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 <i>Pistia stratiotes</i> L. and <i>Lemna minor</i> 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 <i>Lemna minor</i> L (98.1%) and <i>Pistia stratiotes</i> L. (96.8%) which were decreased with increasing EC and acidic pH. <i>P. stratiotes</i> 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 <i>L. minor</i> and <i>P. stratiotes</i> 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 <i>P. stratiotes</i> 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 <i>L. minor</i> 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 <i>P. stratiotes</i> exhibited greater overall adaptability across pH conditions while <i>L. minor</i> performed better under saline environments. Therefore, <i>Pistia stratiotes</i> is recommended for remediation of metal-contaminated waters across variable pH regimes, whereas <i>Lemna minor</i> is better suited for saline and alkaline environments.