Phytoremediation potential of castor (Ricinus communis L.) in the soils of the abandoned copper mine in Northern Oman: implications for arid regions

Contamination levels of copper (Cu) and other heavy metals are very high in the soils of the abandoned copper mine of Lasail in the north western Hajar Mountains of Oman. Environment-friendly approaches such as phytoremediation are needed to clean and rehabilitate these areas to their natural status. In the present study, the phytoremediation potential of castor, Ricinus communis L., was evaluated for copper and other heavy metals by growing it in different types of Cu-mine soils and slags. Growth parameters such as shoot height and biomass weight (fresh and dry) were evaluated. Castor showed a high tolerance index (TI) in Cu-mine soils. The highest TI was calculated for the fresh mass of castor shoots in E soil with a percentage of 405.99. The translocation factor (TF) of all the metals except boron (B) and manganese (Mn) was < 1, which reveals that these metals are stabilised in the root portion of the castor. Bioconcentration factor (BCF) value < 1 for Cu indicates that castor is not a hyperaccumulator plant for copper. In addition to high concentrations of copper, other heavy metals such as arsenic (As), iron (Fe), and zinc (Zn) were observed in the roots than in shoots. Castor grown in slag accumulated Cu in the shoots, roots, and entire plant with the values of 25, 1184, and 1209 mg kg⁻¹, respectively. Similarly, castor cultivated in A soil accumulated 18, 901, and 919 mg kg⁻¹ of copper in shoots, roots, and entire plant, respectively. The calculated plant effective number (PENt) indicated the need for 253 castor plants to remove 1 g of Cu from E soil. The ability of castor to grow well in Cu-mine soils suggests that it can be used for the removal of Cu and other heavy metals. Additionally, the shoot portion could potentially be used for oil production since the phytoaccumulation levels of heavy metal concentration in the shoots were below the standard toxicity limits.