Study On Cd(?) Adsorption Mechanism From Micro-polluted Water For Irrigation By Modified Zeolites

With the rapid growth of population and the development of industrialization and economy,Heavy metal pollution is one of the most serious environmental pollutants,which is also a major global environmental problems.Water is the main channel of heavy metal pollutants diffusion.most of the rivers in the world have been polluted by heavy metal in varying degree,besides the rivers in China.Most of the rivers are burdened with the mission of farmland irrigation,The heavy metal cadmium ions in water easily transfer into vegetables and crops through the soil,and indirectly cause human and animal poisoning.Therefore,some simple,economic and feasible wastewater treatment technologies and materials would be choosen to remove effectively toxic heavy metal pollutant and meet the requirements of farmland irrigation water quality according to tank farm soil and the planting of crops.It is of great significance for the agricultural safety use for the unconventional water resources.In this paper,a typical micro pollution irrigation water containing low concentration of heavymetal cadmium ions was used as water source,The natural zeolites produced in the region of gongyi In Henan province were selected as raw materials.The adsorption performance of natural zeolite were enhanced by acidification heating and chitosan modified respectively natural zeolite successfully.Two kinds of modified zeolite have great application value by column of static test and dynamic adsorption experiments to remove low concentration of cadmium ion respectively and theoretical analysis of the adsorption mechanism.The main results of this paper are as follows:?1?Two kinds of modified zeolite by acidification heating and chitosan modified respectively natural zeolite successfully were applied to remove the micro pollution irrigation water containing low concentration of heavy metal cadmium ions.The results showed that,AZ and CNZ were prepared successful ly and the bes modification condition was obtained in this paper.The best modification conditions of activated zeolite was that natral zeolite was soaked by 5%of dilute hydrochloric acid more than 2 h,then roasted for 1 h in the high temperature 350?.The best modification conditions of natural zeolite loaded chitosan was that the quality ratio of chitosan with natural zeolite was 0.05,the solution concentration of chitosan was 1.0%.?2?In order to confirm the structure and performance of AZ and CNZ improved,The characterization of natural zeolite,actived zeolite and natural loaded chitosan were studied by analyses on BET method,SEM,XRD,FT-IR.Compared with natural zeolite by BET,the specific surface area of AZ is bigger than the natural zeolite,and the pore diameter of AZ also increased.And the average pore size and total pore volume of CNZ slightly decreased,which illustrated the chitosan has been load in natural zeolite,and filled in the channel part of the natural zeolite or covered natural zeolite.The surface of activated zeolite becomes uneven and its internal structure is relatively decentralized and loose with obvious holes by SEM.The surface of zeolite loaded chitosan was covered with finer powder small particulate matters.The crystal structure of modified zeolite didn't changed significantly and the X-ray diffraction spectra of CNZ had a diffraction absorption peak by XRD,the FT-IR pattern of zeolite loaded chitosan contains the virational band of-NH₂ and-OH,which is the further evidence of the presence of chitosan.?3?A series of experiments were done to discusses the additive amount of adsorbent,solution pH,temperature and adsorption time and solution of the influence of initial concentration on the adsorption efficiency using AZ and CNZ respectively adsorption of low concentration of cadmium ions.The test results showed that,the removal rate of activated zeolite was 95.1%or more in 25?,pH=6,dosing quantity 0.6 g/L,adsorption equilibrium time 180 min and 100?g/L of cadmium solution.the removal rate of CNZ was 94.51%or more at 25?,p H=6,dosing quantity 8 g/L,the adsorption equilibrium time 60 min and 100?g/L of cadmium solution.?4?The adsorption mechanism of the two kinds of modified zeolite were analyzed through adsorption isothermal model,adsorption kinetics and adsorption thermodynamic model.The analysis showed that,the adsorption mechanisms of AZ And CNZ on Cadmium?Cd????ions met Langmiur adsorption isothermal adsorption model.The absorption is in monomolecular type.The maximum adsorption capacity of cadmium ions was 149.25?g/g and 112.4?g/g at 25?,.The absorption kinetics process of AZ and CNZ is in lie with the Lagergren second-order rate.According to the calculation of thermodynamic,the adsorption process is a spontaneous endothermic reaction.The entropy increased in the adsorption process.?5?Under similar experimental conditions,the adsorption rate of CNZ is higher than AZ's and the dosing of AZ slightly less than CNZ's by contrast.Two kinds of modified zeolite have good adsorption performance.The adsorption performance of AZ and CNZ was better than others by comparison under similar experimental conditions.It shows that AZ and CNZ are good adsorbent materials,and they have broad application prospects in removing low concentration heavy metal cadmium ions in micro-polluted irrigation water.?6?According to the optimum adsorption conditions by the static adsorption experiment and irrigation mode,the intermittent operation mode was adopt to simulate the dynamic column adsorption.The adsorption medium types,water filtration velocity and the adsorption dose were discussed.The test results showed that,when these factors were considered such as meeting water quality standardand saving the investment cost,it was concluded that the best operating conditions:the adsorption medium could choose a single column of activated zeolite filter or mixed medium?AZ:CNZ=4:1?,the filter speed is 6 m/h,filter column height 500 mm,adsorption dose 30 g when the adsorption filtration column diameter was 30 mm,190 mm high,the adsorption medium particle size was 0.3⁰.45 mm,and the adsorption test was operated intermittently.The test results also showed that,the cadmium ions?100?g/L?were removed obviously by modified zeolite filter and its removal rate was about 99.5%or more.When they were filtered,the concentration of cadmium ions is 0.41?g/L,less than 0.01 mg/L,which was specified in the irrigation water quality standard?GB5084-2005?standards and which reached the requirement of irrigation water quality standards.