Study Of The Cr(?) Reduction By In-situ Bio-FeS Nanoparticles Synthesis By Sulfate Reducting Bacteria

With the discharge of the wastewater from various industrial activities such as electroplating,dyes and leather tanneries,hexavalent chromium(Cr(?))is released into the environment,posing a serious risk to ecosystem.Chromium exists mainly in the forms of Cr(?)and Cr(III)in water,and Cr(?)exhibited stronger toxicity due to the higher solubility and carcinogenicity.Therefore,it is the main strategy to reduce Cr(?)to Cr(III)with the low toxicity and great precipitation potential as Cr(III)(oxy)hydroxide.Bioremediation is a promising technology to deal with chromium pollution in terms of its cost-effectiveness and environmental friendliness.However,the slow metabolic rate of microorganisms is an obstacle to the biological wastewater treatment process.Although nano-materials such as FeS nanoparticles(FeS NPs)are widely used in wastewater remediation owing to their high reactivity,their application in practice was limited due to the high cost,poor stability and low utilization rate.Whereas,biosynthetic FeS(bio-FeS)system integrated the advantages of both microorganisms and FeS NPs,in which nano-materials could enhance the efficiency of biological treatment,and microbes could serve as a good dispersant to prevent aggregation of nano-materials.Therefore,this study first explored the bioreduction mechanism of Cr(?)by sulfate-reducing bacteria(SRB).Then the coupling system of SRB with bio-FeS NPs(bio-FeS@SRB)was constructed for efficient reduction of Cr(?).Particularly,the electron transfer pathway mediated by FeS NPs was further explored to reveal the interaction between nano-materials and microorganisms.This study provided a theoretical foundation for the application of SRB in heavy metal remediation.The research results are as follows:(1)In the remediation of Cr(?)-contaminated wastewater,there was the interactions between the coexisting electron acceptor SO₄^2-and Cr(?).With the addition of Cr(?),a Cr(?)concentration-dependent delay of SO₄^2-reduction and cell growth occurred.Moreover,the moderate amount of SO₄^2-could accelerate the Cr(?)reduction.Once the concentration of SO₄^2-was upon 1500 mg/L,the growth of microorganisms and the removal rate of Cr(?)would be significantly inhibited due to the high acculmulation of H₂S generated from the sulfate.On the other hand,the supplement of Fe(II)in range of 0?200mg/L can effectively shorten the delay period,stimulate theSRB growth,and improve removal efficiencies of SO₄^2-and Cr(?).When the Fe(II)concentration reached 400 mg/L,the cell growth and the removal rate of SO₄^2-were inhibited,while the removal rate of Cr(?)continued to increase.(2)XRD,SEM,and XPS analyses proved that FeS NPs(60±5 nm)were bio-synthesized by SRB(OD₆₀₀=0.1)at Fe:S ratio of 1:2 and p H of 7,and distributed evenly on the cell surface,indicating that the coupling system bio-FeS@SRB was successfully constructed.The kinetic results showed that the removal rate of Cr(?)(2.6×10^-4 s^-1)by bio-FeS@SRB was two magnitudes higher than that of SRB(2.47×10^-6 s^-1)at initial Cr(?)concentration of 10 mg/L,and the completed removal time decreased from120 h to 7 h.By comparison,bio-FeS NPs improved the tolerance and reduction efficiency of SRB to Cr(?).Furthermore,the effects of initial concentration of Cr(?),p H and temperature on the Cr(?)removal efficiency were investigated.The results demonstrated that 10 mg/L Cr(?)could be completely removed within 24 h,but the removal efficiency decreased with the increase of Cr(?)concentration.The removal rate showed a positive correlation with the temperature in a certain temperature range(25?35°C),and the maximum removal efficiency within 24 h reached 77%.Besides,the increase of bio-FeS@SRB and the decrease of initial solution p H also promoted Cr(?)reduction,and the removal efficiency of Cr(?)was 94%when bio-FeS@SRB was 0.4 g/L.(3)FeS NPs in the extracellular presence of SRB on the electron transport properties of cell were analyzed by the electrochemicalmeasurements.The cyclic voltammetry(CV)results showed that bio-FeS@SRB had higher redox currents compared to SRB.In the potentiostatic experiment,the current increased immediately and reached approximately 30?A?with the introduction of bio-FeS@SRB,indicating that FeS NPs embedded could enhance the extracellular electron transfer of SRB.In addition,the bio-FeS NPs could act as an effective electron shuttle to drive the electron to flow towards the extracellular Cr(?)reduction.The dense and regular FeS NPs on the cell surface could switch the Cr(?)reduction from intracellular space to extracellular environment,which have a great detoxification effect on the microorganisms.