# Environmental implications and recovery potential of rare earth elements in solid residues from the incineration of sewage sludge

## TL;DR

This study examines rare earth elements in waste from sewage sludge incineration and finds they are mostly immobile and not hazardous.

## Contribution

The study provides new data on rare earth element mobility and recovery potential in specific waste fractions from sewage sludge incineration.

## Key findings

- REE mobility in SSA and FB is very low, with over 98.5% in immobile fractions.
- REE recovery potential from the tested waste fractions is negligible.
- Significant differences in REE content were observed based on sampling locations.

## Abstract

The presented research aimed to contribute new insights into the chemical composition of solid waste generated during the incineration of sewage sludge, with a particular focus on the occurrence and behaviour of rare earth elements (REEs). In this study, the total content as well as the mobility of rare earth elements (REE) were determined in disposed fluidized beds (FB), sewage sludge ash (SSA), and air pollution control residues (APC) samples obtained from three Polish facilities engaged in thermal sewage sludge treatment. The mobility of REE was assessed using the BCR sequential extraction procedure. Each type of tested material was characterized by different content of REE (SSA > FB > APC). Moreover, statistically significant differences were observed in the REE content in individual waste fractions depending on the sampling places. The mobility of REE in SSA and FB was determined as very low (approx. 98.5% of Sc and > 99% of other REE in the immobile fraction). Based on the results, it can be concluded that the recovery potential of REE from each fraction of the tested waste is negligible. Because of the very low mobility of REE, these elements should not be considered hazardous during FB and SSA management.

The online version contains supplementary material available at 10.1038/s41598-025-32315-0.