Adsorption of humic acid from seawater on organo Mg-Fe-layered double hydroxides: isotherm, kinetic modeling, and ionic strength

The aim of this study is to bring an in-depth understanding of the adsorption phenomenon of humic acid onto calcined layered double hydroxides (LDH) and organophilic LDH in seawater. LDHs were prepared by co-precipitation at constant pH with a molar ration M+2/M+3 = 2. Organophilic LDH were prepared by interposing a surfactant (sodium dodecylsulfate, SDS 0.1 M). All samples were characterized using X-ray diffraction, Fourier transform infrared spectrometry, Brunauer- Emmett-Teller method, and scanning electron microscopy. Several adsorption kinetics models (pseudo-first-order, pseudo-second-order, nth-order, and intraparticle diffusion) were tested against the experimental results. The calculation of the corresponding parameters shows that results are best fitted with the nth-order (n (sic) R+) n not equal 1 model with a determination coefficient close to 1 and a relatively small root mean square error. Among the available mathematical models used to describe the isotherms experimental results (Langmuir, Freundlich, Sips, and many-parameters models). The six models that are proposed show a better performance than Freundlich, Langmuir, and Sips. The calculation of the parameters of the different adsorption models was performed on MATLAB using genetic algorithms. Thermodynamic analysis of sorption isotherms suggests that sorption process of humic acid is spontaneous (Delta G degrees < 0), with a positive enthalpy variation, characteristic of an endothermic process. The positive entropy variation indicates a disorder increase at the solidsolution interface during humic acid sorption. The values of the effective diffusion coefficient and energy activation deduced by modeling range between (10(-13)-10(-9)) m(2)/s and E-a = (28-33) kJ/mol, respectively, as reported in the literature. The materials prepared showed an excellent regeneration ability and were successfully reused after several adsorption cycles.