Biosorption Characteristics For Pb～(2+), Cu～(2+), Mn～(2+) Onto Aquatic Plant: Lemna Minor

A new treatment method of heavy metal removal has gained interest due to its low cost and high efficiency during recent years. The biosorption technique is of broad developing perspective in heavy removal and recycle. Advanced research on this subject will bring great economic value and social benefit.This study mainly investigates heavy metal ions (Pb²⁺, Cu²⁺, Mn²⁺) adsorption of free-floating aquatic plant(Lemna minor) as a biosorbent. Equilibrium isotherm was analyzed using the Langmuir and the Freundlich isotherms. In order to investigate the mechanism of sorption adsorption data was modeled using the intraparticle diffusion model, the pseudo-first-order and pseudo-second-order equations. The results indicate as follows:Firstly, the duckweed(Lemna minor) is able to sorb Pb²⁺, Mn²⁺, Cu²⁺ from aqueous solutions efficiently and rapidly. Sorption of duckweed reached equilibrium in 30 min for Pb²⁺ and 90min for Mn²⁺ and Cu²⁺. The removal efficiency of this plant for Pb²⁺, Cu²⁺ and Mn²⁺ were 97.83%, 1.63%, 87.11% respectively. According to the evaluation using the Langmuir equation, the maximum sorption capacities of metal ions onto duckweed were 34.01 mg·g^-1 for Pb²⁺, 6.09 mg·g^-1 for Cu²⁺, 7.22 mg·g^-1 for Mn²⁺. The maximum specific area for duckweed for binding Pb²⁺, Cu²⁺, Mn²⁺ were 8.26m²Â·g^-1, 1.30m²Â·g^-1, 1.54m²Â·g^-1, respectively. The soption capacity of duckweed follows the patterns of Pb²⁺>Mn²⁺>Cu²⁺. The negative values ofâ–³G (free energy change)of three metals at 25℃indicated that the spontaneous nature of adsorption.Secondly, both methods of linear and non-linear fitting was applied in different fitting equations to describe the equilibrium sorption isotherms respectively . it was found that the Langmuir model is more suitable to describe the sorption isotherms of Pb²⁺, Mn²⁺, Cu²⁺ biosorption by duckweed through the comparison of value of the linear regression coefficient (R²), non-linear regression coefficient (r²), Chi-square test(X²). There is a great difference between the experiment data and the calculated value of the Freundlich model, especially for the non linear form of the Freundlich model. The equilibrium parameter Kr of three metals are all above 0 and below 1, they indicate the shape of the isotherms to be favorable. According to this classification, system favor tends to be in the order of Pb²⁺>Mn²⁺>Cu²⁺.Thirdly, adsorptions of Pb²⁺, Cu²⁺, Mn²⁺ onto duckweed are multi-linear process including three processes. Data analyzed by the intraparticle diffusion model confirm that intraparticle diffusion was not the rate-determining step. The adsorption rate of metals in following order Pb²⁺>Mn²⁺>Cu²⁺.Fourthly, although the values of the linear regression coefficient (R²), non-linear regression coefficient (r²) gained from the the Lagergren equation are high, the calculated Qe values obtained from linear regression of the Lagergren equation is far deviated from the experiment data. In addition, it was observed that the sorption data were well represented by the Lagergren model only for the first 30 min.Fifthly, comparison of error analysis of the intraparticle diffusion model, the pseudo-first-order and pseudo-second-order equations, the result show that the order of deviation was intraparticle diffusion model> the pseudo-first-order> pseudo-second-order. It was confirmed that the pseudo-first-order was the best one in describing the adsorption kinetics of Pb²⁺, Cu²⁺, Mn²⁺ onto duckweed.