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Technology 2005: 2nd Joint Specialty Conference for Sustainable Management of Water Quality Systems for the 21st Century: Adsorption of Basic Dyes onto Peanut Hulls and Kudzu

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In this study, adsorption dynamics for basic dye adsorption from water were determined by observing the adsorption of Basic Blue 3, Basic Red 22 and Basic Yellow 21 onto peanut hulls and kudzu in batch reactions. Three process variables were investigated to determine their individual effects on the adsorption kinetics: (i) the initial concentration of dye in solution; (ii) the mass of adsorbent; (iii) the agitation speed. The peanut hulls and the kudzu had a fixed particle size range of 0.15–0.45mm. All tests were carried out at room temperature (20 ± 2°C).

Four mathematical models were applied to the experimental data to determine the adsorption mechanisms occurring and the factors that influence the overall rate of adsorption. These models correlate the solute uptake rate and are therefore important in process design of coloured water treatment.

Data from kinetic studies have shown that kudzu and peanut hulls removed similar amounts of Basic Blue 3 over the same experimental period, and peanut hulls removed a greater amount of Basic Red 22 and Basic Yellow 21 from solution than kudzu over the same period.

The rate of uptake of the basic dyes onto both materials was rapid. The initial rate of adsorption onto peanut hulls was greater than onto kudzu with peanut hulls removing up to approximately 85% of Basic Red 22 from a 25mg/dm3 solution, in the first thirty minutes of testing. Kudzu removed approximately 80% of Basic Blue 3 from a 25mg/dm3 solution in thirty minutes. The initial uptake of dye was found to be more rapid from solutions with lower concentrations.

The rate of dye adsorption increased with increasing adsorbent mass and agitation speed due to the change in external mass transfer coefficient. The external mass transfer coefficients have been estimated using two methods: first, directly from experimental data, and second, using experimental data coupled with a mathematical model. Both methods failed to predict experimental data over long contact time periods due to the retarding influence of the second or internal mass transfer resistance.

Intraparticle rate parameters were evaluated for the adsorption of the three basic dyes onto both adsorbents. The effects of initial dye concentration, adsorbent mass and agitation speed on the rate parameters were correlated. The results suggested that the overall rate of adsorption was controlled by intraparticle diffusion and/or adsorption onto the adsorbent surface.

A pseudo-first-order model was used to correlate the experimental data for the adsorption of the three basic dyes onto peanut hulls and kudzu. Although correlation coefficients were reasonably high, the model did not predict qe values equal, or even reasonably close to experimental qe values. Any reaction occurring is therefore not likely to be a first order reaction.

A pseudo-second order model was also applied to the experimental data assuming that the external mass transfer limitations in the system could be neglected and that sorption was chemisorption controlled. The correlation coefficients and the accuracy of the model in predicting experimental data strongly suggest that the rate limiting step may be chemical sorption involving valency forces through sharing or exchange of electrons between sorbent and sorbate.

Equations were developed using the pseudo-second order model which accurately predict the amount of the three basic dyes adsorbed at any contact time and initial dye concentration, adsorbent mass, and agitation speed within the given range, onto peanut hulls and kudzu.
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Document Type: Research Article

Publication date: 2005-01-01

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