Control Of Phosphorus Release From Sediments By Magnetic Iron-based Materials And Magnetic Iron/Zirconium-based Materials

Since phosphorus?P?is a limiting factor controlling harmful algae growth,reducing the excessive input of P into the freshwater bodies is vital to controlling their eutrophication.The P in the overlying water?OLY-water?of freshwater bodies is sourced from the external P loading?EX-PL?and the internal P loading?IN-PL?.As we all know,when EX-PL is effectively controlled,IN-PL can delay the recovery of eutrophic freshwater bodies.In recent years,in-situ covering/addition technology,which uses geo-engineering materials to cover sediments or to amend sediments,is regarded as a very promising strategy for blocking P release from IN-PL.The key to control the release of phosphate in sediments by this technique is to select materials with low cost and high efficiency.In this study,magnetite-modified activated carbon?MAC?was synthesized,characterized and used as capping and amendment materials to control sedimentary phosphorus?P?release.Batch experiments were applied to determine the behavior of phosphate adsorption and desorption on/from MAC.Sediment incubation experiments were utilized to evaluate the impact of MAC capping and addition on the mobilization of P in sediments.Sediment capping and amendment with MAC both can greatly reduce the amount of reactive soluble P?SRP?in the overlying water?OLY-water?,with a reduction efficiency of higher than 83%.MAC capping and amendment both can significantly reduce the concentrations of P_DGT measured by diffusive gradient in thin-films?DGT?in the upper sediment,which gives rise to in the formation of the static layer of P?P-S-Layer?in the upper sediment.The forms of P bound by MAC were mainly redox-sensitive P(P_BD),Na OH extractable inorganic P(IP_{Na OH})and HCl extractable P(P_HCl),which accounted for 47.2,18.5 and 32.9%of the total adsorbed P,respectively.Almost half of P adsorbed by MAC existed in the form of mobile P(P_Labile and P_BD),which is easy to be released under anoxic condition,and the retrieval of MAC from the waterbody after its application is very necessary.The concentrations of SRP in OLY-water and mean P_DGT in P-S-Layer under capping condition were much less than those under amendment condition.The reduction of the apparent diffusion efflux of P across the interface between OLY-water and sediment by the MAC capping was much larger than that by the MAC amendment.Results of this work suggest that MAC capping and amendment are very promising methods for blocking the liberation of P from sediments into OLY-water,and MAC capping can achieve a higher efficiency of sedimentary P release control compared to MAC amendment.Secondly,a novel capping material,i.e.,zirconium-modified magnetite/bentonite composite?M-Zr Fe BT?was prepared,characterized and used as a capping material to control the Internal P loading of freshwater bodies.The adsorption performance and mechanism of phosphate on M-Zr Fe BT was studied.The impact of M-Zr Fe BT capping on the mobilization of P in sediments was investigated.The results showed that M-Zr Fe BT possessed excellent phosphate adsorption ability,with a maximum mono layer adsorption capacity of 8.02 mg P/g.Ligand exchange of Fe/Zr-bound hydroxyl groups with phosphate to generate the inner-sphere Fe-O-P and Zr-O-P bonding played a key role in the uptake of phosphate from aqueous solution by M-Zr Fe BT.Sediment capping with M-Zr Fe BT not only significantly decreased the concentration levels of soluble reactive P?SRP?and DGT-labile P(P_DGT)in the overlying water,but also led to a reduction in the P_DGT concentration in the upper sediment.Most of P bound by the M-Zr Fe BT in capping layers was in the form of IP_{Na OH},P_HCl and P_Res,,which were difficult to be released back into the water column under common p H and anoxic conditions.Reduction of pore water SRP and P_DGT in the upper sediment layer induced by the adsorption of SRP on the M-Zr Fe BT based capping layer played a key role in the interception of SRP liberation from the sediment solid into the overlying water.Results of this work indicate that M-Zr Fe BT is a promising capping material for the inhibitation of SRP liberation from sediments.Thirdly,the effect of M-Zr Fe BT addition on the mobilization and species transformation of phosphorus?P?in river sediments was investigated using incubation sediment core experiments.Results showed that,under anoxic condition,P could be released from river sediments to the pore water,and then P in the pore water could be released into the overlying water.The addition of Zr Fe BT into river sediments could greatly suppress the release of P from river sediments into the pore water under anoxic condition,and then the release of P from the pore water into the overlying water could be significantly suppressed by the addition of Zr Fe BT.After the addition of Zr Fe BT into river sediments,the transformation of loosely sorbed P(P_Labile)and BD extractable P(P_BD)to Na OH extractable P(IP_{Na OH})and residual P(P_Res)in the sediments was observed,and the decrease of bioavailable P?BAP?including water soluble phosphorus?WSP?,readily desorbable phosphorus?RDP?,Na HCO₃ extractable phosphorus?Olsen-P?,algal available phosphorus?AAP?and Fe oxide-paper extractable P?Fe O-P?in the sediments was also observed.A certain amount of P in the Zr Fe BT after the incubation experiment existed in the form of mobile P(P_Labile and P_BD),Olsen-P and Fe O-P,which could be re-released into the pore water and overlying water when the environmental condition changes in the future.The control of P release from river sediment into the overlying water by the addition of M-Zr Fe BT could be mainly attributed to the reduction of P in the pore water as well as the reduction of mobile P and BAP in the sediments after the M-Zr Fe BT amendment.Results of this work inidcate that M-Zr Fe BT is a very promising amendment for the control of P release from river sediments into the overlying water.Finally,Sediment incubation experiments were carried out to study the effect of capping with permeable fabric-capsulated M-Zr Fe BT on the upward mobilization of P in sediments.Results showed that,Sediment capping with fabric-wrapped M-Zr Fe BT not only significantly decreased the concentration levels of soluble reactive P?SRP?and DGT-labile P(P_DGT)in the overlying water,but also led to a reduction in the P_DGTconcentration in the upper sediment.Most of P bound by the M-Zr Fe BT in the fabric-wrapped capping layers was in the form of Na OH extractable inorganic P,HCl-extractable P and residual P,which were difficult to be released back into the water column under common p H and anoxic conditions.These observations imply that the release of SRP from the sediment reserve into the pore water could be tremendously inhibited by the fabric-wrapped M-Zr Fe BT capping layer.This could be mainly attributed to the adsorption of SRP from the pore water onto the fabric-encapsulated M-Zr Fe BT capping layer due to the strong phosphate adsorption ability of the M-Zr Fe BT located in the capping layer.Generally,MAC and M-Zr Fe BT all have high capacity on phosphate adsorption and obvious passivation effect of phosphorus in heavily polluted sediment.In order to prevent the destruction of an active capping layer by strong hydrodynamic disturbance and bioturbation,the use of permeable fabrics to provide erosion protection and keep the active capping layer intact is a feasible method.