Study On Interface Distribution And Stability Mechanism Of Arsenic In Pipe Scales And Microplastic Systems In Drinking Water Distribution System

Pipe scales has always been considered as the"source"of heavy metal pollutants in drinking water distribution system（DWDS）.In recent years,microplastics（MP）have gradually been considered as the emerging carrier of various pollutants in the environment.Its existence in DWDS may affect the fate of arsenic pollutants.In this study,the accumulation of As（Ⅲ）and As（Ⅴ）under different water quality conditions in the single carrier system of water supply pipe scales（20-30 years old）,the single carrier system of model MP（polystyrene with-COOH）and the complex system of two carriers coexisting were studied to distinguish the main carrier for enriching arsenic pollutants.The effect of adding MP on arsenic accumulation in pipe scales under different water quality conditions was also clear.Through the accumulation and stability of arsenic on pipe scales,the most conducive water quality to the fixation of arsenic pollutants was obtained.The accumulation mechanism of As（Ⅲ）and As（Ⅴ）at the pipe scales interface was discussed by characterization methods,adsorption kinetics,adsorption isotherm models and the accumulation and stability law of As（Ⅲ）and As（Ⅴ）in single component system and competitive system.The main achievements are as follows:（1）In the single carrier system,under the conditions of pH=3-10 and different concentrations of Cl^-,SO₄^2-,PO₄^3-,the distribution of arsenic at the pipe scales interface is much greater than that at the MP interface（pipe scales:1.08～4.80 mg/g;MP:0.01～0.90 mg/g）.The adsorption of As（Ⅲ）on MP is attributed to hydrogen bonding,while there is electrostatic repulsion between As（Ⅴ）and MP.Ca²⁺,Zn²⁺and Fe³⁺play an important role in promoting the adsorption of arsenic on MP:MP-Mⁿ⁺-As（Ⅲ）/As（Ⅴ）complex may form by cation bridging,so as to significantly promote the adsorption of arsenic by MP.In general,compared with the emerging pollutant carrier MP,pipe scales is still the main carrier for arsenic accumulation.In the complex system,the addition of MP promoted the accumulation of As（Ⅲ）（9.37%）in the range of pH=3～8,but inhibited the adsorption of As（Ⅴ）in varying degrees in the range of pH=3～10（the maximum inhibition percentage was 20.56%）.However,the presence of anion and cation makes the effect of MP less significant.（2）The accumulation rule of arsenic by pipe scales was as follows:the accumulation of As（Ⅲ）first increased,next flattened and then decreased with the increase of pH.The accumulation of As（Ⅴ）decreased with the increase of pH.PO₄^3-significantly inhibited the accumulation of As（Ⅲ）and As（Ⅴ）.Appropriate concentrations of Cl^-and SO₄^2-promoted the accumulation of As（Ⅲ）and As（Ⅴ）.when the concentration increased,the competitive adsorption between arsenic and anions led to the decrease of accumulation.The total accumulation and coprecipitation of As（Ⅲ）and As（Ⅴ）increased with the increase of Fe³⁺,Zn²⁺,Ca²⁺concentration.The accumulation mechanism of arsenic by pipe scales was as follows:the accumulation of As（Ⅲ）was mainly the specific adsorption related to coordination complex,and the proportion of deprotonated inner spherical complex Fe OHAs O₃^-may be large.The accumulation of As（Ⅴ）included specific adsorption and nonspecific adsorption,which was mainly dominated by coordination complexation,electrostatic interaction and chemical precipitation.After accumulation,complex redox reactions would occur on the interface of magnetite in pipe scales,involving the oxidation of As（Ⅲ）and the reduction of As（Ⅴ）.Moreover,MP adsorbed on the surface of pipe scales in complex system may also oxidize the accumulated As（Ⅲ）to As（Ⅴ）and has certain stability.The existence of MP reduced the risk of arsenic release to a certain extent.（3）The results of accumulation and stability experiment showed that strong acidic and strong alkaline,accident concentration levels of SO₄^2-and the existence of PO₄^3-were the most unfavorable water quality for As（Ⅲ）and As（Ⅴ）fixation.Under the conditions of neutral alkaline water quality,appropriate concentration of Cl^-and cation,As（Ⅲ）and As（Ⅴ）had relatively high accumulation and stability.It is conducive to the fixation of arsenic in the DWDS.