenate present in the materials and were particularly low (<4%) from the sample prepared at pH 4.0. Arsenate appeared to be occluded within the network of short-range ordered materials and/or sorbed onto the external surfaces of the precipitates, but sorption on the external surfaces seemed to increase by increasing pH of sample preparation and aging. Furthermore, at pH 4.0 more than in neutral or alkaline systems the formation of aluminum arsenate precipitates seemed to be favored. Finally, we have observed that greater amounts of phosphate were sorbed on an aluminum-arsenate coprecipitate than on a preformed aluminum oxide equilibrated with arsenate under the same conditions (R = 0.1, pH 7.0). In contrast, the opposite occurred for arsenate desorption, which was attributed to the larger amounts of arsenate occluded in the coprecipitate.
土壤中砷的活化主要受吸附/解吸过程控制，但在自然环境中，砷也可能与铝和/或铁共沉淀。尽管砷与铝和铁的氧化物共沉淀是从饮用水中去除砷的有效处理方法，但铝 - 砷或铁 - 砷共沉淀物的性质和反应活性却很少受到关注。我们研究了砷酸铝共沉淀物的矿物学、化学成分和表面性质，以及磷酸盐在这些沉淀物上的吸附和砷酸盐从这些沉淀物中的流失情况。在pH值为4.0、7.0或10.0以及As/Al摩尔比（R）为0、0.01或0.1的条件下合成砷酸铝共沉淀物，并在50℃下老化30或210天。在没有砷酸盐的情况下，形成了三水铝石（pH值为4.0或7.0）和拜耳石（pH值为10.0），而在有砷酸盐的情况下，形成了结晶性很差的沉淀物。在pH值为4.0（R = 0.01和0.1）、7.0和10.0（R = 0.1）时形成了短程有序材料（主要是结晶性差的勃姆石），即使经过长时间老化也没有转变为Al(OH)₃多晶型物。铝沉淀物的表面性质和化学成分受初始pH值、R和老化的影响。用6 mol L⁻¹ HCl和0.2 mol L⁻¹草酸/草酸盐溶液对样品进行化学溶解表明，砷酸盐主要存在于短程有序沉淀物中。磷酸盐在沉淀物上的吸附受样品性质和沉淀物中砷酸盐含量的影响。大量的磷酸盐仅从R = 0.1时形成的样品中部分取代了砷酸盐。这些共沉淀物被磷酸盐解吸的砷酸盐量随着磷酸盐浓度、反应时间和沉淀物老化程度的增加而增加，但始终小于材料中砷酸盐含量的30%，并且从pH值为4.0制备的样品中特别低（<4%）。砷酸盐似乎被包裹在短程有序材料网络内和/或吸附在沉淀物的外表面上，但外表面的吸附似乎随着样品制备和老化pH值的增加而增加。此外，在pH值为4.0时比在中性或碱性体系中更有利于砷酸铝沉淀物的形成。最后，我们观察到，在相同条件下（R = 0.1，pH值为7.0），与预先形成并与砷酸盐平衡的氧化铝相比，更多的磷酸盐吸附在砷酸铝共沉淀物上。相反，砷酸盐的解吸情况则相反，这归因于共沉淀物中包裹的砷酸盐量更大。
