Study On The Distribution And Migration Of Heavy Metals In Sediment By Conditioning

Some pollutants in the water body are deposited in the form of sediment resulting in increased sediment volumes with secondary pollution and subsequent release of pollutants likely.Different sediment conditioning agents affect both the dewatering efficiency of the sediment and the resource utilization of the dewatering cake,especially the leaching behaviors and speciation distribution of heavy metals?HMs?.This thesis carries out a series of studies from two perspectives:"how does the sediment conditioning affect the dewatering efficiency and the risk of HMs in the subsequent recycling of the cake"and"how does the sediment conditioning change HMs content of the sediment composition",and the main research contents are as follows:Taking polyaluminium chloride?PAC?/cationic polyacrylamide?PAM??coagulation precipitation conditioning agent,referred to as P-P conditioning?and ferrous activated sodium persulfate?advanced oxidation conditioning agent,referred to as F-S conditioning?,as examples,the sediments were conditioned.When the dosage of PAC and PAM were 5%dry basis?DS?and 4‰DS,respectively,the specific resistance to filtration?SRF?reduction rate was 91.7%.The dosage of Fe²⁺and sodium persulfate?SPS?were 1.04%DS and 4.8%DS,respectively,the SRF reduction rate was85.3%.The results of diaphragm filter press dewatering tests show that the dewatered cakes moisture content were:F-S?30.3%?<P-P?37.5%?<raw sediment RS?60.3%?,and the dewatering efficiency were:F-S?92.7%?>P-P?89.4%?>?RS,56.2%?.Conditioning makes the nitrogen and phosphorus in the sediment migrate from the liquid phase to the solid phase.The leaching behaviors and speciation distribution of Cd in the dynamic column leaching of dewatered cakes were discussed.As the liquid-solid ratio?L/S?increased,the real-time leaching concentration of the three cakes decreased and the leaching ratio increased.At the same L/S,conditioning reduced the Cd leaching ratio in the cake,and the F-S leaching ratio was the smallest.When L/S was 100 L kg^-1,the leaching ratio of Cd was reduced from 21.5%?RS?to 12.6%?P-P?and 11.7%?F-S?respectively.At this time,the risk assessment code?RAC?of Cd in the P-P and F-S cakes reduced from high risk of RS to medium risk.Conditioning transforms the Cd in the cake from the easy-to-leach speciation to potential-to-leach and difficult-to-leach speciation.Based on the self-designed sand extraction and separation device,according to the order of organic matter content,the sediment was divided into three components:sieved fraction,extracted slurries,sieved sand.F-S conditioning obtained the smallest sieved sand extraction quality(62.1 g kg^-1)and the largest slurries extraction quality(916.1 g kg^-1),and the corresponding minimum particle size?16.3?m and 8.2?m,respectively?.The HMs content of each component was sieved fraction>extracted slurries>sieved sand,conditioning reduces the HMs content of extracted slurries,so that sieved sand and sieved fraction increased.Conditioning did not change the main species of Cr,Cu and Pb in the sieved sand and slurries.The RAC of Cd,Cr and Pb in the slurries were extremely high,low and medium risk respectively,reducing to high,no and low risk after F-S conditioning.The mechanism of different conditioning systems leads to differences in the distribution and risk levels of HMs in each component.This result provides a reference for the hierarchical resource utilization of each component of the sediment.