Determination of metal concentrations in water, soil, and sediment in the KwaZulu-Natal province: comparison of digestion methods, environmental concerns, and human health risk assessment.

Abstract

Metals are environmental natural elements. However, due to the alteration of their geological process and biochemical equilibrium as a result of the metals wide usage for human purpose, they have become a source of environmental pollution which is a global problem. Thus, they need to be evaluated in various environmental compartments, and their human health and environmental toxicity be assessed. Therefore, in this work, a microwave, hotplate and ultrasonic - assisted digestion have been successfully optimised and applied to water, soil, and sediment samples, for the digestion of twelve metals. The inductively coupled plasma – optical emission spectroscopy (ICP-OES) was optimized using the typical analytical figures of merit (correlation coefficient (R2), limit of quantification (LOQ), (limit of detection (LOD)) for metal quantification. The method validation of the mentioned digestion methods was performed by spiked recovery tests using the multi-element ICP standard containing the studied metals (Cd, Cr, Co, Cu, Ga, Li, Mn, Ni, Pb, Sr, Tl, Zn). The percentage recoveries obtained in water, soil and sediments were within the acceptable range of 70 – 120%. All digestion methods indicated a good degree of accuracy as per the calculated figures of merit. The average concentrations obtained ranged from 4.9 – 410.8 μg/L, 5.9 – 465.0 μg/L, 3.6 - 425.4 μg/L, 16.1 - 647 μg/L and 9.7 – 784 μg/L in tap water, river, influent, effluent, and sludge samples for all metal analytes studied, respectively. The influent, effluent and sludge samples were collected from wastewater treatment plants. All metals were below their maximum permissible limits except the Mn in all sludge samples and Pb in all tap water, Umhlathuzana River and Northern works influent samples. This indicates that the tap water from all the sampling areas is not safe for human consumption as Pb is one of the most toxic metals which could result to irreparable harm of the kidney, brain and nervous system in humans, animals. For soil and sediments, metal concentrations obtained ranged from 0.10 – 355.4 mg/kg and 1.50 – 308.3 mg/kg respectively. The environmental contamination was evaluated for the soil and sediments and revealed that heavy metal contamination was not severe however, Cu showed significant contamination. Lastly, agricultural soil metal concentrations ranged from 0.60 – 256.4 mg/kg, however, the metals were all lower than the maximum allowable limits in soil except for Cr. Even though the widely used and recommended method of digestion is microwave, hotplate-assisted digestion proved to be the inexpensive and accessible alternative for routine analysis. Even though the metal pollution was observed to be minor in this study, it is essential to continuously monitor the potential contamination dangers and human health risks in the future.