Acid orange 74 (AO74) is a chromium-complex monoazo acid dye widely used in the textile industry. Due to being highly toxic and non-biodegradable, it must be removed from polluted water to protect the health of people and the environment. The aim of this study was two-fold: to evaluate the biosorption of AO74 from an aqueous solution by utilizing HCl-pretreated Lemna sp. (HPL), and to examine dye desorption from the plant material. The maximum capacity of AO74 biosorption (64.24 mg g^-1) was reached after 4 h at the most adequate pH, which was 2. The biosorption capacity decreased 25% (to 48.18 mg g^-1) during the second biosorption/desorption cycle and remained essentially unchanged during the third cycle. The pseudo-second-order kinetics model concurred well with the experimental results of assays involving various levels of pH in the eluent solution and distinct initial concentrations of AO74. NaOH (0.01 M) was the best eluent solution. The Toth isotherm model best described AO74 biosorption equilibrium data. FTIR analysis confirmed the crucial role of HPL proteins in AO74 biosorption. SEM-EDX and CLSM techniques verified the effective biosorption/desorption of the dye during the three cycles. Therefore, HPL has potential for the removal of AO74 dye from wastewaters.

The biosorption ability of Lemna gibba for removing Ni²⁺ and Zn²⁺ ions in aqueous batch systems, both individually and simultaneously, was examined. The influences of solution pH and initial single and binary metal concentrations on equilibrium Ni²⁺ and Zn²⁺ biosorption was explored. The optimal solution pH for Ni²⁺ and Zn²⁺ biosorption was 6.0, for both the single and binary metal systems. Ni²⁺ and Zn²⁺ biosorption capacities increased with increasing initial metal concentrations. The presence of Zn²⁺ ions more adversely affected the biosorption of Ni²⁺ ions in the binary metal systems than vice versa. The single and binary biosorption isotherms of Ni²⁺ and Zn²⁺ revealed that L. gibba’s affinity for Zn²⁺ ions was higher than that for Ni²⁺ ions. The Redlich–Peterson and Freundlich isotherm models fit well to the experimental equilibrium data of Ni²⁺ ions, whereas Redlich–Peterson and Langmuir models better described the equilibrium data of Zn²⁺ ions in single metal systems. The modified Sips isotherm model best fit the competitive biosorption data of Ni²⁺-Zn²⁺ on L. gibba. FTIR analyses suggest the involvement of hemicellulose and cellulose in the biosorption of Ni²⁺ and Zn²⁺. The presence of Ni²⁺ and Zn²⁺ on the L.gibba surface was validated by SEM-EDX.

This work explored the technical feasibility of using crab shell (CS) as a promising, low-cost biosorbent to individually and simultaneously remove Zn²⁺ and Ni²⁺ from aqueous solutions. It was found that in both monometallic and bimetallic systems, Zn²⁺ and Ni²⁺ biosorption by CS was strongly dependent on the solution pH, with the optimum biosorption occurring at a pH of 6.0 for both heavy metals. The obtained isotherms for Zn²⁺ and Ni²⁺ biosorption onto CS in monometallic and bimetallic systems demonstrated that CS has a higher affinity for Zn²⁺ than for Ni²⁺. The experimental equilibrium data for the bimetallic system revealed that when one heavy metal is present in the system, there is a decrease in the equilibrium biosorption capacity for the other heavy metal; therefore, the combined action of Zn²⁺ and Ni²⁺ was antagonistic. The Sips and Redlich–Peterson isotherm models best fitted the equilibrium biosorption data for Zn²⁺ and Ni²⁺ in the monometallic systems, while the modified Sips model best fitted the binary biosorption equilibrium data. DRIFTS analyses indicated that carbonate ion, chitin, and proteins are mainly involved in the biosorption of Zn²⁺ and Ni²⁺ by CS from aqueous solutions, as confirmed using a range of analytical techniques.