An efficient digestion technique for the analysis of heavy metals, Ca and Al in solid waste incineration ash

Accurately quantifying the metal concentrations in solid waste incineration ash is critical to monitoring the environmental risks of heavy metals and in designing the removal and recovery methods of metals from solid waste resources. In this work, different types of acid mixtures were compared. It was found that current methods could not digest solid waste incineration ash entirely. Typically, HF is essential in destroying the SiO₂ structure in ash samples. However, it generates new precipitates of AlF₃, CaAlF₅, or CaF₂, leading to the incomplete digestion and, hence, inaccurate detection of heavy metals and Ca/Al. To resolve this issue, a two-step digestion method is proposed and tested. This method fully digests many types of solid waste incineration ash, including sewage sludge incinerated ash, incinerated bottom ash, and incinerated fly ash. In the first step, the acid mixture of HCl-HNO₃-HF is used in a microwave heating system at 240 °C. Most heavy metals, Ca and Si, are digested, and a white residue is also generated. In the second step, the residue is completely dissolved by a mixture of diluted acids of 0.5 mol/L HNO₃ and 0.5 mol/L HCl at 100 °C. Subsequently, a digestion strategy for an unknown incinerated ash is developed.