Effect And Mechanism Of Humic Acid On The Structure Memory Effect Of LDHs On Adsorption Of Boric Acid

Layered double hydroxides are a series of artificially synthesized clay minerals with layered structure,which have good anionic adsorption effects due to its structure memory effect.The calcined layered double hydroxides?CLDHs?can be a cheap and efficient adsorbent for borate,while the effect of natural humic acid?HA?cannot be ignored in treating waste water or seawater desalination contain borate.HA widely distributed in the soil and nature water as an important natural ligand and carrier.It could affect the adsorption of borate on CLDHs through changing the structural memory effect,surface properties of minerals,migration of metals and pollutants.However,the influence of humic acid on the structural memory effect and adsorption of boric acid by CLDHs have not been studied in previous studies.The mechanism found out in this work is not only beneficial to apply CLDHs as commerical adsorbents,but also help to clear the interaction between natural organic matter and clay materials.In this work,the natural humic acid HA-B and HA-S were extracted from black soil and sedimentary soil,respectively.The elemental analysis,FTIR and ¹³C-NMR were tested to determine their main groups and the relative contents of them.Then,the hydration and structure regeneration processes of Mg-CLDH,Zn-CLDH and Ca-CLDH in HA-B and HA-S solutions were studied,and the effect of HA on the structural memory effect of LDH was revealed:?1?The dissolution of metal ions on Mg-CLDH were accelerated due to HAs in the solution,thereby facilitating the transformation from Mg-CLDH to Mg-LDH with layered structure,which were more evident in the HA-B than HA-S solution.And the HA adsorbed on the surface adjusted the growth of regenerated Mg-LDH crystal,causing rough and irregular flaky grain with small grain size.?2?In the HAs solution,the surface part of Zn-CLDH particles hydrated then transformed to Zn-LDH rapidly,while HAs were rapidly adsorbed on the surface.Then HA and regenerated Zn-LDH on the surface of the particles hinder the hydration and regeneration of the internal Zn-CLDH.In addition,the transformation rate from Zn-CLDH to Zn-LDH in HA-S solution is lower than that in HA-B solution,and the rate is lower in higher concentrations of both HA-B and HA-S solutions.?3?Ca-CLDH could adsorb HAs and transform into Ca-LDH in HAs solution rapidly.But the interlayer spacing of regenerated Ca-LDH was affected by HA,which was about 8.0?after reaction in 25mg/L HA-B solution for 48 h,while it was about 7.5?in the HA-S solution under the same condition.In addition,the morphology of new generated Ca-LDH was different from that in the deionized water due to HA,and content of CaCO₃ was lower in HAs solution.In the system with the coexistence of HA,B?OH?₃ and CLDH,it was found that HA effected the adsorption of B?OH?₃ by CLDHs containing different divalent metal ions.The results of a series of experiments were analyzed and discussed reasonably,and the conclusions were drawn as following:?1?Due to the complexation between B?OH?₃ and HA before the regeneration of Mg-CLDH,most of active sites for metal ions originally on HA are occupied by B?OH?₃,leading to the poor transformation rate of Mg-CLDH.On the other hand,B?OH?₃ was absorbed quickly at the intial stage of raction by Mg-CLDH since the complexation between B?OH?₃ and HA.The adsorption of HA reduces the crystallinity of the regenerated Mg-LDH,and the morphology of newly LDH becomes rough and irregular so that the adsorption capacity decreased.?2?With the complex compounds of HA and B?OH?₃ adsorbed on the surface of Zn-CLDH with a the spherical morphology,the rate of hydration and structure regeneration of Zn-CLDH insided became slow and less Zn-CLDH transformed into Zn-LDH.In addition,since the inhibition of HA adsorbed on the surface of new regenerated Zn-LDH,the adsorption capacity of B?OH?₃ decreased.?3?In the HA+B?OH?₃ solution,the existence of HA inhibits the reaction between Ca-CLDH with B?OH?₄^-for forming Ca₆[Al?OH?₆]₂[B?OH?₄]₆,so B?OH?₄^-could not immobilized by Ca-CLDH.Meanwhile,the content of CaCO₃ in Ca-LDH regenerated for 48h in the HA+B?OH?₃ solution was lower than both HAs solutions.