Assisted phytoremediation of heavy metal contaminated soil from a mined site with Typha latifolia and Chrysopogon zizanioides

Highlights

• Gold mining activities lead elevated levels of heavy metals.

• EDTA and Al₂(SO₄)₃ amendments enhanced phytoremediation of heavy metals.

• Effects of amendments varied depending on metal and phytoremediation plant in question.

• Amendments had minimal effects on plant growth performance.

Abstract

Chemically assisted phytoremediation is fast gaining attention as a biotechnology to accelerate heavy metal removal from contaminated substrates, but how different chemical amendments affect the process remains an important research question. Here, bioaccumulation factor (BAF), translocation factor (TF), removal efficiency (RE) and uptake of Hg, As, Pb, Cu and Zn by cattail (Typha latifolia) and vetiver (Chrysopogon zizanioides) were quantified in a potted experiment to determine the effects of amendments on the phytoremediation success. Baseline concentrations of heavy metals within the studied mined site were determined. The experiment involved three soil treatments (each comprising 16 samples amended with 0.05 mol/L ethylene di-aminetetraacetic acid (EDTA), 3 g of aluminum sulfate [Al₂(SO₄)₃], and unamended control) transplanted with equal numbers of vetiver and cattail. Growth performance (height) of plant species was monitored every two weeks. Sixteen weeks after transplanting, heavy metal levels in plant and soil samples were quantified following standard protocols, and the biomass and root length measured for each plant species. Results indicated strong negative impact of mining activities on heavy metal levels of soil in the study area. Soil amendment considerably enhanced the BAF, TF, RE and uptake but the effect varied with plant species and heavy metal in question. The amendment also stimulated strong positive correlation between RE and BAF, TF and metal uptake, and generally did not show any negative effects on plant growth performance. In general, soil amendment aided the accumulation and translocation of heavy metals in the plant species studied, and could be explored for cleaning up contaminated sites.

Introduction

The mining industry plays an important role in the socio-economic and technological development of nations around the world (Amponsah-Tawiah and Dartey-Baah, 2011; Mobtaker and Osanloo, 2014). The sector contributes directly to export revenue, gross domestic product (GDP), corporate tax earnings and government revenue, and indirectly through the implementation of corporate social responsibility programmes, technology transfer and provision of employment to people (Aryee, 2012, Dorin et al., 2014). Nevertheless, the mining industry by its very nature is considered a “foot print industry”, and hence leaves significant environmental, social and economic footprints wherever it finds itself (World Bank Group Mining Department, 2002). Mining activities, for example, disturb the natural biogeochemical cycles of metals and subsequently contaminate soil, as well as ground and surface water resources with the mobilized heavy metals (Cobbina et al., 2013, Quarshie et al., 2011).

While most organic contaminants are biodegradable and pose no permanent risk to ecosystems, inorganic contaminants such as heavy metals are non-biodegradable and tend to persist in the environment (Ghosh and Singh, 2005). It has also been established that, though some heavy metals are essential to the function of living things, all metals can present serious health problems to humans and animals above certain threshold limits by causing oxidative stress to living cells (Ghosh and Singh, 2005, Malayeri et al., 2008, Singh et al., 2011). Environmental and health effects associated with heavy metal contamination become even more worrying given their tendency to accumulate and magnify along trophic levels.

Traditionally, these negative effects of heavy metal contamination in the environment have been mitigated with methods such as membrane filtration, electrochemistry, oxidation-reduction processes and soil washing (Hanif and Bhatti, 2015). However, some of these traditional processes sometimes prove too costly and environmentally unfriendly. Consequently, cost-effective and greener biotechnologies such as phytoremediation, which utilizes the natural abilities of plants to immobilize, degrade, reduce or remove heavy metal contaminants from the environment, have gained considerable research attention in recent years (Baker et al., 1994, Nowack et al., 2006). Studies shows that this technology can successfully and effectively remediate contaminated substrates such as industrial wastes, effluents, soil and water (Hegazy et al., 2011, Yeboah et al., 2015).

Nevertheless, application of this biotechnology in remediating heavy metals from soil can be challenging and less successful sometimes even with traditional phytoremediation plants (Raskin et al., 1994). This situation arises because heavy metals in soil are generally bound to organic and inorganic constituents, or are present as insoluble precipitates, and hence, are unavailable for phytoextraction (Raskin et al., 1994, Henry, 2000, Shahid et al., 2014). Reduced availability of heavy metals in soil reduces the efficiency of even traditional phytoremediation plants and also increases considerably the time required for phytoremediation (Ghosh and Singh, 2005, Shahid et al., 2014).

Methods aimed at increasing bioavailability of heavy metal contaminants in soil have therefore become vital for the success of phytoremediation of metal-contaminated soils (Ernst, 1996). Soil amendment with acidifiers, commercial nutrients or some chelates like ethylene di-aminetetraacetic acid (EDTA) and di-ethylene tri-aminepentaacetic acid (DTPA) (Ebbs et al., 1997, Ali et al., 2013, Shahid et al., 2014) have been shown to disassociate heavy metals from soil compartments into soil solution, making them more available for remediation. Though chemically assisted phytoremediation has received considerable attention from researchers, it effects on the success of phytoremediation of different metals using different plant species is still poorly understood. Understanding the effectiveness of these chemical amendments on phytoremediation of heavy metal contaminated soils resulting from mining activities with different plants is thus essential in assessing the viability of the biotechnology for mitigating the environmental impacts associated with gold mining activities.

In Ghana, heavy metal contamination resulting from mining activities has a history as long as the liberalization of the gold mining sector in the mid-1980's and the subsequent use of extraction methods that invariably release mercury and other heavy metals into surrounding water sources and soil (Asante et al., 2007, Essumang et al., 2007, Armah et al., 2010, Obiri et al., 2010). This contamination and its impact on host communities have spurred numerous studies on phytoremediation mostly on the identification of local plants (e.g., Anning et al., 2013). However, studies on how to enhance this process through chemical amendment are lacking in the country. In this study, we evaluated the effects of soil amendments (EDTA and Al₂(SO₄)₃) on phytoremediation of Hg, As, Pb, Zn, Cu by cattail and vetiver. To this end, the effects of these soil amendments on bioaccumulation factor (BAF), translocation factor (TF), metal uptake (MU), removal efficiency (RE) and plant growth of vetiver and cattail were evaluated to ascertain their viability for enhancing phytoremediation. It was hypothesized that soil amendments would enhance the efficiency of phytoremediation depending on the plant species and heavy metal in question.