Adsorption of Phosphate by Goethite and Zeolite: Effects of Humic Substances from Green Waste Compost

Abstract

Compost is widely used as a natural soil conditioner and fertilizer supplement in gardening, planting and agriculture. Ability of compost to retain and release nutrients over time offers potential use for control of excessive nutrient release to the environment; this ability may be further improved by the addition of adsorbents to facilitate rapid retention of nutrients. Therefore, we investigated the effects of humic substances extracted from green waste compost on the adsorption of phosphate by goethite (a-FeOOH) and zeolite (SiO₂.Al₂O₃). Humic acid (4.66% by dry-weight) and fulvic acid (1.26% as carbon/w) were extracted from finished compost and purified. The adsorption capacity of goethite (1.18 mmol/g) was slightly higher than zeolite's capacity (1.03 mmol/g). On the other hand, capacity of humic acid was determined much lower (0.51 mmol/g). The addition of fulvic acid resulted in a substantial reduction of adsorption capacities of all three adsorbents. Addition of 5.0 mmol/L of fulvic acid (as carbon) decreased the adsorption capacity of phosphate by goethite, zeolite and humic acid by 94%, 88% and 82%, respectively. Results of kinetics study indicated that the adsorption data fit the first-order kinetic model well with goethite exhibiting higher kinetic constants. The results of this study suggest that metal oxide adsorbents such as goethite could be applied as additives into compost to improve the nutrient holding ability.