Phytostabilization of heavy metals from rice mill effluent using vetiver grass (Chrysopogon zizanioides) in vertical-flow constructed wetlands

Abstract

The discharge of untreated rice mill effluent (RME) threatens aquatic ecosystems and public health, necessitating sustainable treatment solutions. This study evaluated the phytoaccumulation of heavy metals from parboiled RME by vetiver grass (Chrysopogon zizanioides) in a 90-day mesocosm vertical-flow constructed wetland experiment. RME was characterized for physicochemical parameters and heavy metals (As, Cd, Cr, Ni, Cu, Fe, Pb, Zn) using atomic absorption spectroscopy. Vetiver was exposed to six effluent concentrations (0, 20, 40, 60, 80, 100% v/v). Raw effluent contained elevated nickel (12.78 mg/L) and manganese (15.65 mg/L) levels, substantially exceeding permissible limits. Vetiver exhibited dose-dependent phytotoxicity: plant height, stem diameter, and biomass declined by 25.6%, 19.5%, and 51.1%, respectively, at 100% concentration. Root Bioconcentration Factors (BCF) exceeded 1 for all metals (range: 1.33–2.48), while Translocation Factors (TF) were consistently below 0.20, confirming phytostabilization as the primary mechanism. The system achieved metal removal efficiencies of 39.2% (Pb) to 68.7% (Cd). Strong negative correlations (r = -0.86 to -0.99) linked root metal burden with growth suppression. The 40–60% effluent range was identified as the optimal operational window. Vetiver-based constructed wetlands offer an effective, low-cost strategy for treating RME in developing nations.