Effect Of Organic Matter On The Adsorption Process Of Arsenic On Sediment And Its Main Components

Groundwater contaminated by arsenic usually contains high concentrations of natural organic matter（NOM）,in which humic acid can inhibit the adsorption of arsenic by sediments,mainly through the formation of negatively charged groups to occupy the surface sites of the adsorbent or the formation of As-NOM complexes.The samples of groundwater with high arsenic in the Kuitun River Basin of Xinjiang were collected and analyzed,and the main components of sediment were prepared to study the adsorption properties of arsenic.Five kinds of low molecular weight organic acids（LMWOAs）were selected to study their effects on arsenic adsorption by goethite;The adsorption mechanism was explored by fitting the data with kinetic model and adsorption isothermal model;Infrared spectroscopy and Zeta potential were used to characterize the surface of goethite to verify the conclusion.According to the main structure and functional groups of humic acid,19 kinds of organic matters,such as macromolecular organic acids,natural polysaccharides,alcohols,esters,ethers,amines and sulfonic acids,were selected to study their effects on arsenic adsorption.PHREEQC calculation model was used to analyze the experimental results of arsenic adsorption,and the results were as follows:（1）The groundwater environment in the Kuitun River Basin is neutral to alkaline,belonging to reduction-weak alkaline and high arsenic groundwater（I-2 type）.The main minerals in the sediments are goethite,aluminum phosphate,quartz sand and calcium carbonate.Arsenic adsorption results show that goethite has a good adsorption effect on arsenic,and in acidic and neutral environment（p H 3～7）is conducive to the adsorption of arsenic.（2）LMWOAs can inhibit the adsorption of goethite to arsenic.When the initial arsenic concentration was 2 mg·L^-1,the adsorption of arsenic on goethite in the presence of LMWOAs followed the pseudo-second-order kinetics and Freundlich adsorption model.FT-IR spectra showed that new absorption peaks appeared on the surface of goethite after adding LMWOAs,indicating that the functional groups（such as carboxyl）of organic acids occupied the adsorption sites on the surface of goethite.The results of Zeta potential show that the surface p H_PZC of goethite decreases with the addition of citric acid,and an inner sphere complex with negative charge is formed.（3）The experimental results of the influence of organic matter on arsenic adsorption show that p-benzoquinone and other organic matter have inhibitory effect on arsenic adsorption.In the system of diethylamine and other organic matter and arsenic coexisting,the arsenic adsorption followed the pseudo-first order kinetics,and in the system of methanol and organic matter and arsenic coexisting,the arsenic adsorption followed the pseudo-second order kinetics.In benzoic acid-arsenic system and gallic acid-arsenic system,the adsorption process was in line with the Langmuir isothermal adsorption model（R² was0.95 and 0.94,respectively）.In the fructose-arsenic system,the adsorption process was in accordance with the Freundlich isotherm model（R² was 0.96）.The FT-IR spectra showed that gallic acid,salicylic acid,tannic acid and nitrobenzene were adsorbed on the surface of goethite and competed with arsenic for active adsorption sites.Zeta potential measurements show that arsenic forms negatively charged complexes on the surface of goethite with gallic acid,salicylic acid and tannic acid,respectively.（4）The PHREEQC simulation results of arsenic adsorption behavior under acidic and neutral conditions are consistent with the measured results,indicating that the model has good accuracy.The calculation results showed that the occurrence forms of As in the solution under different p H conditions were mainly HAs O₄^2-,H₂As O₄^-,As O₄^3-and H₃As O₄.Among them,with the increase of p H,H₃As O₄ content decreased rapidly,H₂As O₄^-decreased gradually,As O₄^3-increased rapidly,and HAs O₄^2-increased gradually.The change of arsenic speciation distribution is consistent with the change trend of arsenic adsorption capacity and surface charge of goethite under different p H conditions.