Rare earth and precious elements in the urban sewage sludge and lake surface sediments under anthropogenic influence in the Republic of Benin

Abstract

Nowadays, sewage sludge and water bodies are subjected to heavy pollution due to rapid population growth and urbanization. Heavy metal pollution represents one of the main challenges threatening our environment and the ecosystem. The present work aims to evaluate the contamination state of the sewage sludge and lake sediments in the Republic of Benin. Twenty metallic elements including 15 rare earth elements (Eu, Sb, Cs, Nd, Pr, Gd, La, Ce, Tb, Sm, Dy, Ho, Eu, Yb, and Lu) and five precious elements (Ag, Au, Pd, Pt, and Ru) were investigated using inductive plasma-mass spectrometry. Results showed broad range concentrations of the elements. Ce, La, and Nd were present in both sediments and sewage sludge at concentrations ranging 5.80-41.30mg/kg dry matter (DM), 3.23-15.60mg/kg DM, and 2.74-19.26mg/kg DM, respectively. Pr, Sm, Gd, Tb, Dy, Eu, Er, Yb, Cs, Ho, and Tm concentrations were lower (0.02-5.94mg/kg DM). Among precious elements, Ag was detected at the highest concentration in all sites (0.43-4.72mg/kg DM), followed by Pd (0.20-0.57mg/kg DM) and Au (0.01-0.57mg/kg DM). Ru and Pt concentrations were <0.20mg/kg DM in all samples. Pollution indices and enrichment factor indicated a strong to severe enrichment of the elements, mainly Ce and precious elements in both sediments and sewage sludge. This revealed a growing anthropogenic input which was also implied by principal component analysis. The evaluation of pollution loading index (PLI) indicated a moderate to strong contamination (0.12≤PLI≤0.58; 37≤PLI≤114, respectively, for rare earth elements and precious elements), while the degree of contamination indicated a moderate polymetallic contamination for rare earth elements and significant contamination for precious elements.