Hybrid Immobilization–Phytoremediation Approach for Constructed Wetlands Minimizing the Geoenvironmental Impact of Chromium-Contaminated Soil

Abstract

Soil contamination by heavy metals is a global concern due to their enduring persistence and inherent toxicity. Constructed wetlands represent a cost-effective and dependable green alternative to traditional mechanical remediation systems. This study utilized a hybrid approach, employing Chrysopogon zizanioides (vetiver) root micro powder (VRMP) for chromium immobilization in contaminated soil, in conjunction with Coleus amboinicus (Indian borage) plants to enhance soil remediation efficacy. The findings propose an improved strategy for adoption in constructed wetlands. The bioavailability and phyto availability studies were carried out on unamended and contaminated soil amended with 1%, 5%, and 10% of VRMP. Column leaching studies were conducted using a custom-built setup, and batch investigations using the Toxicity Characteristic Leaching Procedure (TCLP) procedure were employed to assess the bioavailability. Phyto availability studies utilized Indian borage plants in greenhouse pot experiments. Increasing the amendment dosage from 0% to 10% led to a rise in pH from 3.06 to 3.5, an enhancement in organic matter content from 1.31% to 21.86%, and an augmentation in cation exchange capacity from 12.6 to 24.68 cmol/kg. Simultaneously, there was a reduction in electrical conductivity from 3,090 to 1,080 μS/m. These alterations contributed to the diminished mobility of chromium within the soil matrix. For contaminated soil amended with 10% VRMP, chromium immobilization was achieved at rates of 95.32% in the continuous flow leaching experiment, 90.84% in pore volume analysis, 98.6% in the pot leachate study without plants, 99.67% with plants, and 93.48% in the TCLP in comparison to the levels observed in the unamended contaminated soil. Likewise, 97.37% of chromium was sequestered by the application of 10% VRMP, resulting in significantly lower chromium accumulation in the Indian borage plant compared to the untreated contaminated soil. The outcomes from column experiments, TCLP extraction, phytoremediation, and phyto availability studies collectively suggest that the hybrid immobilization phytoremediation approach stands out as a promising method for remediating chromium-contaminated soil and the principle can be adopted on a large scale in constructed wetlands.