Exploring the Potential of Sewage Sludge as Agricultural Fertilizer: Determination of Heavy Metals and Nutrients by Microwave-Assisted Digestion Followed by ICP-OES Analysis

Abstract

Sewage sludge valorization in agricultural fields is in many countries a circular economy strategy to handle the disposal issues of this complex residue. Many countries regulate the content of heavy metals and other agrochemical parameters in sewage sludge, establishing threshold limits for its application in agricultural fields. This requires the employment of robust, timesaving, and cost-effective analytical tools suitable for multielemental analysis at trace levels. Inductively coupled plasma optical emission spectrometry (ICP-OES) is a powerful instrumental technique with proven success in this field. In this laboratory experience, students perform a multielemental analysis of sewage sludge samples, by sample acidic digestion in a closed microwave system followed by ICP-OES analysis. Validation of the analytical method is also proposed, evaluating the linear response of an external calibration with internal standardization and using a certified reference material for evaluation of recovery and precision. The method is to be applied to a real sewage sludge sample to assess its nutrient content and to compare levels of heavy metals with enforced regulations. This work was successfully carried out in an analytical chemistry class with undergraduate students of chemical, environmental, and bioengineering bachelor degree courses. Most students reached the proposed learning goals, obtaining good or very good classifications in all evaluated parameters in the assessments of laboratory performance and final group reports.