Study on preparation of adsorption material (nano FeS loaded on lignite) and its chromium removal performance

Acidic chromium contained wastewater discharged from mining and other processes has caused serious environmental pollution.Although nano FeS can rapidly reduce Cr(Ⅵ) to Cr(Ⅲ) and reduce the toxicity of chromium contained wastewater,it is easy to oxidize and agglomerate.Lignite has good adsorption performance,abundant reserves and stable structure.It can be used as a load to enhance the stability of nano FeS.Therefore,the composite adsorption material of nano FeS loaded on lignite (nFeS-L)were prepared by ultrasonic precipitation method,and its chromium removal performance was tested.The composite materials were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM).The effects of lignite size fraction,concentration of Na2S and Fe/S molar ratio on the removal efficiencies of Cr(Ⅵ) and total chromium were investigated by single factor experiment.The interaction of various factors was studied by Box-Behnken response surface method,and the optimal conditions for the removal efficiencies of Cr(Ⅵ) and total chromium by nFeS-L were determined.The results showed that,①The rod-shaped nano FeS with average length of 40～80 nm can be successfully loaded on lignite particles by ultrasonic precipitation method.②The single factor experiment found that when the reaction was carried out for 90 min,the removal efficiencies of Cr(Ⅵ) and total chromium by nFeS-Lstarted to be stable.The optimal preparation conditions of nFeS-L as follows:lignite size fraction was 0.18～0.25 mm,molar concentration of Na2S was 0.25 mol/L,Fe/S molar ratio was 2∶1.③Under these conditions,the removal efficiencies of Cr(Ⅵ) and total chromium were 79.3% and 71.9%,respectively.The P values of Cr(Ⅵ) and total chromium removal efficiency regression modelsfitted by response surface experiments were less than 0.01,the correction determination coefficient were greater than 0.9,indicating that the models reachedsignificant level,suitable for analyzing,predicting and optimizingthe removal efficiencies of Cr(Ⅵ) and total chromium.The effects of the three single factors on the removal efficiencies of Cr(Ⅵ) and total chromiumreached a highly significant level,and the significant extent of the effects weremolar concentration of Na2S>Fe/S molar ratio>lignite size fraction,but the interaction between the three single factors is not significant.④After further optimization by response surface method,the better preparation conditions of nFeS-Las follows:lignite size fraction was 0.18～0.25 mm,molar concentration of Na2S was 0.30 mol/L,and Fe/S molar ratio was 2.5∶1.Under these conditions,the removal efficiencies of Cr(Ⅵ) and total chromium were 87.5% and 75.6%,respectively.