低磷富硅淡水系统磷酸盐氧同位素测试的前处理方法改进

Modification of Pretreatment Method of Phosphate Oxygen Isotope Analysis in Phosphorus-poor,Silicon-rich Freshwater System

磷是大多数淡水生境营养状态的限制元素,其来源识别是水体磷污染控制及其富营养化防治的关键。磷酸盐氧同位素比值(δ^(18)OP)示踪法可有效甄别磷来源,然而淡水生境的磷含量低且水质成分复杂,如硅酸盐岩分布区水体常具有低磷富硅特征,由于无定形硅易析出并吸附PO_(4)^(3-),通过现有的前处理步骤难以获得有效的δ^(18)OP测试数据。文章针对淡水低磷富硅、沉积物及土壤贫磷砂质特征,基于广泛使用的Mclaughlin前处理方法做出了如下改进:对低磷富硅水体,(1)降低PO_(4)^(3-)富集步骤投加试剂比例;(2)CePO_(4)沉淀前分级调节pH;(3)多步细孔过滤降低硅含量;(4)树脂柱动态交换去除阳离子。对贫磷砂质沉积物及土壤,则增大样品用量和提取液浓度,结合淡水样品的纯化方法,获取足量纯净Ag_(3)PO_(4)用于δ^(18)OP测定。改进优化后的δ^(18)OP前处理方法,沉淀及离子交换的关键步骤回收率均高于80%,重复测量精度为±0.6‰(n=6),通过对比加标回收与KH_(2)PO_(4)试剂直接生成Ag_(3)PO_(4)的δ^(18)OP,验证了改进流程分析的准确性。该前处理方法成功应用于丹江口水库汇流区的水体、沉积物及土壤的δ^(18)OP测定,为其磷的溯源提供了技术支撑。

Phosphorus is the limiting element of the trophic states for most freshwater systems.Therefore,the identification of the main phosphorus sources is essential to the efficient control of phosphorus pollution as well as the prevention of eutrophication.Phosphate oxygen isotope(δ^(18)OP)is proved to be effective in source identification.However,freshwater habitats are always of low phosphorus concentrations and complex compositions.For example,water bodies in silicate rocks exposed areas can be characterized by having little phosphorus while abundant silica.The later can easily precipitate and adsorb PO_(4)^(3-),making it difficult to obtain validδ^(18)OP through existing methods.In this study,the widely used Mclaughlin pretreatment method was modified to deal with the phosphorus-poor,silicon-rich water,the phosphorus-poor sandy sediments and soils.The efficient enrichment and purification of PO_(4)^(3-)in freshwater are achieved with the following improvements:(i)reduction in the proportion of reagents added at the PO_(4)^(3-)enrichment step;(ii)graded pH adjustment prior to CePO_(4)precipitation;(iii)multiple steps of fine pore membrane filtration to reduce the silica content;(iv)dynamic ion-exchange to remove cations.For phosphorus-poor sandy sediments and soils,an increase of the sample amount and the concentration of the extraction solutions,combined with the purification steps of water samples have been used to precipitate sufficient pure Ag_(3)PO_(4)forδ^(18)OP determination.The phosphate recovery of significant procedures relating to precipitation or cation are above 80%,and the precision is±0.6‰.The analytical accuracy was evaluated by comparing theδ^(18)OP of Ag_(3)PO_(4)generated with the modified methods and the pure phosphate solution directly.The modified methods have been applied to theδ^(18)OP determination of samples from Danjiangkou Reservoir basin,providing technical support for tracing phosphorus in this basin.