Adsorption Of Cr(?)in Aqueous Solution By Fe-Modified Crofton Weed Biochar

Heavy metal pollutants in water are harmful to human health and ecological environment.Among many heavy metal pollutants in waste water,chromium?Cr?has attracted much attention due to its toxicity and high carcinogenicity.Generally speaking,waste water containing Cr mainly derived from industrial productive trades such as metal processing and electroplating.The main forms of Cr in waste water are Cr?III?and Cr?VI?,where Cr?VI?is more soluble and has stronger toxicity.For the removal of heavy metals in waste water,adsorption technology with biochar is regarded as a simple and inexpensive method.In the field,there is a trendency to find efficient and low-cost biochar materials to adsorb heavy metal pollutants.In this study,the invasive plant crofton weed was made to biochar at different pyrolysis temperatures?300^oC,400^oC,500^oC,and 600^oC?.The physicochemical properties?including SEM,XRD,elemental analysis,total phosphorus,etc.?of crofton weed biochar?C-BC?were investigated to evaluate its potential application.Then Fe-modified crofton weed biochar?FeC-BC?was prepared with crofton weed and Fe?III?.Furthermore,the effects of FeC-BC on adsorption of Cr?VI?was investigated with batch experiments,which included time,temperature and pH factor.The adsorption kinetics model and adsorption thermodynamic model were applied to fit the adsorption results to explore the adsorption mechanism.Moreover,the adsorption capacity change and reuse of FeC-BC were investigated by using co-existing ion interference experiment and desorption experiment.This study provided a high efficient and low-cost biochar adsorption material for removal of Cr?VI?in waste water.The main research results were as follows:?1?The pH and ash content of C-BC increased with pyrolysis temperature,and the yield had the reverse trend.All C-BC contained a large number of pores.The iodine adsorption values of all C-BC were more than 200 mg/g.The iodine adsorption value of C-BC400 was maximum?381.99 mg/g?.The C content of C-BC presented the increasing tendency accompanying with the increase of pyrolysis temperature,and H,N and O content had the reverse trend.With the increasing pyrolysis temperature,the aromaticity and stability of C-BC increased,while polarity of C-BC decreased.The contents of total Ca,total Mg,total K,and total P ranged 17790.08-29750.08,3230.92-5211.75,31661.67-49553.33,and 3469.08-5453.75 mg·kg^-1,respectively.Overall,the contents of total Ca,total Mg,total K,and total P had an increasing trend from C-BC300 to C-BC600.The soluble Ca²⁺,Mg²⁺,PO₄^3-,NO₃^-and NH₄⁺of C-BC had the decreasing trend with increasing pyrolysis temperature.Compared with the above soluble ions,the soluble K⁺had the reverse trend.The maximum K⁺content was 26293.33 mg·kg^-1.This result indicated that C-BC contained a certain amount of nutrients,especially richness of K.The results of XRD spectrum analysis showed that the main mineral components of C-BC were sylvite,dolomite,quartz,and magnesium diphosphate.Infrared spectrum analysis displayed that C-BC contained multiple surface functional groups,such as O-H,-COOH,C=O,etc.With the increase of pyrolysis temperature,the carboxyl group of C-BC presented the decreasing tendency.The peak strength of 1121 cm^-1?polar C-O?decreased and the peak strength of 873 cm^-1?aromatic C–H?increased with the rise of pyrolysis temperature,confirming that the polarity of C-BC gradually decreased and the aromaticity gradually increased.?2?The screening experiment of preparation conditions for FeC-BC showed that the modification conditions were temperature 55^oC,soaking time 12h,liquid-solid ratio 10,Fe?III?solution concentration 1mol/L.?3?In the time factor experiment,the adsorption amount of Cr?VI?was 25.52mg/g at0.5h for FeC-BC,which was already higher than that of C-BC?23.95mg/g?at 36h.The results showed that the adsorption ability of FeC-BC for Cr?VI?was greater than that of C-BC.The adsorption process of Cr?VI?with FeC-BC was more satisfied with the pseudo second-order dynamics model,indicating that the rate-limiting of the adsorption process was the chemical process.At pH=2,the adsorption amount and adsorption rate of FeC-BC were largest,which were 46.09mg/g and 92.18%,respectively.In the temperature factor experienment,the Langmuir model could better fit the adsorption results of Cr?VI?with C-BC,while the Freundlich model could better fit the adsorption results of Cr?VI?with FeC-BC.The results indicated that Fe modification changed the adsorption process of C-BC.The maximum FeC-BC adsorption amount was 70.40mg/g,which was 1.77 times that of C-BC.With increasing temperature,?G^o value of FeC-BC and C-BC were gradually reduced,?G^o were negative,and?H^o and?S^o were all positive.The results implied that FeC-BC and C-BC adsorption of Cr?VI?were all spontaneous and endothermic.Analysis of the infrared spectra of FeC-BC before and after adsorption indicated that an ion exchange reaction was occurred among O-H,C-H,and Cr?VI?.Furthermore,C=O,C-O,COOH and aliphatic ethers?-O-?were all involved in the adsorption reaction.Coexisting ion experiments showed that NO₃^-,Cl^-,SO₄^2-had competitive adsorption with Cr?VI?,which resulted in the decrease of Cr?VI?adsorption rate of FeC-BC.The influence degree of three coexisting anions on adsorption of Cr?VI?was:Cl^-<NO₃^-<SO₄^2-.After 4 times adsorption-desorption cycles,FeC-BC still maintained highly adsorption rate?63%?.The result demonstrated that FeC-BC had strong stability and repeatability.