The novel kinetics expression of Cadmium (II) removal using green adsorbent horse dung humic acid (Hd-Ha)

Abstract

Humic acid from dry horse dung powder has been prepared and this horse dung humic acid (HD-HA) was then applied as a sorbent to adsorb Cadmium(II) from a solution. Characterization of HD-HA was conducted by detection of its functional group, UV-Vis spectra, ash level, and total acidity. Result of the work showed that HD-HA had similar character compared with peat soil humic acid (PS-HA) and previous researchers. The adsorption study of this work was investigated by batch experiment in pH 5. The thermodynamics parameters in this work were determined by the Langmuir isotherm model for monolayer sorption and Freundlich isotherm model multilayer sorption. Monolayer sorption capacity (b) for HD-HA was 1.329 × 10-3 mol g−1, equilibrium constant (K) was 5.651 (mol/L)−1, and multilayer sorption capacity was 2.646 × 10−2 mol g−1. The kinetics parameters investigated in this work were determined by the novel kinetics expression resulted from the mathematical derivation the availability of binding sites of sorbent. Adsorption rate constant (ka) from this novel expression was 43.178 min−1 (mol/L)−1 and desorption rate constant (kd) was 1.250 × 10−2 min−1. Application of the kinetics model on sorption Cd(II) onto HD-HA showed the nearly all of models gave a good linearity. However, only this proposed kinetics expression has good relation with Langmuir model. The novel kinetics expression proposed in this paper seems to be more realistic and reasonable and close to the experimental real condition because the value of ka/kd (3452 (mol/L)−1) was fairly close with K from Langmuir isotherm model (5651 (mol/L)−1). Comparison of this novel kinetics expression with well-known Lagergren pseudo-first order kinetics and Ho pseudo-second order kinetics was also critically discussed in this paper.