Microbial effects on geochemical behavior of arsenic in As-contaminated sediments

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1 Journal of Geochemical Exploration 88 (2006) Microbial effects on geochemical behavior of arsenic in As-contaminated sediments J.M. Park a, J.S. Lee a, J.-U. Lee b, H.T. Chon a, *, M.C. Jung c a School of Civil, Urban and Geosystem Engineering, College of Engineering, Seoul National University, Seoul , South Korea b Department of Civil, Geosystem, and Environmental Engineering, Chonnam National University, Gwangju, , South Korea c Department of Earth Resources and Environmental Geotechnics Engineering, Semyung University, Jecheon, Choongbuk , South Korea Received 30 March 2005; accepted 19 August 2005 Available online 2 November 2005 Abstract The purpose of this study is to investigate the contamination level and chemical speciation of As in sediments from the Hwachon Au mine area, and to study the effects of indigenous bacteria on geochemical behavior of As in As-contaminated sediments from the mine. Concentrations of heavy metal and metalloids in one composite sample of 9 Hwachon sediments were 24.9 As mg/kg, 16.6 Cd mg/kg, 230 Pb mg/kg and 1080 Zn mg/kg. This indicates that this area was seriously contaminated with As, Cd and Zn. From the result of sequential extraction analysis, most of As (87.9%) existed as a phase of As which coprecipitated with Fe oxyhydroxides. Under aerobic condition, As concentration leached from sediments were 4 times higher in non-sterile than in sterile condition. This enrichment of As leaching could be caused by increase of ph and exudation secreted during microbial metabolism. On the other hand, under anaerobic condition, As concentration has dramatically increased in non-sterile solution with time. This As leaching has begun on 4 5th days of incubation and the highest concentration of 511 As Ag/l was recorded on about 11th day. Arsenic leaching under anaerobic condition was caused by microbial enhancement of Fe leaching. Total Fe concentration increased from the 4th day, and this Fe leaching might cause lixiviation of As which had been coprecipitated with Fe oxyhydroxide. D 2005 Elsevier B.V. All rights reserved. Keywords: Arsenic; Stream sediment; Indigenous bacteria; Microbial metabolism 1. Introduction Arsenic is a carcinogenic substance and drinking of As-contaminated groundwater has been suggested as the main risk for humans (Borum and Abernathy, 1994). There are various sources of As, for example, volcanic lava, geothermal system and uranium and gold mines, coal combustion near smelters and refineries. One of the main anthropogenic source of As is Aumining and smelting (Adriano, 2001). * Corresponding author. Tel.: ; fax: address: chon@snu.ac.kr (H.T. Chon). Metabolic diversity of microorganisms makes them survive at almost every conceivable environment such as heavy metal laden acidic mine waste, thermal hot springs, marine vents, deep surface and polar region. There are various effects of microorganisms on heavy metals. Especially, microorganisms affect oxidation and reduction of metals (Lovley, 1993, 2001; Lovley et al., 1987). The purposes of this study are to investigate the contamination level and chemical speciation of As in sediments from the Hwachon mine area, and to study the effects of indigenous bacteria on geochemical behavior of As in As-contaminated sediments from the mine. Also, to examine the influences of circumstantial /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.gexplo

2 J.M. Park et al. / Journal of Geochemical Exploration 88 (2006) variation on the microbial effects, various elements causing microbial effect were measured. 2. Materials and methods The study area, the Hwachon Au Ag Pb Zn mine, is located in Kyungsangbuk-do, South Korea and its working period was from 1933 to After the mining operation was ceased, mine wastes have been left around the mine area, leading to the secondary contamination of stream and sediments. The stream flowing in front of the tailing was used for agricultural water. To investigate the contaminated level and geochemical behavior of As and other heavy metals, 9 stream sediments were collected along the main stream from the mine area. These samples were combined and mixed to make one composite sample. This composite sample can be a representative for the study area. Composite sediment sample was decomposed by aqua regia and analyzed by hydride generating AAS for As. To know the chemical speciation of As in sediment, sequential extraction method was applied. 300 ml of autoclaved deionized water was mixed with 9 g of dried composite sediment and 9 g of carbon sources (acetate, lactate, glucose, separately). After the ph of this solution was controlled as the ph of sediment (ph 6.9), the inoculum with indigenous bacteria was added. The inculants were incubated in shaking incubator at 30 8C and 150 rpm. Autoclaved control solution without inoculum was also incubated at the same condition. Control solution can be compared as a case without any microbial effect. As stream sediments are usually kept under anaerobic condition, it is essential to make experimental condition anaerobic. To make an anaerobic condition, N 2 gas was injected into the solution for over 20 min, so that the oxygen was eliminated from the solution. To keep the condition, N 2 gas was injected everyday into the solution for over 5 min. The ph and concentrations of arsenic, other heavy metal and anions were measured in order to indicate the bacterial effects. Speciation of arsenite and arsenate was done using silica-based anion-exchange cartridges (500 mg sorbent of 40 Am particle size and 60-Å pore size, Spelco Co.) and measured by hydride generator AAS after preliminary treatment of reduction with KI and GF-AAS with Pd as a matrix modifier. 3. Results and discussion 3.1. Geochemical characteristics of As Arsenic concentration of 9 sediment samples from the Hwachon mine area ranged from 11.0 mg/kg to 61.2 mg/kg, with average value of 32.1 mg/kg. This indicates that this mine area is seriously contaminated with As. This occurrence was considered as contamination due to gold mining. Contamination levels of heavy metal and metalloids in one composite sample of 9 Hwachon sediments were 24.9 As mg/kg, 16.6 Cd mg/kg, 230 Pb mg/kg and 1080 Zn mg/kg. Thus, stream sediments of this area are contaminated with Cd, Zn and As. Arsenic concentration extracted by sequential extraction analysis is shown in Table 1. The most proportion of As was extracted as a phase of coprecipitated As with crystalline Fe oxyhydroxides and sulfides, and remaining recalcitrant As minerals. Arsenic easily makes compound with Fe. This phase is relatively stable, but it could be separated through Fereducible microbial operation Bacterial effects under aerobic conditions In non-sterile solution, total As concentration was almost 4 times higher than in sterile solution (Fig. 1). Arsenite concentration is about 15 ppb, which is very small proportion of total As and close to the background level. This indicates that arsenic speciation leached from the sediments is mostly arsenate. Arsenite Table 1 Arsenic concentration in composite sediment sample by sequential extraction analysis (Ahn et al., 2003) Step Target Phase Extractant Concentration (mg/kg) 1 Ionically Bound As 1 M MgCl 2 (ph 8), 2 H, 25 8C Strongly Adsorbed As 1 M NaH 2 PO 4 (ph 5), 5 H, 25 8C Coprecipitated As With Carbonates 1 M NaOAc (ph 5), 5 H, 25 8C Coprecipitated As With Amorphous Fe M (NH 4 ) 2 C 2 O 4, 0.1 M H 2 C 2 O 4 (ph 3), 2 H, 25 8C, 2.15 Oxyhydroxides In Dark (Wrapped In Al Foil) 5 Coprecipitated As With Crystalline Fe Oxyhydroxides And Sulfides And Remaining Recalcitrant As Minerals Aqua Regia (36% Hcl+62% HNO 3 ), 1 H, 70 8C 19.99

3 136 J.M. Park et al. / Journal of Geochemical Exploration 88 (2006) Fig. 1. Variation of As concentration in the solution incubated under aerobic condition with time. could be ignored as its proportion is minimal. The highest concentration of total As leached out of the contaminated sediment was 94.7 ppb with acetate for carbon source. Arsenic concentration with time has increased much more in non-sterile than in sterile solution (Fig. 1). Clearly, bacteria enhance As leaching from the contaminated sediment. However, variation of Fe concentration cannot find any tendency or differences with time. Arsenic adsorbed on the surface of sediment particles can be easily separated by the change of ph. The initial ph of the solution was controlled at 6.9, however, when glucose was added for carbon source, the initial ph was as high as 10.9 (Fig. 2). Especially, As in stream sediment can be mobilized and desorbed in basic condition. By the result of sequential extraction analysis, 2.73 ppm of As exists as a strongly adsorbed phase, which could be extracted by the change of ph. It can be suggested that As strongly adsorbed on the sediment surface was desorbed by the exudation which is secreted during bacterial metabolism. The exudation can also cause the increase of ph. supplied. This level is 5 times higher than 94.7 ppb in aerobic condition. In aerobic condition, arsenite proportion was relatively small, however, in anaerobic condition, arsenite concentration was detected up to ppb (Fig. 3). Because most As of the Hwachon sediment exists as a phase of coprecipitated As with Fe, it is necessary to observe the change of Fe concentration. On the fifth day, Fe concentration of the solution with lactate and glucose increased (Fig. 4). Especially, Fe concentration of the solution with glucose increased dramatically on the fifth day from 0 ppm to 4.8 ppm, and kept on increasing gradually up to 8.1 ppm at last. Also, Fe concentration 3.3. Bacterial effects under anaerobic conditions Arsenic concentration gradually increased more in non-sterile than sterile solution, similarly to the results obtained in aerobic conditions. In contrast to aerobic condition, As concentration depends on time under anaerobic condition. The tendency could be generalized that As was leached from fourth to sixth day and the highest concentration was recorded at about 11th day (Fig. 3). The maximum concentration of total As in anaerobic condition is ppb when acetate was Fig. 2. Variation of ph in the solution incubated under aerobic condition with time.

4 J.M. Park et al. / Journal of Geochemical Exploration 88 (2006) Fig. 3. Variation of As concentration in the solution incubated under anaerobic condition with time. with lactate began to increase rapidly on the sixth day from 0.15 ppm to 2.7 ppm, and kept on increasing by 4.9 ppm. However, when acetate was supplied, Fe concentration increased up to only 0.35 ppm. There could be microbial effects in this Fe leaching, and the effects also caused As leaching. These bacteria have a preference of carbon source (lactate and glucose). Most of As from the Hwachon sediment exist as a phase of coprecipitated As with amorphous and crystalline Fe oxyhydroxides, so that the bacterial enhancement of Fe leaching could result with As leaching. This microbial effect is limited to the cases that are added lactate and glucose for carbon source. And these two cases showed that As and Fe leaching occur almost simultaneously; there are only 1 2 days of time lag. It seems to be clear that Fe leaching could have induced As leaching. 4. Conclusions The sediments from the Hwachon Au mine area are seriously contaminated with As, Cd and Zn. From the result of sequential extraction analysis, most of As (87.9%) in sediment exists as a phase of As coprecipitated with Fe oxyhydroxides. Under aerobic condition, As concentration leached from the contaminated sediments was 4 times higher in non-sterile than sterile condition. This tendency was more significant when acetate were added for carbon source. The enrichment of As leaching under aerobic condition could be caused by the increase of ph and exude secreted during microbial metabolism. It did not show any tendency related to Fe reduction. Under anaerobic condition, variation of As concentration with time has dramatically increased in nonsterile solution. This As leaching has begun on 4 5th days of incubation and the highest concentration of 511 As Ag/l was recorded on about 11th day. This As leaching under anaerobic condition was caused by microbial enhancement of Fe leaching. Total Fe concentration increased from the 4th day, and this Fe leaching could have induced the leaching of As coprecipitated with Fe oxyhydroxide. Acknowledgements Fig. 4. Variation of total Fe concentration in the solution incubated under anaerobic condition with time. This work was supported by grant (M B ) from Korean Ministry of Environment.

5 138 J.M. Park et al. / Journal of Geochemical Exploration 88 (2006) References Adriano, D.C., Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability, and Risks of Metals, 2nd edition. Springer-Verlag, New York. Ahn, J.S., Chon, C.M., Moon, H.S., Kim, K.W., Arsenic removal using steel manufacturing byproducts as permeable reactive materials in mine tailing containment systems. Water Research 37, Borum, D.R., Abernathy, C.O., Arsenic exposure and health. In: Chapell, W.R., Abernathy, C.O., Cothern, C.R. (Eds.), Science and Technology Letters. Northwood, U.K. Lovley, D.R., Dissimilatory metal reduction. Annual Review of Microbiology 47, Lovley, D.R., Reduction of iron and humics in subsurface environments. In: Fredrickson, J.K., Fletcher, M. (Eds.), Subsurface Microbiology and Biogeochemistry. John Wiley and Sons, New York, pp Lovley, D.R., Stolz, J.F., Nord Jr., G.L., Phillips, E.J.P., Anaerobic production of magnetite by a dissimilatory iron-reducing microorganism. Nature 330,