Metal-tolerant and -sensitive Azolla in synthetic wastewater: Phytoremediation potential and biochemical responses to metal stress

This study evaluated the phytoremediation potential and biochemical responses of Azolla imbricata exposed to 0–10 % multi-metal-contaminated synthetic wastewater (SWW). Azolla biomass decreased by up to 72.6 % in a concentration-dependent manner. Positive and negative relative growth rates at low (0.01–0.1 %) and high (1–10 %) SWW exposures indicate the tolerance and sensitivity of Azolla, respectively. The anthocyanin content increased markedly (by 122–330 %) with increasing SWW concentration, reflecting a stress-induced response. All chlorophyll fractions declined up to 0.1 % SWW but rose unexpectedly (>40 %) at higher concentrations, probably because of pigment release from dying tissues. Carotenoid content, an indicator of antioxidative defense, consistently decreased (up to 56 % relative to the control). The concentration (µg/L) of metals in SWW followed the order Pb (2527.4)> Cu (424.8)> Zn (354.6)> Cd (141.2)> As (114.7). Azolla accumulated these metals in a similar pattern, except for Cu and Pb, for which the accumulation was reversed, likely because of the higher uptake affinity of the plant for Cu. Tolerant Azolla accumulated more metals than sensitive Azolla , except for Cd, which was higher in the sensitive group. Most bioconcentration factor values were < 1, indicating that Azolla primarily acts as a metal excluder. A slight increase in pH and electrical conductivity after exposure suggested a mild modulatory effect of Azolla on water chemistry. Overall, the exposure concentration, plant tolerance, and harvest timing strongly influenced the efficiency of phytoremediation. Azolla is promising for low-strength SWW; however, highly polluted wastewater may require pre-treatment or integrated remediation. • The metal accumulation capacity of Azolla was assessed in this study. • Azolla was exposed to multi-metal-contaminated synthetic wastewater (SWW). • Azolla showed tolerance at low and sensitivity at high SWW levels. • A bioconcentration factor of less than 1 indicates Azolla acted as a metal excluder. • Azolla altered the pH and electrical conductivity of SWW after 7 days of exposure.