Environmental Context. Arsenic is of significant environmental concern in much of the world because of its contamination of waters, from mining, industry, sewage disposal, and agriculture. The environmental mobility of arsenic is controlled primarily by adsorption onto metal oxides, especially iron. Humic substances (natural organic matter), which are ubiquitous in aquatic and soil environments, may interfere with this adsorption and arsenic mobility may be increased. Thus, even if it is assumed that humic substances sorb arsenic less strongly than hydrous iron oxides, they may, nevertheless, influence arsenic sorption and mobility, particularly when the iron oxide content in the environment is low. The environmental mobility of arsenic is primarily controlled by adsorption onto metal oxide surfaces, particularly iron, aluminium, and manganese. Humic acid (HA) may interfere with this adsorption, thereby increasing arsenic mobility. This study has characterized the interaction of arsenic with HA in a system consisting of HA with As(iii), As(v), and dimethylarsinic acid (DMAA). Three sets of batch experiments were performed at varying pH (3–12), ionic strength (0–0.4 mol dm-3), concentration of each arsenic species (0–100 mg dm-3), and HA concentration (0–10 g dm-3). Arsenic species were shown to react with humic acid. The interaction is postulated to involve bridging metals and deprotonated functional groups within the HA. The association is dependent on pH, ionic strength, and arsenic concentration. The extent of the interaction was greater in the pH range 8–10 for As(v) and DMAA, while it extended to pH 12 for As(iii). The strong pH dependency is probably due to the aqueous speciation of arsenic. The logarithmic conditional association constants for the reactions were found to be 1.97 ± 0.02, 1.58 ± 0.07, and 1.50 ± 0.10 for As(v), As(iii), and DMAA respectively. These values indicate the formation of weak complexes with humic acid.

Activated carbon obtained from plantain peels was applied to the optimization of the adsorption process parameters for abstraction of colour from simulated dye effluent. The activated carbon was prepared and characterized using nitrogen adsorption, X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). Equilibrium isotherms were modelled using the Langmuir, Freundlich, Dubinin–Radushkevich and Temkin models; the Temkin and Dubinin–Radushkevich models provided the best fit for the sorption process, with a correlation coefficient greater than 0.95. The D–R model suggested a chemical process. The pseudo second-order kinetic model agreed well for fitting experimental data with the calculated adsorption capacity, qe, (46.5mg/g), which was reasonably close to the experimental value (47.3mg/g). Optimization of the process parameters was achieved using response surface methodology (RSM) – Box–Behnken design, where factors considered are represented on three levels: (−1), (0) and (+1) for high, mean and low levels, respectively. ANOVA fits a quadratic model with prob>F less than 0.05 (<0.0001) at 95% confidence level. From this modelling, significant factors for dye removal have been identified