Study Of Defluoridation Performance And Mechanism Of Aluminum (Hydro) Oxides In Aqueous Solution

Fluorine pollution could make serious damage to the teeth,bones and even nerve of human,so the research for removal of fluoride from aqueous solution has become an important task nowadays.Aluminum(hydro)oxides,such as α-Al2O3,γ-Al2O3,AAH(amorphous aluminum hydroxide)and γ-AlOOH are important fluoride adsorbents.They attract high attention in the field of fluoride adsorbent research.Recently,the relevant researches mainly focus on improving the performance of aluminum(hydro)oxides,while few study has been reported in the mechanism and stability of defluorination in complex solution system.Fluoride can lead to the collapse of the aluminum(hydro)oxide,thus the above two aspects directly affects the application of aluminum(hydro)oxide in defluoridation.In this study,α-Al2O3,γ-Al2O3,AAH(amorphous aluminum hydroxide)and γ-AlOOH were chose to systematically investigate the effects of coexisting ions,surface chemistry,adsorption sites,as well as interaction mechanism involved in the adsorption and the Al3+ release process.In addition,adsorption behavior and mechanism of fluoride on the typical aluminum(hydro)oxides were revealed.Adsorption performance of the aluminum(hydro)oxides and aluminum-containing minerals were compared in simulated fluoride-polluted groundwater.Further,chitosan modified activated alumina was synthesized and was used as an absorbent to remove fluoride from aqueous solution.Detail results are as follows:1.Fluoride adsorption performance of common aluminium(hydro)oxides and the effect of coexisting anions were studied.The initial concentration of F-and coexistence ions were changed to study the effects of F-concentration,pH value,HCO3-,HPO42-and humic acid on the removal of F-,and the interactions between these factors.AAH showed the greatest performance for the removal of F-,followed by γ-Al2O3,α-Al2O3,γ-AlOOH.The increase of pH and coexisting anions concentration decreased the adsorption of F-on α-Al2O3、γ-Al2O3.Among these ions,HPO42-dramatically reduced the adsorption capacity of α-Al2O3,and γ-Al2O3 was severely affected by HCO3-.HCO3-showed slight positive effect on fluoride adsorption on AAH,while no significant effect of pH value was observed in this system.Other anions enhanced the fluoride adsorption capacity of AAH.For γ-AlOOH,humic acid and acid conditions presented positive influence on fluoride adsorption capacity while both HCO3-and HPO42-showed negative influence on fluoride adsorption capacity.2.The changes of chemical composition in adsorption process of adsorption on aluminium(hydro)oxides and the Al3+ release mechanism were studied.Different initial pH values and initial concentrations of F-,coexisting HPO42-,HCO3-,humic acid were set to explore the changes of chemical composition in different fluoride adsosrption system.Interactions between aluminum(hydro)oxides and the solution composition were also studied.It could be concluded that the concentration of OH-was increased in both a-Al2O3 and Y-Al2O3 system,and the OH-increment of γ-Al2O3 was higher than a-Al2O3.The decrease of OH-was observed in the system of AAH and Y-AlOOH,and the decrement of AAH was higher than γ-AIOOH.Increasing pH value of solution could decrease the release of Al3+ on the surface of aluminum(hydro)oxides.The released Al3+ amounts was dececresd in the order of AAH>y-Al2O3>α-Al2O3>y-A1OOH,which was consistent with the order of F-adsorption.The effects of HPO42-,HCO3-and humic acid on OH-and Al3+ variation were investigated.The increase in the concentration of HCO3" and HPO42-could promote the release of Al3+ on γ-A12O3,while the negative effects could be seen in other three systems.With the increasing of humic acid,the release of Al3+ in AAH system was enhanced while the release of A13+was reduced in other three systems.3.F-adsorption mechanism on different aluminum(hydro)oxides active adsorption sites were studied.Per unit mass of adsorbent fluorine adsorption capacity was redueced in the order of AAH>γ-Al2O3>α-Al2O3>γ-AlOOH.Per unit specific surface area was redueced in the order of AAH>γ-AlOOH>a-Al2O3>γ-Al2O3.Zeta potentials of aluminum(hydro)oxide after fluoride adsorption were reduced in different extent.Variation of pHIEP before and after fluoride adsorption was redueced in the order of γ-AlOOH>γ-Al2O3>AAH>a-Al2O3.There are four kinds of F element binding state on the surface of a-Al2O3 and γ-Al2O3 and three kinds of F element binding state on the surface of AAH and y-AlOOH.The adsorption affinity of aluminum(hydro)oxides to F-was redueced in the order of a-Al2O3>γ-Al2O3>AAH>γ-AlOOH.Adsorption sites of F-on aluminum(hydro)oxides are:surface Al-0 group of a-Al2O3 and AAH;surface of Al-OH and Al-0 group of γ-Al2O3;surface-Al(OH)=O group of y-AlOOH.HCO3" competes with F-on the same surface adsorption sites of a-Al2O3,γ-AlOOH,but no obvious influence was showed on the adsorption sites ofγ-Al2O3.Surface active sites for HCO3-and F-on AAH are independent.HPO42-competes with F-on the surface adsorption sites of a-Al2O3 and Y-Al2O3.Surface active sites for HPO42-and F" on AAH and y-AlOOH are independent.Humic acid competes with F-on the surface adsorption sites of a-Al2O3 and Y-Al2O3.Surface active sites for humic acid and F" on AAH and y-AIOOH are independent.F-adsorption had no significant effect on the bulk structure of aluminum(hydro)oxides.4.Fluoride adsorption capacity and solution content change of different aluminium(hydro)oxided and common alumina-contained minerals was studied in simulated fluoride-polluted the groundwater(SGW).a-Al2O3,y-Al2O3,AAH,y-AlOOH,rectorite,montmorillonite and kaolinite showed high affinity to Ca2+ and Mg2+,but only the α-Al2O3,γ-Al2O3,AAH,γ-AlOOH showed adsorption affinity to ainions.The clay minerals showed no obvious adsorption for anions.The ions adsorption capacity of various materials in SGW are as follows.F-adsorption capacity was redueced in the order of in the order of AAH>γ-Al2O3>a-A12O3>γ-AlOOH>kaolinite>rectorite>montmorillonite.The adsorption processes were accompanied by the release of Al3-and increase of H+ concentration.Al3+release concentration was redueced in the order of y-Al2O3>kaolinite>a-Al2O3=AAH>Y-AlOOH=rectorite>montmorillonite The increase amount of H+ concentration was redueced in the order of AAH>rectorite>montmorillonite>kaolinite>γ-AlOOH>α-Al2O3>γ-Al1O3-5.Removal performance and adsorption mechanism of fluoride adsorption on chitosan modified activated alumina(AACB)were studied.The optimum pH range and the removal capacity for F-of AACB were higher than that of activated alumina and chitosan modified adsorbents.Under the natural pH of groundwater,the adsorption quantity of F-achieved 6.70 mg/g,higher than that of AA(2.56 mg/g)and chitosan(CHI,0.37 mg/g).The adsorption process of F-on AACB was a spontaneous endothermic reaction.CO32-and PO43-were more likely to compete with F-for the reaction sites on the solid-liquid interface of AACB,which resulted in the decrease of F-adsorption.CI-,HCO3-,SO42-and N03-had no obvious effects on the adsorption of F-.The adsorption of F-on AACB fit Pseudo-second order,Elovich and inparticle diffusion models well.The adsorption of F-on the surface of AACB was nmultilayer adsorption and a spontaneous endothermic process.The adsorption of F-on AACB was higher than that of AA and CHI.AACB could provide-NH2 and Al-O groups for the adsorption of F-.Complex fluoride adsorption eftfects were observed on-NH2 and Al-O group.2 g/L AACB could reduce the initial concentration of F-to 4.52 mg/L to 0.57 mg/L,which meet the requirement of drinking water,and verified to be better than other chitosan modified materials.