Mercury (Hg) and other heavy metals, such as arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), lead (Pb), manganese (Mn), and zinc (Zn), were analyzed in coal (bituminous), roadside soil, reclaimed mine soil, core zone soil, and reference soil (agriculture soil) along three soil profiles (0–10, 10–20, and 20–30 cm) in a coal mining area (Jharkhand, India) inhabited by economically marginalized dense population. Higher toxic metal concentrations in coal (0.47 mg Hg/kg, 2.81 mg As/kg, and 13 mg Pb/kg, 2.61 mg Cd/kg) can cause ecological hazard in the area. Roadside soil has the highest Hg (2.32 mg/kg), and core zone soil has the highest Pb (15.7 mg/kg) concentration. Hg concentration in roadside soil indicates high potential ecological risk, while Cd contamination in mining area can be put under moderate ecological risk. Roadside soils shows highest Ecological Risk Index (ERI) value of 339, indicating considerable ecological risk, and Hg alone contributes 64% of ERI value. Higher ERI and (Hg) were due to the deposition of coal dust from mining activities and transportation.

A Phytoremediation experimental set up was established by spiking the soil with varying concentrations of mercury (Hg) (Treatment: T1:10; T2:50; T3:100; T4:500 and T5:1,000 mg Hg/kg soil). Hg removal ability of the Indian mustard plant was determined after 30, 60 and 90 days of exposure. Hg accumulation trend in second and third month of exposure was root > leaf > stem, while for the 1st month it was root > stem > leaf. The highest percentage of Hg accumulation (81%) and glutathione (14 mg/kg) was observed in the plants of T4 and T5 treatment, respectively at 90 days of exposure indicating a high level of Hg stress tolerance. At 90 days of exposure the chlorophyll a content in leaves grown on Hg-free soil (control soil) was 1.8, 2.4, 2.8, 3.6 and 4.4 fold higher than T1, T2, T3, T4 and T5 treatment respectively. With increase in exposure duration, translocation factor decreased whereas bioconcentration factor increased signifying Hg is mainly accumulated in the roots. The study suggests that Brassica juncea can withstand under high Hg contamination and can show great potential to phytostabilize Hg when grown under 100 mg/kg of soil Hg without showing any significant detrimental effect on the plant.

This paper reviews research on phytoremediation (2002–2021), particularly for the estimation of plant efficiency and soil pollution indices, examining the extraction of metals from soil and plants growing under both artificial (spiked with specific metal) and natural conditions. From the analysis of >200 published experimental results, it was found that contamination factor and geo-accumulation index as well as translocation and bioconcentration factors are the most important soil pollution and plant efficiency metrices, respectively, which are gaining importance to assess the level of metal pollution and its transfer from soil to plant to find a better metal clean-up strategy for phytoremediation. To access the metal concentration, it was found that the most widely accepted extractants to dissolve and extract the metals from the soil and plant were HNO₃ and HClO₄ (mainly in 5:1; v/v or 4:1; v/v), which are used both in natural and artificial metal contamination studies. Moreover, plants such as Pteris vittata, Monochoria korsakowi, Lolium perenne, Festuca rubra, Poa pratensis, Ricinus communis, and Siegesbeckia orientalis can act as hyperaccumulators under both natural and artificial experiments and can be directly implemented into the fields without checking their further efficiency in phytoremediation.