Treatment of municipal wastewater, which involves multiple steps, produces large amounts of biosolid sludge, which is either incinerated or disposed in landfills. This sludge contains carbon, nitrogen, and phosphorous in appreciable amounts, and hence, it is being recently suggested that it should be used as a fertilizer. However, the biosolid sludge also contains large amounts of heavy metals, which exert harmful effects on the plantation and therefore, they must be removed before it can be used as a fertilizer. In addition, some of these heavy metals are precious such as gold. In this work, heavy metals present in the biosolid sludge produced from municipal wastewater plants were extracted using acidic solutions of different strengths. The method of selected gold extraction using tributyl phosphate (TBP) in kerosene solution from the metal rich acidic solution was also tested. The rate and yield of gold extraction increase with the increase in the acidic strength. The highest extracted gold yield was 0.012 mg/g of biomass. The amount of gold recovery into the TBP solution was 26%, which was much higher than that of other metals extracted into the acid solution. The importance of removing the metals from the biosolid is obvious, as it allows the latter to be used as a fertilizer. In addition, using only one additional step, a valuable product, gold, can be selectively separated, despite being present in lower amounts that other metals found in the sludge.
Keywords: Gold; fertilizer; heavy metal removal; selective extraction; wastewater sludge.