Effect And Mechanism Of Fe On Phosphorus Removal By Biological Carbon Pump In Karst Surface Water

The co-precipitation of soluble phosphate with calcium carbonate（Ca CO₃）during the process of biological carbon pump（BCP）is a self-cleaning mechanism in karst water.At the same time,the Fe（III）oxyhydroxide colloids,which are induced by BCP due to the increasing p H and dissolved oxygen（DO）concentration,can also absorb P to form coprecipitates.Therefore,the P removal by BCP can improve water quality and alleviate the development of eutrophication.Iron element in water is an active electron receptor and electron donor,so Iron could have important effect in the P removal by BCP.The effect of iron on the P removal by BCP can be researched by the settling fluxes of total phosphorus and chemical forms of phosphorus in suspended particulate matter.To further understand the influence and mechanisms of iron element on phosphorus removal by BCP,the spatiotemporal variatios of hydrochemistry,total phosphorus（TP）and phosphorus forms settling fluxes in suspended particulate matter of Pingzhai reservoir,Puding reservoir and Lake Hongfeng in Guizhou Province were studied.and different forms of iron and phosphorus content were also systematically studied.The results show that:（1）The p H and DO of the reservoirs are higher in summer due to the stronger photosynthesis of aquatic plants（BCP effect）.Meanwhile,the concentrations Ca²⁺and HCO₃^-,and electrical conductivity（EC）are lower in summer due to utilization of DIC and the formation of Ca CO₃during the BCP effect.EC decreased synchronously because EC in karst water is mainly controlled by HCO₃^-and Ca²⁺concentration.The TP in surface water in the reservoirs was always lower than that in the inflow rivers,which may be due to P removal by BCP.（2）In summer,the settling fluxes of TP in the three reservoirs have the highest settling fluxes due to the stronger P removal by BCP.The settling fluxes of HCl-P in Puding reservoir is the largest.The P removal through the co-precipitation soluble phosphate with calcium carbonate（Ca CO₃）by BCP is significant in Puding reservoir.The P removal in Pingzhai Reservoir and Lake Hongfeng is mainly due to deposition of BD-P and Na OH-P which were related to Fe hydroxides/oxides.BD-P and Na OH-P were related to BCP,and the higher values of p H and DO in water caused by BCP can promote the formation BD-P and Na OH-P.The difference in settling fluxes of BD-P and Na OH-P between Pingzhai reservoir and Lake Hongfeng may be related to the particulate size of suspended particulate matter and trophic state of reservoirs.The particulate size of suspended particulate in Lake Hongfeng is the smallest,which is not conducive to the settlement of suspended particulate matter.At the same time,P cycling rates were higher in Lake Hongfeng due to the eutrophication.（3）Due to BCP,the concentration of P-Fe_prs in Puding reservoir increased in autumn.The main reason is that some organic phosphorus in water can form organic film during mineralization,which covers the surface of Ca CO₃ and other minerals,thus inhibiting the release of iron.High p H and carbonate in Pingzhai reservoir is beneficial to the formation of Fe_carb and HCl-P can be deposited more stably.The P released from P-Fe_carb and P-Fe_ox2was adsorbed by the precipitated calcite to form HCl-P during migration and transformation in the winter water of Lake Hongfeng,so it can achieve a more stable burial.