Adsorption Of Thiocyanate And Fluorine On Calcined Mg/Al Layered Double Hydroxides

Thiocyanate (SCN-) and fluorine (F-) as a (similar) halogen anion exist inthe biological treatment of coking wastewater, causing a large chroma, solublesolid, etc. Therefore, the purification and treatment of SCN-and F-wastewaterhas attracted people’s attention, and adsorption is an effective way to deal withthis kind of wastewater.Layered double hyclroxides(LDH) is a efficient, reusable, safe andeconomic adsorbent. In this paper, Mg/Al layered double hydroxide (LDH) wassynthesized by using Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH and Na2CO3with co-precipitation method. The LDH was calcined at certain temperature toobtain calcined layered double hydroxides (CLDH). As a adsorbent CLDH wasused to study the influencing factors (Mg/Al molar ratio, calcinationtemperature,contact time, pH, initial concentration of SCN-and F-, adsorbentdosage) of individual adsorption of SCN-and F-. On the condition,thecompetitive adsorption characteristics and mechanisms of SCN-and F-on CLDH were studied in coexistence system. Besides, the feasibility of CLDH inbiological wastewater of coking plant was investigated. The experimental resultsare as follows:(1)Mg/Al molar ratio(2.0,3.0,4.0,5.0) of calcined Mg/A1layered doublehydroxides was judged through SCN-removal effect, which found that theremoval rate of SCN-reached the maximum91.94%at Mg/A1molar ratio4.0and contact time6h.(2)The calcination temperature was a important factor to affect SCN-removal,which found that the removal rate of SCN-reached the maximum91.94%at calcination temperature500℃and contact time6h.(3)It is found that in the single adsorption of SCN-and F-, the adsorptioncapacity of CLDH was related to adsorbent dosage, pH, contact time, initialconcentration of SCN-and F-. In addition, the kinetic studies showed that thepseudo second-order model could describe the individual adsorption of SCN-and F-on CLDH, and the adsorption process is coordinated various mechanismincluded diffusion, intra particle diffusion and other external exchange ofsubstance or chemical reaction. The SCN-and F-adsorption isotherms wasconsistent with the Langmuir isotherm which is single molecule layer adsorptionand the maximum adsorption capacities at40℃is284mg/g and62.46mg/g,respectively. Finally the adsorption process is endothermic, spontaneous,andphysical adsorption process, proved by the adsorption thermodynamics analysis.(4)It is found that the adsorption capacities of SCN-and F-in coexistence system on CLDH were lower than in single system, and the adsorption kineticscould fit pseudo-second-order kinetic, although the equilibrium time is extended.The SCN-and F-adsorption isotherms was consistent with the Freundlichisotherm and Langmuir isotherm, respectively. Meanwhile, the effect of addingorder of SCN-and F-on adsorption capacity are investigated, it is found that thesystem of SCN-adsorption after F-and simultaneous adsorption system have thesimilar results, however, the system of F-adsorption after SCN-was lower thansimultaneous adsorption system,which demonstrated that SCN-have strongbonding force with CLDH than F-and F-is easy to occupy the adsorption sitethan SCN-.(5)The LDH were analyzed by Inductively coupled plasma atomic emissionspectrometry and BET,which showed Mg/Al molar ratio4.0of calcined Mg/A1layered double has big surface area of174.19m2/g. The LDH before and afteradsorption of SCN-and F-were characterized by FTIR and XRD, showed thatLDH has a characteristic structure of CO2-3intercalated hydrotalcite, and CLDHexhibit the disappearance of the peaks of hydrotalcite, and the structure of LDHcollapsed. The FTIR and BET spectra also exhibited that SCN-and F-intercalated into the interlayer space of LDH and CLDH could rebuildedoriginal layered structure by adsorbing SCN-and F-from aqueous solution dueto ion exchange,“memory effect”, the electrostatic attraction force, etc.(6) The CLDH secondary regenerative rate can reach82%using0.5mol/LNa2CO3+NaOH (1:1), proved that CLDH a reusable, efficient adsorbent. (7) To deal with biological wastewater of coking plant using CLDH atcontact time8h,the removal rate of SCN-, F-, chroma, and SO2-4reached70.85%,81.41%,73.91%,75%,respectively, proved CLDH has practicalapplication value.