| Chromium-containing slag is a kind of toxic residue discharged from chromate production.High concentration of Cr(Ⅵ) in the chromium-containing slag,which is known to be toxic to plants and animals,a strong oxidizing agent and potential carcinogen,has been designated as a priority pollutant by US EPA.Chromium-containing slag is usually detoxified using chemical methods including reduction process by sodium sulfide or ferrous sulfate and extraction-exchange process. However,these methods are not widely explored due to high expenditure and incomplete of Cr(Ⅵ) removal.In recent years,more attentions have been paid to microbial method for Cr(Ⅵ) removal.A number of chromium-resistant micro-organisms have been reported to detoxify hexavalent chromium,which include Pseudomonas sp.,Desulfovibrio sp., Enterobacter sp.,Escherichia coli,Shewanella alga,Bacillus sp..Chromium-containing slag has a very complex consititute. Hexavalent chromium(Cr(Ⅵ)) in the chromium-containing slag can be divided into water-solube and acid-solube hexavalent chromium.The acid-solube hexavalent chromium mainly consists of dissociative CaCrO4 and Cr(Ⅵ) existing in the crystal lattice of Ca2SiO4 and 4CaO·Al2O3·Fe2O3 in the form of 4CaO·Al2O3·Cr2O3·12H2O, CaCrO4·Ca2SiO4·CaCrO4 and 4CaO·Al2O3·Fe2O3·CaCrO4.It is widely believed that the leaching of acid-solube Cr(Ⅵ) from the chromium-containing slag is crucial for the slag detoxification.The bio-leaching system involves polyphase interactions,such as microorganism-mineral,mineral-mineral,microorganism-solution, mineral-gas phase.Much effort has been made to enhance the reaction kinetics of bioleaching.The bioleaching effect has been demonstrated to be improved at the addition of some metal ions,surfactants and other additives.In the previous research,one novel Cr(Ⅵ)-reducing strain has been successfully isolated from chromite ore processing residue in Changsha and identified as Achromobacter sp.,naming CH-1.The strain can reduce high concentration Cr(Ⅵ) in alkaline media.And a novel detoxification technique of chromium-containing slag by the above strain has been also developed.To further improve the detoxification of chromium-containing slag by Achromobacter sp.CH-1,the effects of different surfactants on the detoxification of chromium-containing slag were investigated.Moreover, the mixed surfactant systems were used to further improve Cr(Ⅵ) leaching.The predominant outcomes are listed as follows:(1) When relative less surfactants were added to the medium,the growth and Cr(Ⅵ) reduction ability of Achromobacter sp.CH-1 were equal to that when there was no surfactant in the medium.However,the minimal inhibitory concentrations of surfactants were dependent on surfactant species.The minimal inhibitory concentrations of Nonionic surfactants Tween20,Tween60 and Tween80 were 200 mg/L,250 mg/L, 250mg/L,respectively.The minimal inhibitory concentrations of Anionic surfactant Sodium Dodecyl Sulphate(SDS) and Sodium Dodecyl benzene Sulphate(SDBS) were 75 mg/L and 150 mg/L,respectively,while which of Cationic surfactant Dodecyl Trimethyl Ammonium Bromide(DTAB) was 20 mg/L.(2) Anionic surfactant Sodium Dodecyl Sulphate(SDS),Nonionic surfactants Tween60 and Tween80 with optimal dosages of 75,200,and 250 mg/L improved the leaching rate of Cr(Ⅵ) by 8%,9%,and 11%, respectively.The Cr(Ⅵ) concentration of the residues after detoxification meets the national standards of hazard contaminations' leaching toxicity (GB5085.3-2007).However the effects of Anionic surfactant Sodium Dodecyl Sulphate(SDBS),Nonionic surfactants Tween20 and Cationic surfactant Dodecyl Trimethyl Ammonium Bromide(DTAB) on the detoxifcation of chromium-containing slag by A.sp.CH-1 was not distinct.(3) The mixed surfactant system of Anionic surfactant-Nonionic revealed better effectiveness of detoxification than single surfactant system.The synergistic interaction of SDS mixed with Tween80 was the highest among the mixed Anionic surfactant-Nonionic system.In the leaching solution mixed 50mg/L SDS with 50mg/L Tween80,the leaching rate of Cr(Ⅵ) in chromium-containing slag increased by 12% within 5 days as compared with that without surfactant addition. Moreover,the surfactants dosage in mixed systems was less than that in single system at equal effectiveness on the promotion of the Cr(Ⅵ) leaching.The optimal ratio of SDS to Tween 80 was 1:1.Moreover,the leaching rate of Cr(Ⅵ) increased with the increased of NaCl concentration at 2-20g/L.(4) The reduction of Cr(Ⅵ) by Achromobacter sp.CH-1 was subjected to zero order reaction.Tween80 addition elevated the reduction rate constant as compared with control treatment.The effect of the surfactant addition on the morphology of the detoxified slag was not obviously.The diffuse coefficient D of Cr(Ⅵ) in the slags detoxified by A. sp.CH-1 bacteria in the presence of Tween80(7.75×10-8m2/s) was larger than that of the slags detoxified by A.sp.CH-1 bacteria without surfactant addition(2.19×10-8m2/s) and the original slags(1.0×10-8m2/s). The elevated diffuse coefficient D during the detoxification process favor Cr(Ⅵ) leaching from the inner of the slags. |
