Although phosphate (PO₄^3-) may play a decisive role in enriching toxic arsenic (As) in the groundwater of many Asian deltas, knowledge gaps exist regarding its interactions with As. This study investigates the simultaneous immobilisation of PO₄^3- and As in aquifer sediments at a redox transition zone in the Red River Delta of Vietnam. The majority of PO₄^3- and As was found to be structurally bound in layers of Fe(III)-(oxyhydr)oxide precipitates, indicating that their formation represents a dominant immobilisation mechanism. This immobilisation was also closely linked to sorption. In the surface sorbed sediment pools, the molar ratios of total P to As were one order of magnitude higher than found in groundwater, reflecting a preferential sorption of PO₄^3- over As. However, this competitive sorption was largely dependent on the presence of Fe(III)-(oxyhydr)oxides. Ongoing contact of the aquifer sediments with iron-reducing groundwater resulted in the reductive dissolution of weakly crystalline Fe(III)-(oxyhydr)oxides, which was accompanied by decreased competition for sorption sites between PO₄^3- and As. Our results emphasise that, to be successful in the medium and long term, remediation approaches and management strategies need to consider competitive sorption between PO₄^3- and As and dynamics of the biogeochemical Fe-cycle.

尽管磷酸盐（PO ₄ ^3-）在许多亚洲三角洲地下水中富集有毒砷（As）方面可能起决定性作用，但在与砷的相互作用方面仍存在知识空白。这项研究调查了越南红河三角洲氧化还原过渡带中PO ₄ ^3-和As在含水层沉积物中的同时固定化。大部分PO的₄ ^{3 -}并且发现在结构上结合在Fe（III）-（羟基）氧化物沉淀层中，表明它们的形成代表了主要的固定机制。该固定化也与吸附紧密相关。在表面吸附的沉积物池中，总P与As的摩尔比比地下水中的摩尔比高一个数量级，反映出PO ₄ ^3-优先于As吸附。然而，这种竞争性吸附很大程度上取决于Fe（III）-（羟基）氧化物的存在。含水层沉积物与铁还原地下水的持续接触导致弱结晶的Fe（III）-（羟基）氧化物的还原溶解，同时伴随着PO ₄ ^3-之间对吸附位点的竞争减少。并作为。我们的结果强调，要在中长期取得成功，修复方法和管理策略需要考虑PO ₄ ^3-和As之间的竞争性吸附以及生物地球化学Fe循环的动力学。