Study On The Performance Of Water Treatment Residuals Modified Granular Activated Carbon To Remove Nitrate

Nitrate pollution poses a serious threat to the quality of drinking water and promote eutrophication of water bodies.Ion exchange,adsorption,reverse osmosis,electrodialysis,catalytic denitrification and biological denitrification can remove nitrate in water.Among of the methods,the adsorption is simpler,more convenient and easier to operate.Activated carbon has huge specific surface area and tremendous pore structure,and the surface has a variety of functional groups,so it is a widely used adsorbent.Due to the non-selective characteristics of activated carbon adsorption,metal modification is carried on the surface of activated carbon to improve the adsorption capacity of activated carbon to nitrate.The water treatment residual（WTR）in water treatment plants is rich in iron and aluminum elements.Therefore,this study used the metals extracted with HCl from WTR to modify the granular activated carbon（GAC）and enhance its adsorption capacity on nitrate.Single metal loading modification was used to compare the adsorption capacity of GAC modified by different metal ions on nitrate.The optimal modification conditions of GAC were explored.At the same time,XRF,BET,FTIR and Zeta potential analysis methods were used to explore the microscopic changes before and after the modification of GAC,and the mechanisms of modified GACs’adsorption on nitrate were analyzed,the influence factors of nitrate adsorption on modified GAC,the kinetic and thermodynamic model were examined.The optimization test of modification conditions showed that,the lower the pH of the WTR solutionis,the better the dissolving effect of metal ions in the WTR is.The HCl with 400ml of 1+9 was diluted to 1L to dissolve 100g of WTR in the test.The acid-dissolved WTR solution was hold for 3d,and then the supernatant was used as a modified solvent,it can reduce the influence of impurities in the WTR on the modification effect.The optimal modification conditions for the preparation of modified GAC are:a 30mL volume of WTR solution,diluting with deionized water to 100mL and then modifing 10.0g of GAC.The modification solution does not need to adjust the pH again,and the modification time is 12h.The ZnCl₂ solution is used to modify the GAC with a single metal loading.The optimal modification conditions are:modification with a ZnCl₂solution which has a concentration of 2.0g/L,and the modified solid-liquid ratio is1g:10ml.XRF analysis proved that the modified GAC with ZnCl₂ is loaded with a single metal Zn.The WTR modified GAC is loaded with various metals,mainly Al and Fe.BET analysis shows that the modified GAC specific surface area is reduced since the metal ions are loaded in the modified GAC pores.Zeta potential analysis shows that the adsorption sites on the acidic oxygen-containing functional groups on the surface of the GAC were occupied by metal ions,it can lead to an increase in the zero charge point of the modified GAC since the reduction of the acidic oxygen-containing functional groups.The zero charge points of GAC,ZnCl₂ modified and WTR modified GAC are 5.4,5.5 and7.4,respectively.FTIR analysis shows that metal ions were loaded on the surface of GAC after the modification of GAC.The GAC and the modified GAC adsorbed nitrate can be physically adsorbed.The experiments of the effect of initial nitrate concentration and temperature on adsorption shows that when the concentration of nitrate is low,the adsorption amount of modified GAC to nitrate increases rapidly with the increase of concentration,and the temperature only has little effect.As the concentration increases,the rate of the growth of the adsorption capacity is gradually slow and there may be even a decrease in the adsorption capacity at high concentrations.At this time,the temperature has a greater influence on modified GAC.The test of the influence of pH on adsorption shows that under acidic condition,a large amount of H⁺neutralizes the negative charge on the surface of the GAC,so the net positive charge account for a higher proportion on the surface.the GAC and the GAC carbon have a better adsorption effect on nitrate.In an alkaline environment,excess OH^-will form a competitive adsorption with NO^3-,which will hinder the adsorption of nitrate ions by GAC.The test of the influence of modified GAC on the adsorption of ammonia nitrogen,nitrate nitrogen and phosphorus shows that PO₄^3--P,which also a anion as NO^3--N,will compete for adsorption.GAC modified by WTR has almost no adsorption to NH⁴⁺-N,while GAC modified by ZnCl₂ has adsorption to NH⁴⁺-N.Kinetics of adsorption experiments show that the adsorption rate of ZnCl₂ modified GAC is greater in the early stage than the GAC modified by WTR,and the adsorption equilibrium can be reached in a short time.The fitting effect of these two modified GAC using quasi-second-order kinetic equation has a good results,and the correlation coefficient R² are both above 0.99.When the nitrate concentration is low,the fitting correlation coefficient R² can also be above 0.9 using the quasi-first order kinetic equation.The fitting results of Weber-Morris intragranular diffusion model shows that internal diffusion adsorption is not the controlling step of the whole adsorption process,and the adsorption of modified GAC to nitrate mainly occurs in the liquid filmdiffusion stage.Moreover,the fitting results of the Bangham channel diffusion model also indicate that the channel diffusion adsorption is not the main adsorption process.Isothermal adsorption experiments show that the Langmuir isotherm adsorption model has the best fitting effect on the modified GAC adsorption process,and the correlation coefficients are all above0.99.The correlation coefficients of the Freundlich and Temkin isotherm adsorption models are around 0.90.The Langmuir theoretical saturated adsorption capacity of modified GAC by WTR and ZnCl₂ are 2.75mg/g and 2.15mg/g,respectively.According to the Langmuir isotherm adsorption model,the adsorption of nitrate by modified GAC is surface monolayer adsorption,and a positive value of k₁ indicates that the adsorption is a spontaneous reaction.Adsorption thermodynamic analysis shows that the adsorptions of nitrate by both two kinds of modified GAC have spontaneous characteristics,and entropy increases and exothermic,what’s more,physical adsorption is the main adsorption mechanism.