Study On The Silver Occurrence Form And Release Of Silver In AgNPs Floc

With the wide application of silver nanomaterials,silver nanoparticles(AgNPs)is inevitably accompanied by waste(sewage)water discharge into the water treatment facilities.Coagulation process,as one of the most widely used water treatment processes,can efficiently remove AgNPs from water and waste water.In recent years,the transformation and release of AgNPs during the process of water treatment and sludge final disposal is considered to be an important step in evaluating the environmental toxicity of AgNPs.In this paper,the simulated water samples containing AgNPs were treated by coagulation process,and the flocs leaching experiments and flocs heat treatment experiments were carried out.The occurrence forms(chemical components,crystal structures and particle size)of AgNPs in floes and the dissolution and rerelease of silver during floc disposal were analyzed by various characterization methods.The effects of coagulation process conditions and sludge disposal environmental conditions on the immobilization of AgNPs were clarified,which provided a theoretical basis for the evaluation and control of environmental toxicity in the whole life cycle of AgNPs.The main research and results of this paper are as follows:1.Coagulation experiment.AgNPs simulated water samples were treated by coagulation process.The occurrence of silver in AgNPs floc was analyzed by means of characterization,and the effect of coagulation conditions on the transformation of silver form was discussed.The results show that AgNPs will enter the floc in the form of small particle size dispersion and large particle size aggregation,and most of them will exist as precipitated silver.Due to oxidation-dissolution,part of Ago will be transformed into AgCl and Ag2O during coagulation process.This process was affected by the characteristics of AgNPs and water quality,but the type of coagulant was less affected.The results of XRD and XPS show that,AgNPs without stabilizer,noted as AgNPs-P,exists in the chemical form of AgO crystal(68.8%)and AgCl crystal(31.2%)in floes,while AgNPs coated with PVP,noted as AgNPs-L,xwas exists in the chemical form of Ag0(33.4%),Ag20(30.6%)and AgCl(36%)in flocs.2.Floc dissolution experiment,include static dissolution and dynamic leaching experiment.The rerelease process of silver in contact between the actual landfill sludge and infiltrated rain,groundwater or seawater was simulated by flocs dissolution experiment.The effect of pH;water hardness and the concentration of chloride ion on the dissolution of silver in floc was studied and the release mechanism of heavy metals in floc was clarified.The results show that the leaching solution with high calcium ion or chloride ion content will lead to a great increase in silver dissolution from floc.When the concentration of chloride ion in the extract was 19000.00 ppm,10.04 mg silver could be released from 1.00 g AgNPs floc within 120 h.This means that seawater or high hardness water will lead to a large amount of silver dissolution in landfill sludge.In addition,in the process of static dissolution,the concentration of silver in floc quickly reached the highest value,and then it was unstable and precipitated,while in the process of dynamic leaching,the dissolution of silver in floc showed an upward trend.This indicates that under the condition of flowing water,silver in floes will continue to be released and may migrate over a long distance in the environment.3.Floes heat treatment experiment.AgNPs floc was calcined in muffle furnace,and the effect of calcination temperature on silver occurrence in floc was discussed,so as to clarify the effect of sludge heat treatment on environmental risk.The results showed that after calcining the sludge at 600?,part of Ag0 in AgNPs floc could be further converted into A02O.AgNPs-P water samples with acid,calcium concentration of 200.00 mg/L or humic acid concentration of 100.00 mg/L were treated by coagulation.For AgNPs-P floc,produced by coagulation with acidity,high HA content or high calcium ion content in water,the percentage of dissolved silver and colloidal silver will increase to 68.26%,42.57%and 78.23%after calcining,respectively.However,the content of dissolved silver and colloidal silver in AoNPs-L ash did not increase,which indicated that AgNPs protected by PVP had better thermal stability.