Adsorption behaviour of methylene blue onto Jordanian diatomite: a kinetic study.

The effect of initial concentration, particle size, mass of the adsorbent, pH and agitation speed on adsorption behaviour of methylene blue (MB) onto Jordanian diatomite has been investigated. The maximum adsorption capacity, q, increased from 75 to 105 mg/g when pH of the dye solution increased from 4 to 11. It is clear that the ionisable charge sites on the diatomite surface increased when pH increased from 4 to 11. When the solution pH was above the pH(ZPC), the diatomite surface had a negative charge, while at low pH (pH<5.4) it has a positive charge. The adsorption capacity increased from 88.6 to 143.3mg/g as the initial MB concentrations increased from 89.6 to 225.2mg/dm(3). The experimental results were also applied to the pseudo-first and -second order kinetic models. It is noticed that the whole experimental data of MB adsorption onto diatomite did not follow the pseudo-first order model and had low correlation coefficients (R(2)<0.3). The calculated adsorption capacity, q(e,cal), values obtained from pseudo-first order kinetic model did not give acceptable values, q(e,exp.) The maximum uptake capacity seems to be independent of the particle size of the diatomite when the particle size distribution is less than 250-500 microm. While at larger particle size 250-500 microm, the maximum uptake capacity was dependent on the particle size. It would imply that the MB adsorption is limited by the external surface and that intraparticle diffusion is reduced. The effect of the agitation speeds on the removal of MB from aqueous solution using the diatomite is quite low. The MB removal increased from 43 to 100% when mass of the diatomite increased from 0.3 to 1.7 g.