Plants as Biosorbents 1281 Vascular Plants

Plant materials such as living plants (Hoffman et al. 2004), their parts or their dried, seized, and chemically treated seeds (Kumari et al. 2006; Koivula et al. 2009; Pandey et al. 2009) and also the residue of vascular plants from industry or agriculture, such as rice polish, and orange wastes (Ranjan et al. 2009; Ghimire et al. 2003), were tested as biosorbent material (Table 12.3) for arsenic.

Maximum sorption capacity for arsenic depends on the three factors; sorbate solution characteristic, biosorbent characteristic and process duration. Sorbate solution characteristics such as initial arsenic concentration, pH of the solid solution, occurrence of other co-ions, sorption time duration, and temperature, control the quantity of adsorption. Important biosorbent characteristic are: material porosity, particle size distribution, specific surface area, cation exchange capacity and surface functional groups. Arsenic sorption is limited by phosphate concentration (Kumari et al. 2006; Ghimire et al. 2003; Ranjan et al. 2009). Cadmium, selenium, and carbonate ions in solution above 200 mgL-1 showed an improved arsenic biosorption. Kumari et al. (2006) reported that optimum pH for Moringa oleifera seeds as a biosorbent is different for arsenite and arsenate ion sorption, and was respectively 2.5 and 7.5 mg AsL-1.

Maximum biosorption capacity for living plants was observed to be a two-stage process i.e. rapid first phase, and a slow second phase. The first, rapid phase of reaching equilibrium was found for arsenate sorption by Salvinia minima, to be 6-12 h (Hoffman et al. 2004) ; and for arsenite sorption by Salvinia natans 48 h,

(Mukherjee and Kumar 2005), with different arsenic ions concentrations. The second phase was up to 96 and 12 h (Table 12.3). The adsorption of metal ions onto a plant surface was conducted in a relatively short-time, while the uptake of metal ions took a long-time and was more complex (Veglio and Beolchini 1997; Hoffman et al. 2004; Mukherjee and Kumar 2005). Pandey et al. (2009) reported chelation of As(III) with the -OH groups for fresh different parts of the biomass of Momordica charantia.

Most of these experimental results were in good agreement with the Langmuir and Freundlich sorption models. However the Dubinin-Radushkevich (D-R) sorption isotherm, was applied to evaluate the nature of sorption and was used to explain the heterogeneity of surface energies (Ranjan et al. 2009).

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