Elucidating the phytoremediation potential of aquatic macrophytes for Cd, Cr, and Pb under varying pH and salinity

Water contamination by heavy metals threatens human and environmental health. This study evaluated Pistia stratiotes L. and Lemna minor L. for remediation of cadmium (Cd), chromium (Cr VI), and lead (Pb) applied at the rate of 0, 5, 10, and 20 mg L^-1 under varying pH (6-8) and electrical conductivity (EC; 1,450 and 2,150 µS cm^-1) in Hoagland solution. Higher moisture content was observed at neutral pH in Lemna minor L (98.1%) and Pistia stratiotes L. (96.8%) which were decreased with increasing EC and acidic pH. P. stratiotes sustained comparatively higher moisture between pH 6-8, proving its adaptability to pH stress. Heavy metal stress significantly decreased growth of both aquatic macrophytes. Higher growth (75% and 54.5%), tolerance index (41.9% and 62.1%), and plant growth rate (0.16 and 0.07) were recorded in L. minor and P. stratiotes respectively, at neutral pH. However, acidic pH and increasing heavy metal content adversely affected mentioned parameters. The potential of macrophytes for heavy metal absorption/uptake significantly varied between the species depending upon pH and EC. The P. stratiotes absorbed higher Cr (1,614 mg kg^-1), Pb (1,469 mg kg^-1), and Cd (487 mg kg^-1) at pH 6-7, while L. minor showed higher accumulation (1,644 mg Cr kg^-1, 617 mg Cd kg^-1 and 593 mg Pb kg^-1) under salinity stress. Both species showed hyper-accumulation having a bio concentration factor (BCF) > 1,000) of Cd, Cr, and Pb under acidic pH and elevated EC. The P. stratiotes exhibited greater overall adaptability across pH conditions while L. minor performed better under saline environments. Therefore, Pistia stratiotes is recommended for remediation of metal-contaminated waters across variable pH regimes, whereas Lemna minor is better suited for saline and alkaline environments.