Alkali-activated material Palm oil fuel ash Hydroxide precipitation a b s t r a c t Heavy-metal-contaminated wastewater is harmful to human beings as well as to the environment. Heavy metals are easily soluble in acidic conditions; thus, chemical treatment is necessary for this type of wastewater. Hydroxide precipitation of heavy metals is a simple and low-cost process that was used in this research. Palm oil fuel ash (POFA) was used to synthesize an alkali-activated material (AAM) for use as a

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... pH modifier and hydroxide (OH À ion) releasing agent in Cu and Zn contaminated solutions. The released hydroxide reacts with Cu 2þ and Zn 2þ ions and forms metal hydroxide, which was removed from the wastewater by precipitation. The final concentrations of Cu 2þ and Zn 2þ ions in the solution were lower than the standard allowances of 5 and 2 ppm, respectively. The retention time and pH value of the solution played an important role in the final concentration of metal ions. The hydroxide precipitate was removed from the solution by a simple filtration process. Tests also showed that the POFA alkali-activated material could be reused. The precipitation method using POFA alkali-activated material is, therefore, a simple and cost-effective method for wastewater treatment. Heavy-metal-contaminated agricultural wastes can be converted into useful composite, leading to green and sustainable products.