| Groundwater pollution has become one of the global environmental problems.Nitrate and hexavalent chromium are two important pollutants in groundwater pollution problems.As an important part of chemical fertilizers,nitrate is widely used in agricultural production.With the development of intensive agriculture,excessive fertilizer is added to farmland,resulting in a large part of nitrate not being absorbed by crops,and then with irrigation and precipitation.Infiltrated into the ground and pollutes the groundwater.Chromite and chromium salts are important industrial raw materials.They are widely used in the metal processing industry,the leather industry and the printing and dyeing industry.However,it is difficult to get the hexavalent chromium waste.The treatment,long-term accumulation in the open air environment,hexavalent chromium will infiltrate into the ground with precipitation and various human activities,and seriously pollute the groundwater.Drinking water containing excessive levels of nitrate can cause methemoglobinemia.In severe cases,suffocation or even death will occur.Hexavalent chromium,as a highly toxic substance,can cause a variety of cancers.The presence of two pollutants in groundwater is for people.The safety of life poses a great threat.In this context,the organic modification of sodium bentonite is carried out by using the adsorption method with cetyltrimethylammonium bromide as a modifier.The modified bentonite adsorbed nitrate and hexavalent chromium in water,and explored the removal effect of pollutants and the adsorption mechanism of organic modified bentonite from macroscopic and microscopic perspectives.The cation exchange capacity of bentonite was determined by ammonium chloride-50%ethanol method.According to the determination results,the bentonite was modified to different degrees by using cetyltrimethylammonium bromide,and the nitrate was simulated in the laboratory.Groundwater contaminated with chromium is adsorbed by organically modified bentonite,and the effects of various variables on the adsorption effect are affected.Through the single factor experiment,it was found that when the organic modification amount of bentonite was 0.8CEC,the reaction temperature was 5?,and the pH was 6.5,the removal effect of nitrate and hexavalent chromium was the best.As the initial concentration of water sample increased,The removal effect is reduced.According to the processing parameters,the adsorption amount,adsorption time and adsorbent regeneration were investigated.It was found that the dosage increased from 5g/L to 30g/L,and the removal rate always showed an upward trend.As the dosage increases,the rate of removal increases.In terms of adsorption time,the adsorption rate in the first 5 minutes is very fast,and the removal rate of hexavalent chromium is increased from 0 to 52.33%.The adsorption equilibrium was from 5 min to 540 min.At this stage,the adsorption rate slowed down and the removal rate increased from 52.33%to 69.13%.Regarding the regeneration of the adsorbent,it was found that the regeneration effect was the best with 1.0 mol/L hydrochloric acid,and the optimum number of regenerations was three.In the process of adsorption mechanism research,mathematical models such as adsorption kinetics,adsorption isotherm,adsorption thermodynamics,and scanning electron microscopy-EDS analysis,X-ray diffraction,Fourier infrared spectroscopy and other characterization methods were used.The adsorption kinetics of nitrate and hexavalent chromium by organically modified bentonite accorded well with quasi-secondary kinetics,and the adsorption isotherms were consistent with Langmuir adsorption isotherm.In the adsorption thermodynamics experiment,the reaction of organic modified bentonite on the adsorption process of nitrate and hexavalent chromium is ?H<0,the entropy change ?S<0,and the Gibbs free energy AG>0 in the adsorption process,indicating the adsorption process of the two substances.Both are exothermic reactions,and the adsorption system proceeds in the direction of reduced chaos.Both adsorption reactions are non-spontaneous reactions. |
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