ARSENIC TRANSFORMATION AND TRANSPORT STUDY IN

CONTAMINATED REGION OF THE GOLD RECOVERY PLANT

Olga V.Shuvaeva* ( Institute of Inorganic Chemistry , Siberian Branch of Russian

Academy of Sciences, Ac.Lavrentyev Pr., 3, Novosibirsk, 630090, Russia,

 E-mail:Olga@che.nsk.su), Svetlana B.Bortnikova, Elena V.Lazareva

( United Institute of Geology, Geochemistry and Mineralogy, Siberian Branch of Russian Academy of Sciences, Ac. Koptug , 3, Novosibirsk, 630090, Russia)

 

Abstract.  Arsenic behaviour in the region of tailing impoundment after the cyanidation process of gold - arsenopyrite - quartz ores  has been investigated.

It  has been  concluded that in gold recovery plant tailing impoundment the As pre-concentration in   ground  water with respect to the surface  water of the lake takes place. The last is  due to the sediment leaching procedure where As is predominantly present as residual arsenopyrite and partly as co-precipitate with iron hydroxide. In surface water arsenate and arsenite are the main arsenic species, but in pore (ground ) waters methylation processes play a significant role: arsenic transport is accompanied with transformation into the less  toxic compounds co-existing with the most toxic specie - arsenite. 

 

Introduction.

 

 The high mobility of As makes it  a potential environmental hazard with groundwater leaching from the tailing dam material. The stored sulphide waste of the ore mineral processing are the constant sources of the metals dispersed over the environment (Wiliams, 1975). The main factors influencing on the rate and the scale of the elements dispersion are the substances deposition, type of mineral processing and the storage conditions. The object under consideration is the collection pond of the Komsomolsk gold  recovery plant, which has been put into operation during 1937-1940 (Kemerovo region, South West of Siberia). The tailings impoundment is located at 800 m above sea level. The square of the pond is 146 000 m2, the volume - about 1.1 million m3. The average depth is 2 m. After purification which includes mechanical settling of solid and chemical treatment of drainage to remove toxicants the waters  drain to the river. It has been found that arsenic content in pore waters of the Komsomolsk pond is much higher then in the surface water (Bortnikova et al., 1999). In natural lakes the diffusive release of arsenic from the sediments mainly results from the reductive dissolution of iron and manganese oxides (Hamilton-Taylor, 1995). As for the tailings impoundment under investigation the process of arsenopyrite oxidation have to be taken into consideration.

The goals of investigation:

- arsenic distribution in solid and water samples of tailing impoundment

- arsenic speciation in  sediment as well as in surface and ground water

- arsenic transformation and transport in tailing impoundment

 

Experiment

 

Sampling. Sediment cores were taken in the bottom of the tailings impoundment. The cores were located just near the pulp-line closed to aerobic conditions.

Pore water have been taken from the bottom of the prospecting pits (2 m depth) corresponding to the water table level.

Surface water samples were taken in different parts of the pond and the average value has been taken into consideration.

All of water samples were filtered through the filter 0,45 mm. The fresh waters have been analyzed using field analysis methods. The samples for laboratory analysis have been preserved with nitric acid (pH < 2) after filtration. For arsenic speciation the probes have been froze fast and kept frozen.

The collected core and sediments samples were fast froze, kept closed before analysis and than sliced.

Analytical chemistry included:

Field experiment: the rapid test kit ( Mercoquant Arsenic, Merck, Germany)

 Eh and pH measurements have been done with a conductometer «Anion-210» (Russia) using platinum and glass electrodes calibrated against standard solutions

The laboratory determination of total arsenic concentrations has been done using electrothermal atomization atomic-absorption spectrometry (ETA AAS) in the presence of Pd (NO3)2 as a matrix modifier at 193,7 nm,  Hitachi 8000 Zeeman instrument has been applied.

Arsenic  species have been determined using the combination of  microcolumn HPLC with ETA AAS detection  ( under the same conditions as for total As analysis) has been applied

Arsenic speciation in sediment cores has been done using the sequential leaching procedure, developed by Bombach et al. (1994). The same procedure has been also applied to the arsenopyrite-containing ore for the comparative study. The leachate solutions have been analyzed using flame AAS.

 

Results and discussion

 

. The gold cyanidation process includes arsenic release from arsenopyrite:

FeAsS +CN-+O2 ® Fe(CN)63- + SO42- + AsO43-

and arsenic precipitation in alkaline media:

CaO + AsO43- ® Ca3(AsO4)3 + OH-

The resulting arsenic distribution in the water components of the tailings impoundment is shown in Tables 1.

Table 1. Geochemical parameters and   comparative arsenic content in water components of      

              tailings impoundment Komsomolsk (Summer,1998), mg/L

 

Sample

As, mg/L

 

pH

Eh, v

O2, mg/L

Surface water

100±20

 8,5±0,05

0,420±0,01

7,5±0,1

 

Background water

Collection pond

Settler

River

Pore water

 

surface

pore

surface

pore

 

 

1,9±0,8

100±20

1700±70

35±8

130±25

20±6

500±80

 

 As may be seen there is little arsenic weathering during the refining cycle, but the arsenic content of the pore is much higher than for surface waters of the tailings impoundment. It appears that arsenic re-distribution during solid - liquid interactions is accompanied by arsenic reduction and methylation reactions. The arsenite content in water samples is correlated with oxygen concentration (Fig.1).

 

 

 

Figure 1. Arsenic speciation as a function of oxygen content

 

According to the Red-Ox diagram for As (Wagman et al., 1982) the only form of As in the surface water should be arsenate, however arsenite has been found as well. This may mean that not only chemical but biochemical processes have to be taken into account for the speciation study. The data produced in the present investigation are in agreement with the hypotheses of Ballin et al. (1994) concerning intermediate pre-reduction of arsenate to arsenite during the methylation stage.

The  arsenic species distribution in  sediments is presented in Figure 2.

 

 

Fig.2Arsenic species profile in sediment

 

It is  seen that in large part the sediment is composed of residual mineral. At the same time the share of exchangeable arsenic and arsenic bound to organic or sulphide matter is uniformly distributed in the core and does not exceed 10% of total element. Arsenic bound to the moderately reducible fraction is mainly concentrated in upper layers (0-25 cm). It is probable that this part of the sediment contains iron oxides and hydroxides resulting from the arsenopyrite oxidation process in the aeration zone:

FeAsS + 3,5O2 +4H2O = Fe(OH)3 solid + HAsO42- +SO42- + H+

At the pH value of the surface water of the pond (pH=8,5) iron hydroxides would be negatively charged and attracted by positively charged species. Therefore arsenic species bound to the moderately reducing fraction may be calcium arsenate co-precipitated with iron hydroxides. The appearance of high sulfate ion concentrations in surface waters (about 900 mg/L) may be considered as a confirmation of this process. For the sediment layers, there were no great differences in iron content except for the residual fraction  where Fe is presumably concentrated through bonding to sulfide and organic matter.

 

Conclusions

 

-  in gold recovery plant tailing impoundment of the As pre-concentration in pore

    ( ground )water with respect to the surface  water of the lake takes place

 

-  in lake sediments As is predominantly present as residual arsenopyrite and partly as co-

    precipitate with iron hydroxide

 

-  in surface water arsenate and arsenite are the main arsenic species, but in pore (ground )   

    waters methylation processes play a significant role

 

-  arsenic transport is accompanied with transformation into less  toxic compounds co-

   existing with the most toxic specie (arsenite) 

 

References

 

Ballin U., Kruse R., Russel H.A., 1994. Determination of total arsenic and speciation of arseno-betaine in marine fish by means of reaction -headspace gas chromatography utilizing flame -ionization detection and element specific spectrometric detection. Fresenius J.Anal.Chem, 350, 54-61.

Bombach G., Pierra A., Klemm W. , 1994.Arsenic in contaminated soil and river sediment. Fresenius J.Anal. Chem., 350, p.49-53.

Bortnikova S.B., Lazareva E.V., Androsova N.V., O.L.Gaskova, L.Andre. 1999. The geochemical peculiarities of technogenic lake at the Komsomolsky recovery plant. Proceedings of ISGS (GEOENV'97) Istanbul, Turkey 1-5 September 1997, Ed.: Prof. I.Yilmazer, pp. 59-66.

Hamilton-Taylor R.and Davison W., 1995. Redox-Driven Cycling of Trace Elements in Lakes. In: Physics and Chemistry of Lakes, NY, p.218 - 262.

Wagman D.D., Evans H.H., Parker V.B., 1982. The NBS tables of chemical thermodynamic properties. Selected values for inorganic and organic substances in SI units. J.Phys. Chem. Ref. Data II, supp.2: 392 p.

Williams R.E., 1975.Waste production and disposal in mining, milling and metallurgical industries. USA, Miller Freeman Publication, 489 p.