A holistic approach for the recovery of rare earth elements and scandium from secondary sources under a circular economy framework - A review.

Limited natural resources and a continuous increase in the demand for modern technological products, is creating a demand and supply gap for rare earth elements (REEs) and Sc. There is therefore a need to adopt the sustainable approach of the circular economy system (CE). In this review, we defined six steps required to close the loop and recover REEs, using a holistic approach. Recent statistics on REEs and Sc demand and the number of waste generations are reported and studies on more environmentally friendly, economic, and/or efficient recovery processes are summarized. Pilot-scale recovery facilities are described for several types of secondary sources. Finally, we identify obstacles to closing the REE loop in a circular economy and the reasons why secondary sources are not preferred over primary sources. Briefly, recovery from secondary sources should be environmentally and economically friendly and of an acceptable standard concerning final product quality. However, current technologies for recovery from for secondary sources are limiting and technology needs will vary depending on the source type. The quality/purity of the recovered metals should be proven so that they do not result in any adverse effects on the product quality, when they are being used as secondary raw material. In addition, for industrial-scale facilities, process improvements are required that consider environmental conditions.

Heteroaggregation, transformation and fate of CeO2 nanoparticles in wastewater treatment.

Wastewater Treatment Plants (WWTPs) are a key pathway by which nanoparticles (NPs) enter the environment following release from NP-enabled products. This work considers the fate and exposure of CeO2 NPs in WWTPs in a two-step process of heteroaggregation with bacteria followed by the subsequent reduction of Ce(IV) to Ce(III). Measurements of NP association with solids in sludge were combined with experimental estimates of reduction rate constants for CeO2 NPs in Monte Carlo simulations to predict the concentrations and speciation of Ce in WWTP effluents and biosolids. Experiments indicated preferential accumulation of CeO2 NPs in biosolids where reductive transformation would occur. Surface functionalization was observed to impact both the distribution coefficient and the rates of transformation. The relative affinity of CeO2 NPs for bacterial suspensions in sludge appears to explain differences in the observed rates of Ce reduction for the two types of CeO2 NPs studied.

Monte Carlo simulations of the transformation and removal of Ag, TiO2, and ZnO nanoparticles in wastewater treatment and land application of biosolids.

The use of nano-enabled materials in industry and consumer products is increasing rapidly and with it, the more imperative it becomes to understand the consequences of such materials entering the environment during production, use or disposal. The novel properties of engineered nanomaterials (ENMs) that make them desirable for commercial applications also present the possibility of impacting aquatic and terrestrial environments in ways that may differ from materials in bulk format. Modeling techniques are needed to proactively predict the environmental fate and transport of nanomaterials. A model for nanoparticle (NP) separation and transformation in water treatment was parameterized for three metal and metal-oxide NPs. Functional assays to determine NP specific distribution and transformation were used to parameterize the model and obtain environmentally relevant concentrations of NPs and transformation byproducts leaving WWTPs in effluent and biosolids. All three NPs were predicted to associate >90% with the solid phase indicating significant accumulation in the biosolids. High rates of transformation for ZnO and Ag NPs resulted in ~97% transformation of the NPs that enter the plant despite differences in transformation rate in aerobic versus anaerobic environments. Due to high insolubility and negligible redox transformation, the only process predicted to impact TiO2 NP fate and transport in WWTPs was distribution between the solid and liquid phases. Subsequent investigation of ZnO NP species fate and transport when land applied in biosolids indicated that steady state concentrations of ZnO phases would likely be achieved after approximately 150days under loading conditions of biosolids typical in current practice.

Modeling nanomaterial fate in wastewater treatment: Monte Carlo simulation of silver nanoparticles (nano-Ag).

Wastewater effluent and sewage sludge are predicted to be important release pathways for nanomaterials used in many consumer products. The uncertainty and variability of potential nanomaterial inputs, nanomaterial properties, and the operation of the wastewater treatment plant contribute to the difficulty of predicting sludge and effluent nanomaterial concentration. With a model parsimony approach, we developed a mass-balance representation of engineered nanomaterial (ENM) behavior based on a minimal number of input variables to describe release quantities to the environment. Our simulations show that significant differences in the removal of silver nanoparticles (nano-Ag) can be expected based on the type of engineered coatings used to stabilize these materials in suspension. At current production estimates, 95% of the estimated effluent concentrations of the nano-Ag considered to be least well-removed by the average wastewater treatment plant are calculated to fall below 0.12 µg/L, while 95% of the estimated sludge concentrations of nano-Ag with coatings that increase their likelihood of being present in biosolids, fall below 0.35 µg/L.

Estimating production data for five engineered nanomaterials as a basis for exposure assessment.

The magnitude of engineered nanomaterials (ENMs) being produced and potentially released to the environment is a crucial and thus far unknown input to exposure assessment. This work estimates upper and lower bound annual United States production quantities for 5 classes of ENMs. A variety of sources were culled to identify companies producing source ENM products and determine production volumes. Using refining assumptions to attribute production levels from companies with more reliable estimates to companies with little to no data, ranges of U.S. production quantities were projected for each of the 5 ENMs. The quality of data is also analyzed; the percentage of companies for which data were available (via Web sites, patents, or direct communication) or unavailable (and thus extrapolated from other companies' data) is presented.

Ultrasonic dispersion of nanoparticles for environmental, health and safety assessment--issues and recommendations.

Studies designed to investigate the environmental or biological interactions of nanoscale materials frequently rely on the use of ultrasound (sonication) to prepare test suspensions. However, the inconsistent application of ultrasonic treatment across laboratories, and the lack of process standardization can lead to significant variability in suspension characteristics. At present, there is widespread recognition that sonication must be applied judiciously and reported in a consistent manner that is quantifiable and reproducible; current reporting practices generally lack these attributes. The objectives of the present work were to: (i) Survey potential sonication effects that can alter the physicochemical or biological properties of dispersed nanomaterials (within the context of toxicity testing) and discuss methods to mitigate these effects, (ii) propose a method for standardizing the measurement of sonication power, and (iii) offer a set of reporting guidelines to facilitate the reproducibility of studies involving engineered nanoparticle suspensions obtained via sonication.

Estimates of upper bounds and trends in nano-TiO2 production as a basis for exposure assessment.

An upper bound is estimated for the magnitude of potential exposure to nano-TiO2 with the purpose of enabling exposure assessment and, ultimately, risk assessment Knowledge of the existing bulk TiO2 market is combined with available nano-TiO2 production data to estimate current nano-TiO2 sources as a baseline. The evolution of nano-TiO2 production as a percentage of the total TiO2 market is then projected based on material and market information along with a method that combines observations from scientific articles and patents as predictive indicators of the rate of innovative transformation.

Relative risk analysis of several manufactured nanomaterials: an insurance industry context.

A relative risk assessment is presented for the industrial fabrication of several nanomaterials. The production processes for five nanomaterials were selected for this analysis, based on their current or near-term potential for large-scale production and commercialization: single-walled carbon nanotubes, bucky balls (C60), one variety of quantum dots, alumoxane nanoparticles, and nano-titanium dioxide. The assessment focused on the activities surrounding the fabrication of nanomaterials, exclusive of any impacts or risks with the nanomaterials themselves. A representative synthesis method was selected for each nanomaterial based on its potential for scaleup. A list of input materials, output materials, and waste streams for each step of fabrication was developed and entered into a database that included key process characteristics such as temperature and pressure. The physical-chemical properties and quantities of the inventoried materials were used to assess relative risk based on factors such as volatility, carcinogenicity, flammability, toxicity, and persistence. These factors were first used to qualitatively rank risk, then combined using an actuarial protocol developed by the insurance industry for the purpose of calculating insurance premiums for chemical manufacturers. This protocol ranks three categories of risk relative to a 100 point scale (where 100 represents maximum risk): incident risk, normal operations risk, and latent contamination risk. Results from this analysis determined that relative environmental risk from manufacturing each of these five materials was comparatively low in relation to other common industrial manufacturing processes.

Laboratory assessment of the mobility of nanomaterials in porous media.

The production of significant quantities of engineered nanomaterials will inevitably result in the introduction of these materials to the environment. Mobility in a well-defined porous medium was evaluated for eight particulate products of nanochemistry to assess their potential for migration in porous media such as groundwater aquifers and water treatment plant filters. Contrary to the assertion that nanomaterials present monolithic environmental risks, here we show that these nanomaterials exhibit widely differing transport behaviors. Fullerene-based nanomaterials that had been functionalized to facilitate dispersal in water displayed the highest mobilities, with a calculated potential to migrate approximately 10 m in unfractured sand aquifers. Colloidal aggregates of C60, which have been the focus of recent toxicity studies, were among the least mobile of the nanomaterials evaluated.