Study On The Efficiency And Mechanism To Reduce Of Cadmium And Arsenic Pollutants During Boiling Drinking Water

Chinese residents have a drinking habit of boiled water since ancient times.At first,it was believed that boiled water can only reduce the risk of water-borne diseases by inactivating pathogenic microorganisms in the water.In recent years,people have found that drinking boiled water can simultaneously reduce the concentration of disinfection by-products,sulphonates and other pollutants,and improve the quality of drinking water.Heavy metal is a common pollutant when drinking water facing polluted,especially when the water is polluted by heavy metal,even low-dose will pose a great threat to residents’health without effectively treatment.In this paper,from the perspective of reducing the cadmium and arsenic contaminants in drinking water,the simple treatment of boiling drinking water is explored to reduce the concentration of cadmium and arsenic in water,and try to strengthen the effect of pollutant reduction by extra brewing tea,repeatedly boiling and cooling,so as to provide scientific guidance for residents to drink healthy water.This paper takes cadmium and arsenic ions as examples to carry out the following three parts of research:（1）Establish a mathematical model for reducing the concentration of residual pollutants in the solution after boiling and cooling operation with alkalinity,hardness,and initial contaminant concentration as the influencing factors,find out the main influencing factors of contaminant reduction,in order to predict the effect of the operation on the improvement of water quality under specific water quality conditions;（2）Explore the effect of boiling and cooling operation on the reduction of pollutant concentration under different water quality conditions,and the reduction effect of pollutants was enhanced by brewing tea or repeatedly boiling and cooling;（3）The reduction mechanism of pollutants is explored by calculating thermodynamic parameters and characterizing crystals.The study draws the following conclusions:（1）Through three-dimensional response surface and variance analysis,it is found that for the cadmium concentration reduction effect is the reaction of cadmium ions with carbonate ions in water to produce Cd CO₃ insoluble matter,so that free cadmium ions are immobilized in carbonate.Arsenic reduction is mainly achieved by precipitation reaction with calcium ions or co-sedimentation in the process of scale crystallization.（2）The reduction model of cadmium and arsenic pollutants is obtained from the three-dimensional response surface experiment,which is expressed in the algebraic sum of quadratic polynomial.The reliability of the mathematical model of pollutant reduction is verified by reducing the concentration of the actual pollutants in the water and testing the fitting results.After verification,it is found that the model fitting is relatively accurate and can be used to estimate the concentration of residual pollutants after boiling treatment under specific water index.（3）Evaluated the impact of the coexistence of four anions,four cations,and humic acid in the case of boiling and cooling operation treatment.Found that the coexistence of anions and cations concentration in the normal range of drinking water,cadmium,arsenic reduction effect is not significant,higher concentrations of SO₄^2-ions,Zn²⁺ions,Cu²⁺ions coexistence will inhibit the reduction effect of cadmium,Cl^-ions,SO₄^2-ions,Zn²⁺ions or high concentrations of CO₃^2-ions coexistence will inhibit the reduction effect of arsenic.When humic acid exists,it will seriously inhibit the reduction effect of cadmium and arsenic elements in drinking water.Solution in the cooling stage after boiling,at 100℃-70℃found that cadmium,arsenic contaminants have a significant reduction effect.（4）Repeatedly boiling and cooling can further reduce the concentration of pollutants by strengthening the heterogeneous crystallization in the solution.15μg/L cadmium contaminated drinking water after three times treatment,the remaining cadmium concentration is 3.35μg/L,which has met the requirements of China’s standards for drinking water quality;20μg/L arsenic contaminated drinking water after three times treatment,the remaining arsenic concentration is 9.17μg/L,which also has met the requirements of China’s standards for drinking water quality.Recommended repeatedly boiling and cooling treatment for 3 to 4 times,cadmium and arsenic both have a significant immobilization effect.（5）The drinking habit of brewing tea can strengthen the reduction effect of cadmium,but can not strengthen the effect of arsenic.This is due to tea polyphenols,organic acids,proteins and other substances,ionized to make the solution negatively charged,can be chelated with positively charged cadmium ions.When the amount of tea brewed is 4 g/L,the remaining cadmium ion concentration can reduce from 15μg/L reduced to 4.03μg/L.However,the electrostatic repulsion between the tea extract and arsenic can not strengthen the arsenic reduction effect.（6）The chemical reactions that may occur in the solution system are preliminarily analyzed,and the thermodynamic parameters of each reaction are calculated by using the PHEEQC software to deduce the pollutant reduction path.After characterizing the water scale crystals,the evidence that cadmium and arsenic pollutants are stabilized in the scale of drinking water has been found.Finally,the reduction mechanism of cadmium and arsenic pollutants is determined.