KIÒU QUúNH HOA¹, PETER SOTTNIK², L¹I THÚY HIÒN¹,
L· THÞ LÀI³, JOERN KASBOHM⁴

Unlike organic compounds, metals generally cannot be degraded to gaseous products or otherwise destroyed. It is probably that they present a very stable chemical form for the metal in environment. Consequently, the objective of metals treatment is often to return mobile contaminants to their stable, immobile mineral forms.

Nowadays, the variety of passive treatment systems have been replaced gradually by physico-chemical treatment systems due to some following advantages:

- It is simple to collect and realize experiments.

- It takes advantage of naturally occurring chemical and biological processes used for cleaning contaminated waters.

- Low cost and high effect that can meet the practical conditions of V©n Chàng village.

The best condition for metal bioremediation is a microbial ecosystem approach but not only a specific genus or species of microorganism.

Now we would like to refer to mixture of sulfate-reducing bacteria (SRB) that play significant role in anaerobic condition by its metal removal as precipitate sulfides.

 In anaerobic condition SRB can decompose organic compound using SO₄^2- as terminal electron acceptor producing H₂S. It reacts with dissolved metal to form sulfide precipitation (1), (2)

2CH₂O + SO₄^2- ® H₂S + 2HCO₃^- (1)

H₂S + M²⁺ ® MS¯ + 2H⁺ (M: metal) (2)

* Heavy metal contaminated water samples (most of them are Cr, Al and Fe) were collected from three different places in V©n Chàng, Nam §Þnh.

* The composition and role of substrate and additive in experimental modelling include:

- Straw constituting stable substrate for bacterial growth

- Cow shit served as organic carbon source for bacterial growth.

- Crushed limestone providing buffering capacity to increase the pH.

* Media for inoculation of sulfate-reducing bacteria (g/l):

Desulfobulbus: Na₂SO₄ 3, KH₂PO₄ 0.2, NH₄Cl 0.3, NaCl 1, KCl 0.5, MgCl₂ O.4, CaCl₂.2H₂O 0.15, FeSO₄.7H₂O 0.5, yeast extract 1, vitamins 1, trace element 0.1, water up to 1 liter, pH 7.2-7.4.

Desulfovibrio: KH₂PO₄ 0.5, NH₄Cl 1, NaCl 1, CaSO₄ 1, MgSO₄.7H₂O 2, sodium lactate 3.5, FeSO₄.7H₂O 0.5, yeast extract 1, vitamins 0.1, trace elements 0.1, water up to 1 liter, pH 7.2-7.4.

- Analyzing the content of heavy metals and measuring pH and Eh value during the experiment by atomic absorption spectrometry (AAS) and pH-Eh measuring instrument.

- Assessment the growing heavy metal removal bacteria by most number probably method (MNP)

- Studying morphological, physiological and biochemical properties of SRB.

Sample of wastewater was analyzed in ViÖt Nam. Results showed high concentration of Fe, Al and Cr. Then, wastewater sample was transported to Slovakia, we measured pH, Eh and analyzed the content of metals. The result showed high concentration of Cr only. Apparently during the transport period Fe, Al and Cr were precipitated as oxyhydroxides. The proof was a sediment on the bottom. Analysis of the same sample after decreasing of pH<2 (by HNO₃) showed high concentration of Cr, Al and Fe. Acid caused the dissolution of oxyhydroxides (Figure 1).

Figure 1. The content of heavy metal in the original wastewater
in V©n Chàng before and after reducing pH value

Three experimental containers (1liter) were set up with different ratio of composition (Table 1). All of them were added wastewater to wet the substrate and were incubated in static condition for 12 days (30^oC).

Table 1. Composition of substrate and additive in the experimental containers (wt%) and amount of inlet wastewater (ml)

First two weeks kept static stage for bacterial growth, then add water inflowing and taking out with the same volume (Table 2) during the operation process. Content of Fe, Al, Cr, Cu, Ni, NH₄⁺, NO₃^-, and SO₄^2- were measured during the experiment.

Table 2. Amount of wastewater added and taken out during the experimental time.

At the beginning of experimental period the pH value decreased greatly in all containers (Figure 2). After that the pH value continuously increased to closely approximating the original value. Decreasing of pH was caused by organic acids, which were produced by fermentation.

Eh value was almost in minus during the experimental period. It expresses that the reduction condition was always in modelling, which is very necessary for effective growth of SRB

Table 3. Concentration of metal during the experimental period (mg/l)

At the final stage of experiment the concentration of metals (Cr, Al, Fe, Cu, Ni, Cd, Zn…) were expressively reduced in comparison with original value in wastewater: Fe from 11.4 mg/l to 2-3 mg/l, Al from 131 mg/l to 15 mg/l, efficiency of passive treatment is especially high with Cr, one of main pollutant factors in V©n Chàng, from 100 mg/l to 1 mg/l. The result of two containers No.1. and No.2 were better than one No.3 after 49 day treatment because of the different composition of substrate and additive.

Sulfate-reducing bacteria were very active throughout the experimental process. The highest number zeaches up to 10⁷ CFU/ml (Table 4).

Table 4. Number of sulfate-reducing bacteria on selected media

The experiment also showed the excellent removal of heavy metal that was obtained at two containers No.1 and No.2.

The results showed the ability of applying microorganism for minimizing the content of metal elements in wastewater. Our laboratory investigation proved to be very beneficial in examining the potential of modelling in heavy metal removal from wastewater and the prospect of expanding the large-scale treatment in Vân Chàng.

The authors would like to thank BMBF and Vietnamese-German cooperating Project VMN / 004 for support and also to thank the members of Department of Mineral Deposit, Faculty of Natural Sciences, Commemius University, Bratislava, Slovakia and Department of Petroleum Microbiology, Institute of Biotechnology, Hà Néi, ViÖt Nam for their help in our study.

References