Metals accumulation and as releasing during interaction of clay and iron minerals with heterotrophic bacteria in soil and sediment bioleaching

Soil and sediment are major reservoirs for contaminants as they possess an ability to bind various metals. In general many of them contain a wide range of heavy metals with varying concentration ranges depending on the surrounding geological environment and anthropogenic and natural activities occurring or once occurred [1]. Clay minerals are among the major materials that interact with almost all soils and sediments contaminants [2]. Clays often represent a short-term sink of heavy metals in soils and sediments, because of their adsorptive properties [3]. 

Clay minerals and microbes are ubiquitous in soils, sediments, and sedimentary rocks and they play important roles in environmental processes. Microbes interact with clays via a variety of mechanisms, such as reduction and oxidation of structural iron and mineral dissolution and precipitation through the production of siderophores and organic acids [4]. Along with microorganisms, clays provide some of the most catalytic surfaces in sedimentary environments, which are important to a variety of biogeochemical cycles. Iron (Fe+3) bound in clay minerals should be considered an important electron acceptor supporting the growth of bacteria in soils or sedimentary environments [5]. Clay minerals often account for about half the iron in soils and sediments. Much of the structural iron in clay minerals is ferric iron, which can be reduced either chemically or biologically [4] The reduction of structural Fe(III) to Fe(II) tends to decrease the surface area, interlayer spacing, water swellability, and hydraulic conductivity of clay minerals. In general, reduction increases the negative layer charge and cation exchange capacity [6], [7], while the interlayer cations become less exchangeable. These physical and chemical changes of clay minerals would have profound effects on soil fertility and contaminant mobility. The transport mechanism of heavy metals through soil has long presented great interest to both environmental and soil scientists because of the possibility of groundwater contamination through metal leaching [4]. Many heavy metals are nonessential and potentially toxic to microorganisms. At higher concentrations these heavy metal ions form unspecific complex compounds within the cell, which leads to toxic effects, making them too dangerous for any physiological function. Bioremediation is an option that offers the possibility to destroy contaminants or remove them innocuous using natural biological activity [8]. The presence of bacteria capable of tolerating heavy metals from soil and sea water samples from heavily contaminated sites in Mauritius was  investigated by Hookoom and Puchooa [9]. No isolate was able to grow in the presence of copper at concentration higher than 3.0 mM. 

The mining waste, mineral processing and industrial treatment processes represent potential source of heavy metals in the soil in close proximity of the industrial area KOVOHUTY Krompachy with production of Cu and Zn. The soil contamination investigated here is located close to the river Hornád which flows into reservoir Ružín and contaminated sediment. Despite the fact mentioned the reservoir is situated in a different aquatic environment and catch contaminants from several sources. In the reservoir, these metals are retained in the sediment layer and during changes of physical-chemical conditions, their immobilization or mobilization can negatively impact aquatic biota through the food chain.

 This study was conducted to determine the distribution of heavy metals in two different contaminated samples of soil and sediment in dependence on the particle size distribution of clay and iron minerals. The purpose of the present test was to verify the resistance of bacteria to As and to investigate the effect of indigenous bacteria on mobilization of Fe, Cu, Zn, As by bioleaching.