Mechanism Of Contaminant Stepwise Capture And Enhanced Removal By Hydroxide Microflocs

Coagulation is an important water treatment technology,which plays a vital role in the removal of colloidal particles and organic matters.To address the issue that coagulation has good performance in removing hydrophobic and high-molecular-weight organic matters but poor performance in removing hydrophilic and low-molecular-weight organic matters（i.e.,low-coagulability organic matter,LCOM）,the interaction between coagulants and LCOM in the low-turbidity secondary effluent water was investigated in this study.Based on this,a novel coagulation model was proposed in which the flocs grew stepwise.A continuous dosing coagulation（CDC）process was built,and its removal performance on LCOM and phosphorus in secondary effluent water was investigated.The stepwise growth process of hydroxide microflocs was investigated.The mechanism of contaminant stepwise capture and enhanced removal was revealed,as was the mechanism of the initial p H value affecting the enhanced removal of contaminants.The main research content and conclusions of this study are as follows:（1）The coagulation behavior and mechanism of LCOM were revealed:under charge neutralization conditions（weak acidic p H and low dosage）,the coagulants could not form hydroxide flocs by themselves.When treating LCOM,no flocs formed.The charge neutralization mechanism did not apply to LCOM coagulation.Under sweep flocculation conditions（neutral p H and high dosage）,the coagulants formed hydroxide flocs,and LCOM flocs were also observed.Due to its simple structure,low molecular weight,and low charge,LCOM hardly influenced the properties of hydroxide flocs and could only be removed by the adsoprtion of bulk hydroxide surfaces.Based on this and referring to the stepwise growth model of flocs,a novel coagulation model was proposed in this study:regulate the hydroxide floc formation process and enhance the capture of LCOM in its stepwise growth process to improve the efficiency of the coagulation process.（2）A CDC process was built:the coagulant and alkali were continuously dosed into water that was pre-acidified,rather than dosing all the reagents at once as in the conventional coagulation（CC）process.The treatment performance of the CDC process and the optimal operation parameters were investigated.Compared with the CC process,the CDC process increased the removal rates of salicylic acid and phosphate in simulated water by up to 18.02%and 24.34%,respectively.In addition,the CDC process increased the removal rates of organic matter and total phosphorus in wastewater treatment plant secondary effluent water by up to 11.68%and 21.46%,respectively,showing its enhanced removal ability.An initial p H of 6 and a coagulant dosing time of 5–10 min were selected as the optimal operation parameters.The CDC process could improve the removal of various model organic matters,which showed its general applicability.The phosphate removal performance in the CDC process was steadier than the CC process and less affected by the temperature and coexistence of organics and ions.（3）The stepwise growth process of hydroxide microflocs and the mechanism of LCOM stepwise capture and enhanced removal in the CDC proess were revealed.In the early stage,the dosed coagulant formed 100-nm medium polymers,which could bind with LCOM to form primary complexes.The subsequent doses of coagulant could one by one adhere to the primary complexes and form coral reef-like surfaces with more positive charges and a larger specific surface area.Each freshly formed surface could capture LCOM as well as cover previous surfaces.As a result,more LCOM were included in the interior of microflocs.LCOM was stepwise captured and got enhanced removed in the stepwise growth process of hydroxide microflocs.（4）The mechanism of phosphate stepwise capture and enhanced removal in the CDC proess was revealed.Due to the“low p H and low Al³⁺concentration”environment in the early stage,there was enhanced co-precipitation of Al³⁺and PO₄^3-,which promoted the formation of insoluble Al PO₄ particles.The destabilization of phosphate was obtained.The coral reef-like flocs that formed during the continuous dosing process had more positive charges and a larger specific surface area,which realized the stepwise capture and enhanced removal of undissolved Al PO₄ particles and dissolved phosphate anions in their stepwise growth process.（5）The mechanism of the initial p H value affecting the enhanced removal of contaminants was revealed.Using salicylic acid（SA）as an example,when the initial p H value was 5,monomeric Al（Al_a）formed in the early stage,which could complex with SA to form 1:1 complexes SA-Al_a,whose chemical formula was Al（OH）（C₇H₄O₃）（H₂O）₂.The 1:1 complexes were very steady and could not polymerize with other Al polymers.Finally,they remained in the water,and the removal promotion was limited.When the initial p H value was 6,medium polymer Al（Al_b）formed in the early stage,which could bind with SA to form SA-Al_b complexes.The subsequent doses of coagulant could one by one adhere to it and realize the stepwise growth process of hydroxide microflocs and the stepwise capture and enhanced removal of SA.When the initial p H value was 7,high polymer Al（Al_c）formed throughout the CDC process.Only floc surfaces could absorb SA,and the interior of the flocs did not work,which was similar to the CC process.A lower initial p H promoted the phosphate removal in the CDC process because more Al_aformed in the early stage,which was beneficial for forming more undissolved Al PO₄particles.The flocs had more positive charges,which was beneficial for the attraction towards undissolved Al PO₄ particles and dissolved phosphate anions.