Equilibrium, Kinetic, and Thermodynamic Studies on the Adsorption of Cadmium from Aqueous Solution by Modified Biomass Ash

Abstract

Natural biomass ash of agricultural residuals was collected from a power plant and modified with hexagonal mesoporous silica and functionalized with 3-aminopropyltriethoxysilane. The physicochemical and morphological properties of the biomass ash were analyzed by ICP-OES, SEM, TEM-EDS, FTIR, and BET analysis. The adsorption behavior of the modified product for Cd²⁺ in aqueous solution was studied as a function of pH, initial metal concentration, equilibrium time, and temperature. Results showed that the specific surface area of the modified product was 9 times that of the natural biomass ash. The modified biomass ash exhibited high affinity for Cd²⁺ and its adsorption capacity increased sharply with increasing pH from 4.0 to 6.0. The maximum adsorption capacity was 23.95 mg/g in a pH 5 solution with an initial metal concentration of 50 mg/L and a contact time of 90 min. The adsorption of Cd²⁺ onto the modified biomass ash was well fitted to the Langmuir model and it followed pseudo-second-order kinetics. Thermodynamic analysis results showed that the adsorption of Cd²⁺ was spontaneous and endothermic in nature. The results suggest that the modified biomass ash is promising for use as an inexpensive and effective adsorbent for Cd²⁺ removal from aqueous solution.

Figure 1

(a) SEM of biomass ash at 20 KeV; magnification 500. (b) SEM of synthesized matrix at 20 keV; magnification 500. (c) EDS analysis of elemental composition of biomass ash. (d) EDS analysis of elemental composition of modified biomass ash and (e) FT-IR spectra: (a) biomass ash and (b) synthesized matrix.

Figure 2

Effect of pH on the adsorption of Cd²⁺ on biomass ash and modified biomass ash (initial concentrations of Cd²⁺, 50 mg/L; biomass ash concentration, 4 g/L; T = 30°C).

Figure 3

Langmuir model fit of modified biomass ash at different temperature.

Figure 4

Sorption kinetics: concentration and removal (%) of Cd²⁺ from aqueous solution versus sorption time (initial concentrations of Cd²⁺, 100 mg/L; biomass ash concentration, 2 g/L; T = 30°C; initial pH 5.0).

Figure 5

Plots of the pseudo-second-order kinetics for the adsorptions of Cd²⁺ on biomass ash and the synthesized matrix (initial concentrations of Cd²⁺, 100 mg/L; biomass ash concentration, 2 g/L; T = 30°C; pH 5.0).