Performance Of Zn/Fe-LDH Modified Ceramsite In Removing Phosphorus In Agricultural Runoff

The phosphorus?P?concentration in agricultural runoff is generally lower than that in other sewage.Therefore,challenges such as the low driving force for P uptake,the difficult adsorbent recovery,and the high cost often obstruct its P removal process.Layered double hydroxides?LDHs?are promising reusable adsorbents with high capacity and selectivity for phosphate,however,their real applications have always been hindered due to their inconvenient powder form.In this study,based on the excellent P selectivity and regenerative properties of Zn/Fe-LDH,and the better mechanical strength and the mesoporous surface of the porous ceramsite,the Zn/Fe-LDH modified ceramsite was fabricated by alkaline in-situ coprecipitation synthesis method.Subsequently,the cyclic adsorption performance of Zn/Fe-LDH modified ceramsite was explored,and the process parameters of the P removal module using the Zn/Fe-LDH modified ceramsite as the adsorption substrate were obtained,which provided technical supports for solving low concentration P pollution in agricultural runoff.The main research findings of this study were as follows:?1?The Zn/Fe-LDH modified ceramsite fabricated in this study?solid-liquid modification ratio 20%;Zn/Fe:2:1?could obtain a P adsorption capacity of 460 mg-P kg^-1.When the dosage was 20 g L^-1,the influences of various factors on the P adsorption capacity?indicated by the slope?were:p H value?-22.6?>Ca²⁺?21.5?>Mg²⁺?19.9?>?0.32?>NH₄⁺?0.19?.The P adsorption behaviors of the modified ceramsite were chemical processes that occurred on the heterogeneous surface,including interlayer anion exchange,ligand exchange,and electrostatic attraction.?2?Through the 55 cyclic adsorption/regeneration experiment,the operation mode was optimized as:dosage:20 g L^-1,adsorption time:2 h?p H=7.2?,desorption time:1 h?p H=13.0?.During the cyclic-adsorption under this mode,the total P?TP?concentration in agricultural runoff samples could be steadily reduced from 0.58 mg-P L^-1 to below 0.10 mg-P L^-1,and P species adsorbed on the modified ceramsite was gradually transformed from Fe/Al-P to Residual-P;the maximum Zn release amount was 0.033 mg L^-1,which was below the Zn concentration limit of surface water(0.05mg L^-1),and the highest luminescence toxicity inhibitory rate was 17%,which was also below the acute toxicity limit?20%?,indicating that the modified ceramsite has a low environmental risk.?3?When applying the Zn/Fe-LDH modified ceramstie as the interception zone in the P removal module for treating agricultural runoff flow,we found that with an interception zone length of 30 cm,the module could reduce the initial P concentration of 0.5 mg-P L^-1in the agricultural runoff flow with a copresence of 5 mg L^-1 humic acid and a velocity of 0.04 m³ d^-1 to below 0.1 mg-P L^-1,and the effective operating time was 28 days?4 weeks?.Using Parallel Sigmoidal?PS?model analysis,it was found that after 28 d,the adsorption behavior of the module was shifted from ligand exchange to interlayer anion exchange.From the artificial neural network?ANN?sensitivity analysis,the relative influences of different parameters on the effective operating time was:velocity?48.6%,-?>interception zone length?26.0%,+?>coexistent organic matter?14.4%,-?>P concentration?10.9%,-?.Based on the 9-week cyclic operation and its economic analysis,the cost of Zn/Fe-LDH modified ceramsite for treating P in agricultural runoff was 742 CNY kg^-1-P.Therefore,if treating 300 m³ of drainage every unit of farmland with an initial P concentration of 0.5 mg-P L^-1,the average cost of this modified ceramsite packing module within two months is about 111 CNY,indicating its economic advantages compared with the same types of P adsorption substrates,and could be promoted for P pollution treatment in agricultural runoff.