Assessment of the occurrence, spatiotemporal variations and geoaccumulation of fifty-two inorganic elements in sewage sludge: A sludge management revisit

Tracing Gadolinium levels throughout wastewater treatment: Insights from a yearly assessment in northern Spain

Gadolinium (Gd) is a rare earth element (REE) used in the formulation of contrast agents for Magnetic Resonance Imaging (MRI) due to its paramagnetic properties. The growth in population and the improved quality of the healthcare systems over the last years, has promoted the use of MRI as an effective diagnostic tool thus increasing the consumption of gadolinium and its release into the wastewater treatment network. Therefore, the tracking and quantification of this metal in sewage treatment plants and water bodies, is of paramount importance since there are currently no specific rare earth treatment technologies installed in WWTPs, and consequently gadolinium is finally discharged into the environment. In this work, the presence of gadolinium and all other rare earth elements was monitored during a year in three WWTPs in northern Spain (Vuelta Ostrera and San Román in Cantabria and Galindo in País Vasco). These WWTPs are located close to urban centres with hospitals where MRI tests are performed. By tracing Gd throughout the wastewater treatment facilities, its presence was confirmed in water streams, in the order of ng per litter, and in sludge and ashes, in the order of mg per kilogram. A significant human influence was observed, with Gd anomaly values between 3.14 and 79.2 and anthropogenic Gd percentages above 90 %. The presence of Gd in water streams is affected by the sampling period due to the variations of the activity periods of the hospitals nearby the treatment plants. On the contrary, its content in sludge and ashes remains almost constant along the year. The concentration of this metal found in the ashes opens the door to its possible recovery together with other critical raw materials in the context of the circular economy.

Characterization and ecotoxicological risk assessment of sewage sludge from industrial and non-industrial cities

The present study highlights the occurrence and the temporal variations of physicochemical properties, and heavy metals in the sludge from sewage treatment plants (STPs) located in industrial (two sites) and non-industrial (one site) cities of Haryana, India. The sludge was acidic (5.59) to neutral (7.21) with a mean EC of 7.4 dS m-1. Prominent heavy metals present in the sewage sludge from industrial sites were Cd, Ni, and Cr with maximum values of 2.83, 1449.0, and 3918.5 mg kg-1, respectively. The contamination and enrichment factor better explained the buildup of Ni, Cr, and Cu in the sewage sludge from industrial sites. The pH, total carbon, phosphorus, and other water-soluble anions, viz. SO42-, Cl-, HCO3-, and PO43-, were the most important attributes of sludge controlling the binding and removal of the metals with particulate matters during the phase separation in STPs. These attributes explained about 90% of the variation in Cd, Ni, Cr, Cu, Mn, and Zn content of the sludge from different STPs. Sludge from the non-industrial site had a low potential ecological risk index of 74.0 compared to a very high-risk index of 2186.5 associated with the industrial sites. This study concludes that besides the concentration of the heavy metals, the enrichment factor coupled with geo-accumulation or ecological risk index can effectively categorize the sludge. However, these indices need to be linked with bioaccumulation, bioaccessibility, and biomass quality under different agroecologies for guiding the safer use of sewage sludge in agriculture.

Co-transport of biochar nanoparticles (BC NPs) and rare earth elements (REEs) in water-saturated porous media: New insights into REE fractionation

The present study investigated the co-transport behavior of three REEs³⁺ (La³⁺, Gd³⁺, and Yb³⁺) with and without biochar nanoparticles (BC NPs) in water-saturated porous media. The presence of REEs³⁺ enhanced the retention of BC NPs in quartz sand (QS) due to decreased electrostatic repulsion between BC NPs and QS, enhanced aggregation of BC NPs, and the contribution of straining. The distribution coefficients (K_D) in packed columns in the co-transport of BC NPs and three REEs³⁺ were much smaller than in batch experiments due to the different hydrodynamic conditions. In addition, we, for the first time, found that REE fractionation in the solid-liquid phase occurred during the co-transport of REEs³⁺ in the presence and absence of BC NPs. Note that the REE fractionation during the co-transport, which is helpful for the tracing application during earth surface processes, was driven by the interaction of REEs³⁺ with QS and BC NPs. This study elucidates novel insights into the fate of BC NPs and REEs³⁺ in porous media and indicates that (i) mutual effects between BC NPs and REE³⁺ should be considered when BC was applied to REE contaminated aquatic and soil systems; and (ii) REE fractionation provides a useful tool for identifying the sources of coexisting substances.

Variation of the element composition of municipal sewage sludges in the context of new regulations on phosphorus recovery in Germany

Phosphorus (P) recovery is obligatory for all sewage sludges with more than 20 g P/kg dry matter (DM) from 2029 in Germany. Nine wastewater treatment plants (WWTPs) were chosen to investigate variations of phosphorus contents and other parameters in sewage sludge over the year. Monthly sewage sludge samples from each WWTP were analyzed for phosphorus and other matrix elements (C, N, H, Ca, Fe, Al, etc.), for several trace elements (As, Cr, Mo, Ni, Pb, Sn) and loss of ignition. Among the nine WWTPs, there are four which have phosphorus contents both above and below the recovery limit of 20 g/kg DM along the year. Considering the average phosphorus content over the year, only one of them is below the limit. Compared to other matrix elements and parameters, phosphorus fluctuations are low with an average of 7% over all nine WWTPs. In total, only hydrogen and carbon are more constant in the sludge. In several WWTPs with chemical phosphorus elimination, phosphorus fluctuations showed similar courses like iron and/or aluminum. WWTPs with chamber filter presses rather showed dilution effects of calcium dosage. As result of this study, monthly phosphorus measurement is highly recommended to determine whether a WWTP is below the 20 g/kg DM limit.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s12302-022-00658-4.

Quantification of individual Rare Earth Elements from industrial sources in sewage sludge

Rare Earth Elements (REEs) are used in increasing amounts in technical applications and consumer products. However, to date, the contribution of industrial sources to the loads of individual REEs in wastewater streams have not been quantified. Here, we determine the REE contents in sludge collected from 63 wastewater treatment plants (WWTPs) across Switzerland. To quantify the industrial fraction of individual REEs in the sewage sludge, we develop two complementary approaches, based on REE ratios and REE pattern fitting. Unspecific (background) inputs, with REE patterns similar to the averaged REE pattern of soils collected across Switzerland, dominate the REE budget of most WWTPs. A few WWTPs receive significant REE inputs from specific industrial sources. Based on population equivalents of Switzerland, we estimate a total annual load of 4200 kg Cerium (Ce, 0.5 g Ce year^-1 capita^-1), with an industrial contribution of 2000 kg year^-1. The latter agrees with estimates of probabilistic mass flow models for engineered nanoscale CeO₂ particles discharged to the sewer network. About 7 kg year^-1 of Samarium (Sm,total for Switzerland: 184 kg year^-1 or 0.02 g Sm year^-1 capita^-1) and 3 kg year^-1 of Europium (Eu,total for Switzerland: 44 kg year^-1 or 0.005 g Eu year^-1 capita^-1) are assigned to industrial inputs from single WWTPs. Gadolinium (Gd) is used in the form of a stable complex as contrast agent in magnetic resonance imaging. Assuming 10% removal of Gd during wastewater treatment, we calculate an annual discharge of 90 kg of Gd from one individual WWTP to surface waters. WWTPs with exceptionally high industrial inputs of specific REEs warrant detailed investigations to identify the respective sources and to assess whether REE concentrations in effluents are elevated to the same degree.

Quantification of anthropogenic and geogenic Ce in sewage sludge based on Ce oxidation state and rare earth element patterns

Emissions of Ce from anthropogenic activities (anthropogenic Ce) into urban wastewater systems and the environment result from its widespread industrial use (abrasives, catalysts, nanotechnology). Because Ce in sewage sludge can also be of geogenic origin, the quantification of anthropogenic Ce in sewage sludge remains elusive. In this study, we evaluated the suitability of Ce oxidation state and rare earth element (REE) patterns for the quantification of anthropogenic Ce fractions in sewage sludge. A diverse set of soil samples served to gain baseline information on geogenic Ce. Geogenic Ce in the soils was characterized by high Ce(III) fractions (≥70%) and their REE patterns were comparable to the REE patterns of the upper continental crust. The sewage sludges contained on average ∼80% Ce(IV) (range 18-108%), pointing to the importance of anthropogenic inputs of Ce(IV). The quantification of the anthropogenic Ce fraction based on Ce oxidation state, however, was associated with considerable uncertainty because geogenic and anthropogenic Ce cannot exclusively be assigned to Ce(III) and Ce(IV), respectively. The REE patterns of most sewage sludges indicated a clear enrichment of Ce compared to heavier REE. Based on the assumption that the industrially used Ce is free of (most) other REE, we estimated the fraction of anthropogenic Ce in the sludges based on individual Ce/REE ratios. For the individual sludges the anthropogenic contributions were very variable (10-100%) but consistent fractions were obtained for individual sludges when calculated based on Ce/Dy (dysprosium), Ce/Er (erbium) and Ce/Eu (europium) ratios. Electron microscopy analysis of sludges dominated by anthropogenic Ce revealed that the Ce was mostly contained in nanoscale particles devoid of elements characteristic of Ce-bearing minerals. Thus, anthropogenic Ce contents derived from REE patterns may be used to validate current mass flow models for engineered CeO2 nanoparticles.

Occurrence, geochemical fractionation, and environmental risk assessment of major and trace elements in sewage sludge

Industrialization and accelerated population growth have created a huge amount of sewage sludge. Many studies have reported the sewage sludge as a sink of major and trace elements, but less is known about their geochemical fractionations. In order to assess the mobility, the distribution, bioavailability, and toxicity of those elements in sludge, we collected the sewage sludge samples from all the seven wastewater treatment plants in Xiamen City, China. Results revealed a strong spatial variation and the occurrence of 48 elements with concentrations ranging from 1.00×10^-2 mg kg^-1 (Re) to 9.03×10¹ g kg^-1 (Fe) on the basis of dry sludge weight. Sequential extraction procedure showed that residual and oxidizable fractions were the main geochemical fractions of most studied elements. However, Ca, Mn, Sr, and Ni were mainly bound to acid-exchangeable fractions, while Fe, Zn, Cd, Cr, Co, and V were mainly distributed in the reducible fractions. The contamination factor and risk assessment code indicated that Ni, Cu, Zn, Cd, Cr, Co, Sr, Ca, Mn, Mo, Re, and W were highly mobile with less retention time and exerted high environmental risks through sludge land application. The sludge disposal strategy should consider not only the total concentrations of a broad range of elements but also their bioavailability.

Preparation of Glass-ceramics Using Chromium-containing Stainless Steel Slag: Crystal Structure and Solidification of Heavy Metal Chromium

It is a useful way to stabilize the elements of heavy metal in the glass-ceramics with the form of ions due to the environmental pollution of heavy metal, such as Cr. The glass-ceramics of excellent combination properties were prepared, and the effect of stabilizing Cr-containing stainless steel slag with different concentrations of nitric acid leaching test were investigated. It was found that the major crystalline phase was diopside or anorthite with or without the amount stainless steel slag. Moreover, the continuous refinement of grains exhibited with the increase of amount of stainless steel slag. The results indicated that the excellent physical and mechanical properties, including density (2.9 g/cm3), hardness (729.27HV0.3), bending strength (222.9 MPa), and the solid solution of Cr in excess of 0.00057% for the glass-ceramics were related to the change of microstructure and phase structure. There showed the potential for reusing and detoxifying stainless steel slag.

Diagnosis and ecotoxicological risk assessment of 49 elements in sludge from wastewater treatment plants of Chongqing and Xiamen cities, China

Limited information about the sludge quality is a major constraint for its usage and proper disposal. This study investigated the occurrence of 49 elements in sludge from 11 wastewater treatment plants (WWTPs) in Chongqing and Xiamen cities of China. The concentration of 46-detected elements ranged from 16.2 μg kg^-1 (Pt) to 55.0 g kg^-1 (Al) on dry solid basis in the sludge. The enrichment factor of most of the elements was > 1.5, indicating their possible anthropogenic origin. The precious metals had considerably higher enrichment factor ranging from 56.3 to 200,000. Principal component analysis clustered the samples from Chongqing and Xiamen separately to suggest strong spatial variations. Contamination factor, pollution loading index, and integrated pollution degree were calculated to evaluate the elemental pollution risk. The pollution loading index indicated unpolluted to highly polluted levels of the elements in the sludge. In addition, results from the ecotoxicological risk index showed an individual low to very high ecotoxicological risk posed by eight metal(loid)s (As, Cd, Cr, Cu, Mn, Ni, Pb, and Zn) in the WWTPs.