HE De-wen，LIU Huan，JIANG Chong-wen，DUAN Hao-peng，LIU Lei

(School of Metallurgical Science and Engineering，Central South University，Changsha 410083，China)

Microbial Detoxification of Hexavalent Chromium from Chromium-Containing Slag*

HE De-wen，LIU Huan，JIANG Chong-wen，DUAN Hao-peng，LIU Lei

(School of Metallurgical Science and Engineering，Central South University，Changsha 410083，China)

In this paper，treatment and recovery of hexavalent chromium(Cr(Ⅵ))from chromium-containing slag by bacteria was studied.The work showed that a bacterial strains of Ch-1 isolated from chromium-containing slag can efficiently accelerate Cr(Ⅵ)leaching and remove it.Moreover，bacteria Ch-1 can effectively change the structure of chromium-containing slag by Scanning Electron Microscopy(SEM)and the Energy Dispersive X-Ray (EDX)，which is benefitial to Cr(Ⅵ)leaching and its detoxification by changing Cr(Ⅵ)to Cr(Ⅲ).The effect of detoxification by bacteria of Ch-1 is obvious，the toxicity in leached slag was 3.3 μg/g lower than the national standard of 5 μg/g，and the recovery rates of hexavalent chromium are more than 90%.

Chromium-containing Slag;bacteria;treatment;recovery

Chromium-containing slag is usually treated by using solidification and then safely filled in specified location［12］.But the leaching Cr(Ⅵ)is still harmful to environment，humans，crop，and livestock.In general，for getting rid of the hazard to the environment，chemical methods such as pyro-based reduction and hydro-based reduction detoxification are adopted to detoxify the slag by changing Cr(Ⅵ)to Cr(Ⅲ).In pyro-based reduction，it is utilized to manufacture aggregate and refractory material，but the treatment processes consume a great deal of energy，causing a high treatment cost.In hydro-based reduction，different reduction solutions were selected as Na2S，NaHS，F(xiàn)eSO4or Na2SO3，but they will cause the secondary pollution.

On the other hand，the Cr(Ⅵ)in chromium-containing effluent can be efficiently removed by chromate resistant bacterial consortium［13-15］.Biotechnological methods are usually cheaper and do not need application of chemical reagents as compared with physicochemical processes［16-17］.The rate of leaching Cr(Ⅵ)from the slag is controlled by its diffusion process.Higher concentration gradient is beneficial to the enhancement of Cr(Ⅵ) leaching.The Cr(Ⅵ)presents in liquid phase，which is leached from chromium slag and can be continuously removed by biosorption and bioaccumulation of bacteria［18］.This process helps to maintain a higher concentration gradient between the surface of chromium slag particles and buck liquid phase，thus facilitating the detoxification process controlled by diffusion.In this paper，detoxification of leaching hexavalent chromium from chromium-containing slag by using bacteria was studied，optimized culture medium of bacteria Ch-1 were obtained，and experiment results showed that a bacterial strains of Ch-1 isolated from chromium slag can efficiently accelerate Cr(Ⅵ) leaching rate and remove it.

2 Materials and Methods

2.1 Chromium-Containing Slag Sample and Pretreatment

Chromium-containing slag samples were collected from Changsha Chromate Plant，Changsha，China.The fresh chromium-containing slag samples were dried in an oven at 100℃，then ground in a ball mill and passed the 150 mesh sieve.The treated chromium-containing slag was sterilized in an oven for 2 h at 150℃ before use.

2.2 Bacterial Strains

The strain was isolated from chromium-containing slag in Changsha，China，which was identified as bacterial strains of Ch-1 by gene sequencing of 16S rRNA and nominated as Ch-1.The 16S rDNA sequence was deposited at GenBank with accession no.EF 362 778.The isolated bacterial strains of Ch-1 was inoculated in modified Luria Broth(LB)medium containing 10 g tryptone，10 g NaCl，5 g yeast extract，0.1 g glucose，and 4 g sodium lactate in 1 L distilled water at pH 9.The medium was autoclaved at 121℃ for 18 min.The cultured bacterial strains of Ch-1 was kept in a refrigerator at 4℃ prior to experiment［19］.

2.3 Detoxification by Bacteria of Ch-1

To simplify the testing，the experimental devices are shown in fig.1，in which filtration column is made of PVC，the high and low slot tanks are made of glass.At the bottom of the column with a porous filtration plastic plate，with a layer of gravel and fine sand about 3 cm，in the middle a representative of chromium slag after sterilization by heating 100℃ 2 h are loaded，and the loop spraying speed is 1 40 mL/min.

Fig.1 Experiment Scheme of Leaching of Chromium Slag

3 Results and Discussion

3.1 Detoxification of Hexavalent Chromium

Fig.2 Change of Cr(VI)Concentration in Solution During Column Leaching

Fig.2 shows that Cr(VI)concentration increased at the beginning stage，then in the next 1～3 days in a steady phase with concentration about 450 mg/L because of adaptation of bacteria treatment in a slow period.From the fourth day，C r(VI)concentration increased;in the fifth day，it reached the peak value.It shows obviously bacterial detoxification effect.When the effect of detoxification by bacteria is getting stronger，Cr (VI)concentration fell to less than 20 mg/L，and the water-soluble Cr(VI)leaching rate was 98.82%.The toxicity in leached slag was 3.3 μg/g，lower than the national standard of 5 μg/g.Cr(VI)leached from chromium-containing slag under bacteria of CH-1 was changed into C r(Ⅲ)，which was precipitated as the formation of Cr(OH)3in the alkaline condition.The recovery rates of hexavalent chromium was more than 90%.

3.2 SEM of Chromium Slag

The Scanning Electron Microscopy(SEM)was used to make 10 000 times the surface morphology compared chromium slag to chromium-containing slag by bacteria(fig.3 and fig.4).The results showed that the surface of chromium residue before bacteria treatment was messy sheets.After bacterial leaching action the messy disappeared and gully erosion was obvious overlapping blocks.

Fig.3 SEM of Chromium Slag

Fig.4 SEM of Chromium Slag by Bacteria

3.3 EDX of Chromium Slag

In fig.5 and fig.6，the Energy Dispersive X-Ray(EDX)showed that the element wt%Cr is 7.77%before bacteria，while fell to 1.95%of its value after bacteria leaching action.

Fig.5 EDX of Chromium Slag

Fig.6 EDX of Chromium Slag by Bacteria

4 Conclusions

The mechanism ofbacteria detoxification is quite complex and is restricted by all experimental factors，so it is very important to keep bacterium alive in high concentration chromium(Ⅵ).The experimental conclusions in the paper can provide guidance to further research thoroughly bacteria detoxification of chromium(Ⅵ).

(1)By Scanning Electron Microscopy(SEM)and the Energy Dispersive X-Ray(EDX)，bacteria of Ch-1 change the structure of chromium-containing slag，which is benefitial to Cr(Ⅵ)leaching and its detoxification.

(2)The effect of detoxification by bacteria of Ch-1 is obvious.The toxicity in leached slag was 3.3 μg/g，lower than the national standard of 5 μg/g，and recovery rates of hexavalent chromium are more than 90%.

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(責(zé)任編輯 易必武)

鉻渣中6價鉻的微生物解毒研究

何德文，劉 歡，蔣崇文，段浩鵬，劉 蕾
(中南大學(xué)冶金科學(xué)與工程學(xué)院，湖南 長沙 410083)

詳細(xì)研究了微生物處理鉻渣中6價鉻Cr(Ⅵ)及其回收Cr(Ⅲ)的方法.研究成果表明，來自鉻渣場的一株名為Ch-1的細(xì)菌能有效地加速Cr(Ⅵ)浸出和去除;此外，通過掃描電子掃描電鏡(SEM)和X射線儀器(EDX)觀察，Ch-1細(xì)菌能有效改變浸出后鉻渣的結(jié)構(gòu)，這有利于鉻渣中Cr(Ⅵ)浸出及其解毒為毒性低的Cr(Ⅲ);最后經(jīng)浸出毒性試驗(yàn)，淋溶渣的毒性為3.3 μg/g，遠(yuǎn)低于國家標(biāo)準(zhǔn)5 μg/g，且6價鉻Cr(Ⅵ)的回收率高達(dá)90%以上.

鉻渣;微生物;解毒;回收

TQ136.11

B

TQ136.11

B

10.3969/j.issn.1007-2985.2013.02.016

1007-2985(2013)02-0072-05

1 Introduction

Recently some industries using chromium compounds，such as electroplating，chemical industry，printing and dying，leather，etc，had made good benefits，which promoted the scale-production of chromium salt plant and made environmental problems worse［1-3］.Metallurgical and chemical industries in China discharged 200～300 thousand of chromium-containing slag per year and the accumulative total amount was not lower than 2 million tons in the past［4-7］.Chromium-containing slag from chromate plants is one of the most hazardous solid wastes because of its high density of dissolvable Cr(Ⅵ)，a strong oxidizing agent and potential carcinogen［8-10］，which is toxic to plants and animals.It can induce pathologic change if absorbed by stomach and intestines，resulting in asphyxia.Therefore，the Cr(Ⅵ)is a kind of carcinogenic substance，also one of 129 chief pollutants recognized by EPA in US.Some effective measures must be taken to solve leaching C r(Ⅵ)of chromium slag［11］.

* Received date:2013-01-29

Supported by the National Advanced Technology Research and Development Project(863 Project 2010AA6492080)

Biography:HE De-wen(1968-)，male，was born in Yongzhou City，Hunan Province，professor of Central South University，majoring in environmental material research

JIANG Chong-wen(1968-)，male，was born in Loudi City，Hunan Province，associate professor of Central South University，majoring in environmental material research;E-mail:jcwcsu@csu.edu.cn