Comparison of ultrasonic and microwave assisted digestion methods for the determination of heavy metals in soil and sediment: The effect of seasonal variations on metal concentrations and risk assessment

This study presents the analysis of twelve trace elements in soil and sediment samples using microwave-assisted and ultrasonic-assisted digestion prior to analysis with inductively coupled plasma optical emission spectroscopy. The recoveries obtained for the microwave-assisted ranged between 81–101% and 80–98% while ultrasonic-assisted recoveries were 80–108% and 79–103% for soil and sediment samples respectively. The metal concentrations obtained ranged from 0.10–355.4 mg/kg and 1.50–308.3 mg/kg in soil and sediments respectively. Most of the studied elements were below the maximum permissible limits in soil except for zinc. Both digestion methods revealed similar accuracy, indicating that both can be used for accurate determination of the target metals. However, ultrasonic-assisted digestion can be recommended as an alternative method to the conventional microwave-assisted digestion since it can successfully digest without the use of extreme temperatures and pressures, and it requires inexpensive technique. Further geo-statistical analysis for heavy metal contamination in soil and sediment were assessed. The enrichment factor and geo-accumulation index ranged between 0.1–18.9 and −2.6–2.5 respectively. The potential ecological risk index showed the overall biological hazard to be the highest at Woodhouse soil classified as risk level C (strong pollution level), indicating a need for continuous monitoring of these metals.