Study On Covalently Bound Hybrid Coagulants In Dewantering Of Dredged Sediments

Sediment dewatering is the core of sediment treatment processes.One of the most effective methods of sediment dewatering is chemical flocculation dewatering,which can realize deep dewatering of sediments.However,now there is little research on coagulants with deep dewatering performance.Therefore,it is of great significance to develop a new and highly efficient coagulant.In this study,the dredged sediments from Maozhou River in Shenzhen were taken as a target,and a novel inorganic-organic covalent bond-type coagulant?CBHyC?and its optimal ratio was determined.At the same time,the conventional coagulants FeSO₄,FeCl₃,Al₂?SO₄?₃,polymerized ferrous sulfate?PFS?,cationic poly?N-isopropyl-acrylamide??CPAM?and CBHyC were used to investigate the dewatering effect of sediments.The solidification effect of CBHyC on heavy metals,the dewatering and solidification mechanisms were investigated.The main results are as following:?1?According to the results of the zeta potential and flocculation performance of CBHyC,the optimal ratio of was determined.When the alkalization degree?B value?was 1.5 and Si/Fe was 0.4,CBHyC had the highest electric neutralization ability and flocculation performance.?2?The effect of coagulants on the dewatering performance of sediments were studied from the determination of settling rate,specific resistance to filtration?SRF?,water content of sediment cakes and turbidity of supernatant.CBHyC achieved a setting rate of 86%-334%faster than conventional coagulants,its SRF decreased by 67%–83%and the water in the dewatered cakes reduced by 14%-24%compared with conventional coagulants.At the same time,CBHyC had a good effect on the turbidity removal.In addition,the sediment solidification conditioned with CBHyC through the analysis of the leaching behavior of heavy metals and the concentration of pollutants in leaching liquid was examined.It is shown that CBHyC had a significant effect on solidifying heavy metals in the sediments.The leaching concentration of Cr,Cu,Zn,Ni,Pb,and Cd was reduced by 55.8%,92.1%,89.2%,61.4%,34.2%,and56.3%,respectively.The concentration of total phosphorus,ammonia nitrogen,total nitrogen and chemical oxygen demand was greatly reduced in the sediment leaching solution after treatment.The leaching solution could meet the first class B standard of GB18918-2002,and be directly discharged.?3?The dehydration mechanism of the sediments treated by coagulants was explored by studying the changes of zeta potential,flocs size,bound water content,extracellular polymeric substances?EPS?and microstructure of the sediments before and after flocculation.Firstly,the iron salts and quaternary ammonium salts in CBHyC neutralized the colloids with negative charges and destroyed the stability of the colloids.Secondly,the long carbon chains in CBHyC made sediment particles to aggregate into large and compact flocs due to the bridging effect,which enhanced sedimentation performance of sediments.Finally,CBHyC dissolved the EPS and released the boubd water encapsulated by the EPS into the free water,promoted sediment to form a porous structure and improved the dewatering performance.In addition,the mechanism of CBHyC curing heavy metal was speculated based on the effect of surfactant and sodium dimethyl dithiocarbamate on the solidification of heavy metals.The surfactant in CBHyC firstly discharged the heavy metals in sediment particles,which could transform the heavy metal from the adsorption state into the dissociation state.Secondly,the anion group of dimethyldithiocarbamate in CBHyC was combined with the heavy metal to form an insoluble organic chelate,which transformed the heavy metal into a chemical binding state,thereby enhancing the stability of heavy metals in the sediments.