共存阴阳离子对针铁矿表面磷固存机制的影响研究

磷素(P)兼具重要养分元素的利和潜在面源污染的弊,其在土壤环境中的固存行为及其迁移转化过程受到广泛关注。该研究选取黄、红壤中典型矿物(针铁矿、赤铁矿及高岭石)为模式矿物,在排除pH干扰的条件下,开展了共存阴(As(Ⅴ))阳(Cd(Ⅱ))离子对矿物表面P(Ⅴ)固存机制的影响研究。结果表明:P(Ⅴ)在不同矿物表面的吸附效率表现为针铁矿>赤铁矿>高岭石;阴离子As(Ⅴ)、阳离子Cd(Ⅱ)与P(Ⅴ)共存并不会改变P(Ⅴ)在针铁矿表面的吸附动力学特征,仍符合准二级动力学模型,化学吸附为其控速步骤;P(Ⅴ)与As(Ⅴ)共存时,随着As(Ⅴ)浓度的增加,P(Ⅴ)的吸附等温线呈现从Langmuir型向Freundlich型转变的趋势,吸附量减小,但吸附速率增大,As(Ⅴ)主要通过静电排斥作用和吸附位点竞争作用降低P(Ⅴ)在针铁矿表面的吸附量;P(Ⅴ)与Cd(Ⅱ)共存时,P(Ⅴ)的吸附速率先增加后减小。具体来说,Cd(Ⅱ)/P(Ⅴ)物质的量比值<0.5时,Cd(Ⅱ)主要通过静电吸引略微促进P(Ⅴ)在针铁矿表面的吸附;Cd(Ⅱ)/P(Ⅴ)物质的量比值>0.5时,静电吸附和形成Fe-P(Ⅴ)-Cd(Ⅱ)三元络合物是促进针铁矿表面P(Ⅴ)固存的主要调控机制;进一步增加Cd(Ⅱ)/P(Ⅴ)物质的量比值,P(Ⅴ)固存的主要调控机制逐步转变为形成P(Ⅴ)与Cd(Ⅱ)的表面共沉淀。该结果可为研究P在土壤环境中固存的微观机制及其关键影响因子提供基础研究数据,也有望为提高P的利用率以及为调控P的面源污染问题提供有益帮助。

Influence of Co-existing Anions and Cations on Phosphate Sequestration onto Goethite

Phosphorus(P) is an important nutrient element, but is also a potential non-point source pollution. Retention behavior,migration, and transformation process of P in soil environment have attracted extensive attention. In this study, typical minerals(i.e.,goethite, hematite and kaolinite) in yellow soil and red soil were selected as the experimental model minerals. By excluding p H interference, the effects of coexisting anion As(Ⅴ)] and cation Cd(Ⅱ)] on the P(Ⅴ) surface retention mechanism onto the surface of minerals were studied. The adsorption efficiency of P(Ⅴ) on different mineral surfaces shows that goethite > hematite > kaolinite. The coexistence of anionic As(Ⅴ) and cationic Cd(Ⅱ) does not change the P(Ⅴ) adsorption kinetic characteristics onto the goethite surface.This finding is consistent with the pseudo-second-order kinetic model when chemical adsorption is the rate-controlling step. When P(Ⅴ) coexists with As(Ⅴ), as the As(Ⅴ) concentration increases, the P(Ⅴ) adsorption isotherm changes from Langmuir type to Freundlich type, the adsorption capacity decreases, and the P(Ⅴ) adsorption rate increases. As(Ⅴ) mainly reduces the P(Ⅴ) adsorption capacity on the goethite surface through electrostatic repulsion and adsorption site competition. When P(Ⅴ) coexists with Cd(Ⅱ), the adsorption rate of P(Ⅴ) first increases and then decreases. Specifically, when the Cd(Ⅱ)/P(Ⅴ) molar ratio is less than 0.5, Cd(Ⅱ) slightly promotes the adsorption of P(Ⅴ) on the goethite surface, mainly through electrostatic attraction. When Cd(Ⅱ)/P(Ⅴ) molar ratio is more than 0.5, electrostatic adsorption and the formation of Fe-P(Ⅴ)-Cd(Ⅱ) ternary complex are the main regulatory mechanisms that promote P(Ⅴ) sequestration on the goethite surface. By further increasing the Cd(Ⅱ)/P(Ⅴ) molar ratio, the main regulation mechanism of P(Ⅴ) storage is gradually transformed into the surface coprecipitation of P(Ⅴ) and Cd(Ⅱ). The purpose of the study is to provide basic research data for studying the micro mechanism and crucial influential factors for P sequestration in a soil environment. Findings are expected to help improve the P utilization rate and regulate the P non-point source pollution.