Distribution of rare earth elements in sewage-irrigated soil profiles in Tianjin, China

The concentrations of rare earth elements (REEs) in the soil extracts and soil solutions from two different alluvial soil profiles irrigated with sewage were measured using inductively coupled plasma-mass spectrometry. The results showed that the REE concentrations in the soil extracts from soil samples affected by sewage irrigation were much higher than those in virgin soil samples. The REE concentrations in the soil extracts of the rhizophere soil were higher than those of the non-rhizophere soil. The shale-normalized REE patterns in the soil extracts from the upper soil layer affected by sewage irrigation shown middle REE (MREE) enrichment relative to light REE (LREE) and heavy REE (HREE). This result was attributed to the MREE-rich organic colloidal material, REE-HM (humic and fulvic acids) and phosphate-REE complexations. The REE patterns in the soil extracts from deep soil little-affected by sewage irrigation showed HREE enrichment relative to LREE, which might have been caused by the preferential complexation of HREE with carbonate. The normalized La/Yb ratios in the soil extracts increased with the decrease in depth. The sewage irrigation did not affect the total REE contents and REE pattern in the soil profiles.