Withania somnifera Dunal-mediated dissipation of lindane from simulated soil: implications for rhizoremediation of contaminated soil

Lindane is an organochlorine chemical that has been used both as an agricultural insecticide and as a treatment for head lice and scabies. It is a neurotoxin that interferes with GABA neurotransmitter function. In humans, lindane primarily affects the nervous system, liver, and kidneys and may be a carcinogen and/or endocrine disruptor. Currently, India is the largest consumer and producer of lindane in the world. Due to its continuous use and indiscriminate industrial production, lindane-contaminated soils are widespread in the country. Apart from India, historical lindane production sites were found in Austria, France, Spain, Bulgaria and in China, Turkey, and the former USSR. Before 1984, lindane was also manufactured in the German Democratic Republic, Poland, Yugoslavia, Romania, and Hungary; since then, all production has been stopped in Germany, Japan, The Netherlands, the UK, and the USA. Because of its worldwide use for more than 50 years, lindane-contaminated soils can be found in most countries of the world. Although many countries have restricted or eliminated its usage, obsolete stock piles continue to pose a threat to various ecosystems and human health. Physical, chemical, and biological methods can all be used for the remediation of contaminated sites, but phytoremediation is now recognized as a cost-effective method for the decontamination of soil sites. The present study examines the potential of Withania somnifera Dunal (previously shown to accumulate lindane from contaminated industrial area; Abhilash et al., Chemosphere 72:79-86, 2008) to take up lindane (gamma-HCH) and the subsequent plant-mediated dissipation of lindane from an artificially contaminated soil. The study species was grown in four simulated concentrations (5, 10, 15, and 20 mu g g(-1)) of lindane. Each treatment was prepared in triplicate. In addition, two control treatments were established: vegetated control (non-contaminated soil planted with W. somnifera) and non-vegetated control [contaminated soil (prepared in above said concentrations) without plants]. Pots were harvested after 21, 50, and 145 days. Plant growth, biomass, chlorophyll, protein, carotenoids content, microbial biomass carbon, lindane concentrations in plant parts, residual lindane concentrations in soil, and percentage lindane dissipation from soil were determined after every harvest. Lindane accumulation potential of W. somnifera per acre was calculated based on the mean dry matter production of the plant multiplied by mean lindane accumulation potential and the number of plants that can be planted per unit area to optimum planting density. Plant growth (root length, shoot length, and dry matter production) decreased with increasing lindane concentration. At 145 days, the dry matter production in 5, 10, 15, and 20 mu g g-(1) of lindane was reduced to 7%, 9%, 11%, and 20% of control plants, respectively. Similarly, there was a significant reduction in chlorophyll contents and soluble proteins in various treatments at each harvest. In contrast, carotenoids content increased with exposure time and lindane treatments. After 145 days, the accumulation of lindane in four spiked concentrations reached up to 8.4, 14.2, 26.8 and 45.0 A mu g g(-1) dry matter, respectively. Regardless of the lindane treatment, maximum accumulation occurred in roots followed by stems and leaves (p < 0.01). In contrast, lindane was not detected in the roots of control plants. However, low levels of lindane were detected in shoot and leaf (0.98 and 1.35 A mu g g(-1) dry matter) matrices of control plants. Although the growth of the plants was affected by lindane, W. somnifera survived in all spiked soils without any visible toxic symptoms. After final harvest, lindane concentrations in the 5-, 10-, 15-, and 20-mu g g(-1) treatments were reduced to 0.83, 2.0, 3.53, and 5.38 mu g g(-1), respectively. This corresponds to a dissipation of 83%, 80%, 78%, and 73% in the four different lindane treatments. In contrast, a significantly (p < 0.001) lower dissipation was observed in non-vegetated controls: 40%, 35%, 32%, and 30%, respectively. These differences in lindane dissipation between vegetated and non-vegetated soils were correlated with their respective microbial biomass carbon, suggesting that W. somnifera assisted in the enhanced dissipation of lindane due to an enhanced rhizospheric microbial activity. Based on the present study, it was estimated that W. somnifera can accumulate 764-944 mg of lindane per acre after 145-day cultivation. However, the plant-mediated dissipation of lindane (phytostimulation) is the major contribution of this species, leading to the enhanced remediation (rhizoremediation) of contaminated soil (> 73%). However, other processes such as volatilization or adsorption cannot be discarded (Kidd et al., Plant Soil 302:233-247, 2008). W. somnifera can be used for the remediation of lindane contaminated soils. However, suitable agronomic practices are essential for the successful implementation of this venture. Density of planting is a key factor determining the successful growth of plants. It is obvious that plants cannot grow well in contaminated area. Therefore, overcrowding will cause a negative effect on plants growth which will ultimately reduce their remediation potential. A spacing pattern of 1 x 1 m is suggested so that a maximum of 4,000 plants can be planted per acre (however, more agronomic trials are required to get an optimum planting density). Further, the accumulation and dissipation potential of plants can be enhanced by suitable soil amendments (e.g., addition of organic acids; White et al., Environ Pollut 124:71-80). However, field trials are needed to establish the on-site remediation potential of W. somnifera. Furthermore, additional investigations are needed to understand the catabolic degradation of lindane in W. somnifera.