The comparison of fresh and dry duckweed to remove Cr, Cd and Zn from wastewater

Duckweed is recognized as a promising plant for the phytoremediation or biosorption of metal-contaminated water due to its rapid growth and high biomass. However, there is limited research on the simultaneous removal of multiple metals and the comparison of the efficiency between fresh and dried duckweed in wastewater treatment systems. This study investigated the effectiveness of both fresh and dried Lemna minor in removing metals such as Cr⁶⁺, Cd²⁺, and Zn²⁺ from contaminated water.

Fresh and dried duckweed were cultured in metal solutions with varying concentrations (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 168 hours to assess metal uptake and biosorption processes. The results indicated that metal uptake in fresh duckweed conformed to a zero-order model, while metal adsorption to dried duckweed followed a pseudo-second-order model, reaching saturation between 4 and 48 hours. This further supports the idea that living duckweed removes metals through active physiological uptake, whereas dry biomass adsorbs metals via passive biosorption mechanisms on its surface. The adsorption of metals onto dried duckweed was further characterized by the Langmuir isotherm model, indicating monolayer adsorption on a homogeneous surface with uniform binding sites

Overall, fresh duckweed removed a greater amount of metals compared to dried duckweed over the 168-hour period, with removal efficiency ranging from 5.59% to 63.48%, depending on the type and concentrations of metals in the solution. These findings provided valuable insights for optimizing the use of duckweed in the remediation of metal-contaminated wastewater. However, since duckweed serves as food for many wildlife species, it is also important to find effective ways to harvest and manage duckweed biomass contaminated with heavy metals to reduce ecological risks.