The Comparison of Fresh and Dry Duckweed (Lemna minor L.) on Metal (Cr6+, Cd2+, and Zn2+) Removal from Wastewater

Heavy metals contaminating the environment is a global concern. Duckweed (Lemna minor) is a promising plant for the phytoremediation and biosorption of metal-contaminated water. Although studies have shown that duckweed can remove multiple metals, there is limited research comparing the efficiency of fresh and dried biomass for wastewater treatment. To evaluate the performance of both forms, fresh and dried duckweed were exposed to metal solutions containing varying concentrations of Cr6+, Cd2+, and Zn2+ (5 mg/L Cr6+ + 1 mg/L Cd2+ 10 mg/L Zn2+; 10 mg/L Cr6+ + 5 mg/L Cd2+ + 50 mg/L Zn2+; or 50 mg/L Cr6+ + 25 mg/L Cd2+ + 250 mg/L Zn2+) for a duration of 168 h. Metal uptake in fresh duckweed followed zero-order kinetics for Cr6+, Cd2+, and Zn2+ sequestration or Michaelis–Menten kinetics for Cd2+ and Zn2+ uptake, rather than a first-order model. In contrast, dried duckweed reached equilibrium more rapidly, within 4–48 h, exhibiting pseudo-second-order kinetic and fitting the Langmuir isotherm model. Zn2+ reached equilibrium the fastest (4 h), Cd2+ required 4–24 h, and Cr6+ required up to 48 h to reach equilibrium. In general, fresh duckweed uptakes more metals over the 168 h period, depending on the metal type and concentration. However, dried duckweed demonstrated a rapid remediation capability. The findings highlight the complementary potential of applying both fresh and dried duckweed for wastewater treatment.