Research On The Assembly And Properties Of Hybrid Adsorbents Derived From Humic Substance And Layered Double Hydroxides

As an adverse impact of industry development,various pollutants have been found in wastewater in China.Adsorption is an effective and economical way to remove such pollutants from wastewater;therefore,it is of significance to synthesize new type of adsorbent,especially for the wastewater contaminated by complex pollutants.In this study,two kinds of hybrid adsorbents derived from humic substance and layered double hydroxide have been synthesized.Based on the coagulation function of positively charged LDH crystal nucleus and flocculation function of macromolecular FA anion.A multifunctional hybrid adsorbent,fulvic acid anchored layered double hydroxide?LDH-FA?has been prepared by a very simple co-precipitation method.Besides,according to the non-biological catalytic humification theory,a process started with low molecular precursors followed by in-situ humification to prepare humic-like substances?HLS?loaded LDH nanocomposite?LDH-HLS?has been also proposed in this study.As for the the hybrid adsorbent LDH-FA,the results show that the large hydrodynamic radius of FA does not favour its intercalation into LDH and the exchangeable Cl^-is retained as well as the anion exchange capacity.Therefore,this novel hybrid adsorbent possesses the ability for anion exchange and toxic metal complexation.The adsorption capacity of Orange II can reach 1.9 mmol/g,which is almost as much as stoichiometric anion exchange capacity of pure LDH.pH and ionic strength do not influence the adsorption of Orange II.Moreover,the hybrid adsorbent's adsorption capacity of heavy metals which are also the stable isotopes of radionuclides ⁶⁴Cu?²¹⁰Pb?⁶³Ni and ¹⁰⁹Cd,can get to maximum 2.5 mmol/g with an adsorption preference order of Cu²⁺>Pb²⁺>Ni²⁺>Cd²⁺.In addition,Orange II and toxic metals are able to be simultaneously removed by this composite adsorbent,and the adsorption of heavy metals can be enhanced by the synergetic adsorption of Orange II.The removal ratio of these two pollutants can respectively reach 99%and above.With regard to LDH-HLS,the results show that after decomposition of anion exchange product?LDH-Fe?C₆H₄O₇?^-?at 550?C for 1 h and rehydration in 0.015 M HNO₃-30 wt%NaNO3 mixture aqueous solution,lamellar structure is successfully recovered with Fe3O4and NO3^-remaining in the interlayer gallery of LDH.The self-assembly process of hybrid adsorbent includes four steps,which are diffusion,in-situ humification by the catalysis of Fe3O4 and loading,humification in solution,humification on the solid and loading.During the humification reaction,catechol as the main consumption reagent,experiences the oxidation and ring cleavage reaction,followed by polycondensation reaction to form the macromolecular HLS.The HLS generated from different conditions such as different organic loading rate and pH,possess the similar feature.The molecular weight is around1000-2000 g/mol and the content of acidic functional groups can reach to approximate8.5 mmol/g.The loading amount of HLS increases as the organic loading rate increases,while the humification productivity decreases.Although alkalic pH benefits the polycondensation reaction,it will reduce the loading amount of HLS,which is not good to LDH-HLS.After self-assembly,this adsorbent retains the lamellar structure and is negatively charged in the pH range of 2-11.This process can be employed to treat the phenolic wastewater and meanwhile produce hybrid adsorbent for heavy metal.The adsorption of heavy metals on LDH-HLS follow the monomolecular layer adsorption model such as Langmuir and Freundlich isotherm models.And the adsorption capacity can get to maximum 1.41 mmol/g with an adsorption preference order of Pb²⁺??slightly larger than?Cu²⁺>Ni²⁺?Cd²⁺.Competitive adsorption and increasing pH can prompt the adsorption of heavy metals.