Raw sawdust utilization for the removal of acid red57 and basic fuchsin dyes from aqueous solution: equilibrium, kinetics, and thermodynamic investigation

The aim of work is the use of a cheap adsorbent; sawdust (SD), for the adsorption of anionic dye, acid red57 (AR57), and cationic dye, basic fuchsin (BF). The adsorption of AR57 and BF on SD increased by increasing contact time and adsorbent dosage, while decreased with the increase of temperature and initial dye concentration. The increase in pH decreased the adsorption of AR57 and increased the adsorption of BF. The effective pH, adsorbent dose, and contact time on the AR57 removal efficiency were found to be 3, 0.1 g and 50 min, respectively. While for BF it was found to be 6, 0.2 g and 50 min, respectively. Fitting equilibrium data to Langmuir, Freundlich and Temkin isotherms showed that Freundlich model is the most suitable to describe the acid red57 and basic fuchsin. The kinetic studies indicate that the adsorption of AR57 and BF follows a pseudo-second-order model. The adsorption of both AR57 and BF is spontaneous (Delta G = -3.97, -3.27 kJ/mol) and exothermic (Delta H = -22.9, -8.3 kJ/mol) with negative values of Delta G and Delta H. The negative values of Delta S of both AR57 and BF (Delta S = -64.6, -16.7 J/mol. K) showed that the randomness decreases through the adsorption process. SD is locally available in Egypt - Damietta, as solid residue and it is very cheap, therefore there is no need for its regeneration and reuse because of its nearly free cost and safe disposable. On the other hand, the treated waste liquids reach satisfied limits for reusing in the agriculture of palm trees.