Modeling the effect of salts in single and binary biosorption of acid dyes by residual diatomaceous earth for sustainable treatment of textile wastewaters

Biosorption is a well known technology for the reduction of the hazardous nature of textile wastewaters. However, its efficiency and mechanisms should be evaluated on complex conditions to be properly applied. In this work, biosorption of the textile dyes Acid Blue 277 (AB277) and Acid Red 361 (AR361) onto residual diatomaceous earth from brewing filtration (RDE) was investigated in single and binary systems with the addition of inorganic salts (NaCl, CaCl2 and MgSO4). RDE showed a maximum biosorption capacity of 6.16 mg g1 for AB277 and 6.75 mg g-1 for AR361 for single solutions and a maximum biosorption capacity of 8.42 mg g-1 for the sum of dyes in binary systems, which is associated to a synergistic effect of one dye over the other. The presence of salts reduced biosorption capacity in single solutions due to a competitive effect of anions for the binding sites, but different electrostatic interactions by the cations increased biosorption capacity in binary solutions. Modeling indicated that the salts increased the number and density of adsorbed molecules on the binding sites. Adsorption energy values, ranging from 14.2 to 16.7 kJ/mol, demonstrate that physical interactions contribute most to the biosorption process, and the energy is independent of the addition of salts. The physical monolayer model describes the adsorption phenomena accordingly to the experimental results, showing that biosorption with the interference of salts has different mechanisms in single and binary solutions.