Induced Phytomanagement of Multi-Metal Polluted Soil with Conocarpus erectus Supported by Biochar, Lignin, and Citric Acid

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Induced heavy metals (HMs) phytoextraction from heavily contaminated soils is challenging, as high HM bioavailability causes phytotoxicity and leaching. This study introduces a novel approach for HM immobilization with biochar (BC) and lignin (LN), and later their controlled mobilization with citric acid (CA) in soil. Conocarpus erectus was grown for 120 days in shooting-range soil (SS) polluted with Pb, Cr, Cd, Ni, and Cu. HM concentrations in parts of the plants, their percentage removal, and leaching from SS were measured. Moreover, plant biochemical parameters such as the contents of chlorophyll a (Chl-a), chlorophyll b (Chl-b), protein, ascorbic acid (AsA), amino acids, and total phenolics, along with biophysical parameters such as relative water content (RWC) and water uptake capacity (WUC), were also inspected. Adding BC, LN, and BC+LN to SS improved biomass, as well as the biophysical and biochemical parameters of plants, while efficiently reducing HM concentrations in plant parts, DTPA extract, and leachates compared to the control (CK). However, the greatest amplifications in plant height (82%), dry weight of root (RDW) (109%), and dry weight of shoot (SDW) (87%), plant health, and soil enzymes were noted with the BC+LN+CA treatment, compared with the CK. Moreover, this treatment resulted in Pb, Cr, Cd, Ni, and Cu removal by 68, 30, 69, 59, and 76% from the SS compared to the CK. Surprisingly, each HM concentration in the leachates with BC+LN+CA was below the critical limits for safer water reuse and agricultural purposes. Initial HM immobilization in HM-polluted soils, followed by their secured mobilization during enhanced phytoextraction, can enhance HM removal and reduce their leaching without compromising plant and soil health.