Which one controls the absorption of DR 80 and DR 81 dyes by diatomite? Transport of pollutant in pores or adsorption on surfaces

The adsorption steps of Direct Red 80 (DR 80) and Direct Red 81 (DR 81) dyes from a synthetic aqueous solution were investigated using diatomite as a low cost, abundant and naturally available adsorbent. The equilibrium and kinetics studies were first carried out. The effects of pH, contact time, temperature, initial concentration, calcinations and sorbent dose on the adsorption process were conducted in order to obtain the maximum dyes removal. The highest adsorption was obtained at pH 2. The maximum adsorption of DR 80 and DR 81 dyes onto diatomite were 90.19 and 93.08 % respectively. It was further found that both investigated steps of adsorption including diffusion and adsorption on surfaces are important. However, the size of dye molecule is more effective, especially, on the adsorption capacity. It means that diffusion through the micro-pores is the most important step of the adsorption mechanism and adsorption on the adsorbent surfaces has the second place of significance.