Phytoremediation of Chromite Mining Contaminated Soils

Abstract

Several anthropogenic and natural sources are considered as the primary sources of toxic metals in the environment. The present study was conducted to investigate heavy metal concentrations in soil and plants collected from the mafic and ultramafic rocks situated in Heroshah (Malakand Agency), Qala (Mohmand Agency) and Alpuri (Shangla District). In the study area, mafic and ultramafic rocks are hosting chromite deposits; therefore, both soil and plant samples were collected for chemical analyses. For the reclamation of mine impacted soil, different adsorption materials such as activated carbon powder (ACP), activated carbon granules (ACG) and farmyard manure (FYM) at different ratio (2 and 5%) were randomly mixed with the desired soil samples and plants were cultivated in the pots. Samples were analyzed for heavy metals i.e. arsenic (As), aluminum (Al), chromium (Cr), cadmium (Cd), copper (Cu), iron (Fe), nickel (Ni), manganese (Mn), cobalt (Co), zinc (Zn) and lead (Pb) using atomic absorption spectrometer (Perkin Elmer, AAS-PEA-700), ICP–OES (Perkin Elmer Optima 7000 DV, USA) and ICP-MS (Agilent Technologies, 7500 CX, USA). Currently, the most burning issue is the contamination of soil ecosystems and associated human health risks of heavy metals because of their adverse effects and toxicity. The findings of the current study were compared with the maximum allowable limits (MAL) set by various international organizations such as State Environmental Protection Administration of China (SEPA), World Health Organization (WHO) and Food Agriculture Organization (FAO). Different statistical tools were used for the identification of sources of selected heavy metals in the study area. Furthermore, the study aimed to select the best hyperaccumulative plant species and create awareness among the local population regarding potential health risk of heavy metal contamination. The current study investigates the level of heavy metals contamination in the flora associated with serpentine soil adjoining the Mafic and Ultramafic rocks Northern-Pakistan. Soil and wild native plant species were collected from chromites mining affected areas and analyzed for heavy metals (Cr, Ni, Fe, Mn, Co, Cu and Zn) using atomic absorption spectrometer (AAS-PEA- 700). The heavy metal concentrations were significantly (p<0.001) higher in mine impacted soil as compared to reference soil, however Cr and Ni exceeded maximum allowable limit (250 and 60 mg kg-1, respectively) set by SEPA for soil. Inter-metal correlations between soil, roots and shoots showed that the sources of metals contamination were mainly associated with chromites mining. All the plant species accumulated significantly higher concentrations of heavy metals as compared to reference plant. The open dumping of mine wastes could create serious metals contamination of food and drinking water sources for local community of the study area. The native wild plant species (Nepeta cataria, Impatiens bicolor royle, Tegetis minuta) growing on mining affected soils can be used for phytoremediation of affected soils contaminated with heavy metals. Medicinal, fodder plants and soil samples collected from the study area were analyzed for Pb and Cd concentrations using AAS Perkin Elmer, AAS-PEA-700. Soil pollution load indices (PLI) values more than 2 showing high contamination levels for both Cd and Pb. Furthermore, Cd concentrations in the soil surrounding the mining sites exceeded MAL (0.3 mg kg-1), while the concentrations of Pb were lower than its MAL set by SEPA for agriculture soil. The Pb and Cd concentrations were significantly higher (p<0.001) in soil of mining contaminated sites as compared to the reference site, which can be attributed to the dispersion of toxic heavy metals, present in the bed rocks and waste of the mines. The concentrations of Pb and Cd in majority of medicinal and fodder plant species grown in surrounding areas of mines were higher than their MALs set by WHO /FAO for herbal and edible plants. The high concentrations of Cd and Pb may cause contamination of the food chain and health risk. Phytoremediation is an environmentally friendly and economically feasible technique for the reclamation of environment polluted with toxic heavy metals. Therefore the next study was designed to evaluate the heavy metal concentration (Cr, Ni, Co, Fe, Mn, Cu and Zn) in chromite mining affected soil and native plant species by using AAS. The contaminated soils from Qala and Heroshah site soil showed multifold enrichment in heavy metals and PLI > 5. Soil and plant species from chromite affected areas have significantly (p<0.001) high metal concentrations as compared to reference site. Among the contaminated sites, Heroshah was highly contaminated than Qala. These high metal concentrations could be due to chromite mining and open dumping of mine wastes. High metal concentrations in the study area may cause potential threat to the local community and grazing animals. Phytoremediation potential of plant species like Solanum. surrattense, Dedonia. viscosa, Raziya. stracta, Calotropis. procera and Artemisia scoparia can be used to remediate chromite contaminated soils, though further investigation is still needed in terms of field and lab experiments. Organic amendment in soil has been proposed to immobilize heavy metals and minimize bioaccumulation. Another study was conducted to investigate the effects of different amendments (ACP2, ACP5, ACG2, ACG5, FYM2 and FYM5) on heavy metals such as (As, Cd, Ni, Cr, Pb, Mn, Cu, Fe, Zn, Co and Al) in mine impacted soil and plants cultivated under controlled conditions . The results showed that almost all the amendments significantly (p≤0.001) reduced the bioavailability of heavy metals in soil. The bioaccumulation of heavy metals by Penisitum americanum and Sorghum bicolor significantly (p≤0.001) reduced with all the amendment except (Zn and Cd) concentration increased with FYM2 and FYM5. The plant growth was significantly reduced with ACP2, ACP5, ACG2 and ACG5 amendment, while FYM2 and FYM5 significantly (p≤0.001) increased the plant growth as compared to the control. Among the amendments, ACP5 was the best one in reducing bioavailability and bioaccumulation of toxic metals in plants.