Innovative amendments derived from industrial and municipal wastes enhance plant growth and soil functions in PTE-polluted environments

Highlights- Water Treatment Residuals, Red Muds, Municipal Solid Waste Compost and Biochar can reduce labile PTE in contaminated soils.- When used as amendments, WTR, RM, MSWC and BCH improve soil chemical fertility of PTE-polluted soils.- WTR, RM, MSWC and BCH stimulate soil enzyme activity and heterotrophic bacterial abundance in PTE-polluted soils.- WTR, RM, MSWC and BCH can be used as strategic amendments to enhance plant growth in environments polluted by PTE. Potentially toxic elements (PTE), e.g. As, Sb, Cd, Cu, Pb, Zn, can severely impact soil element cycling, organic matter turnover and soil inhabiting microbiota. Very often this has dramatic consequences for plant growth and yield which are greatly restricted in PTE-contaminated soils. The use of innovative amendments to reduce the labile pool of such soil contaminants, can result as a feasible and sustainable strategy to improve the fertility and functionality of PTE-contaminated soils as well as to exploit these latter from an agronomic point of view. Water treatment residuals (WTR), red muds (RM), organic-based materials originating from the waste cycle, e.g. municipal solid waste compost (MSWC) and biochar (BCH), have emerged in the last decades as promising amendments. In this paper, we report a synthesis of the lessons learned from research carried out in the last 20 years on the use of the above-mentioned innovative amendments for the manipulation of soil fertility and functionality in PTE-contaminated soils. The amendments considered possess physico-chemical properties useful to reduce labile PTE in soil (e.g. alkaline pH, porosity, Fe/Al phases, specific functional groups and ionic composition among the others). In addition, they contain organic and inorganic nutrients which can contribute to improve the soil chemical, microbial and biochemical status. This is often reflected by a higher organic matter content in amended soils and/or an increase of the cation exchange capacity, available P and total N and/or dissolved organic C. As a result, soil microbial abundance, in particular heterotrophic fungi and bacteria, and enzyme activities (e.g. dehydrogenase, urease and β-glucosidase) are commonly enhanced in amended soils, while plant growth can be significantly stimulated. Overall, the obtained results suggest that the studied amendments can be used to reduce PTE bioavailability in polluted soils, improve soil microbial status and functionality, and enhance the productivity of different crops. This can offer a precious opportunity for the productive recovery of PTE-polluted soils.

Obtaining of Maghemite Containing Red Mud for Effective As(V) Adsorption

This paper describes the studies of the use of red muds as adsorbents for cleaning solutions from As(V). The red mud is a waste that contains a large amount of iron oxides and hydroxides, which are excellent adsorbents of arsenic, especially those possessing magnetic properties and large specific surface area. The purpose of the experiment was to study the possibility of obtaining an effective adsorbent by direct extraction of alumina from bauxite using the caustic alkali fusion method and optimization of the process. The main iron-containing phase of the red muds obtained by fusing bauxite with caustic alkali was maghemite, which has a large specific surface area. Arsenic adsorption experiments were carried out using red muds obtained through bauxite alkali fusing at different temperatures and time of fusion, as well as the mass ratio of caustic alkali to bauxite. The red muds obtained by fusing bauxite with caustic alkali at 400∘ C and NaOH to bauxite mass ration 1.5 within 70 minutes have the highest effectiveness removing arsenic. Their As(V) uptake capacity was over than 37 mg/g. Keywords: red mud, maghemite, nanoparticles, As(V) adsorption, optimization

Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions

The main purpose of this study was to characterize the mineral and chemical composition of typical red muds in China. Changes in the physicochemical and microstructural properties of red muds collected from the Shanxi and Shandong provinces were investigated after they were immersed in an alkaline NaOH or an acidic HCl solution for 7, 28, and 120 days. The results showed that red mud has a high cation exchange capacity and active physicochemical properties, which can be closely related to its extremely high alkalinity and complex microstructure. The neutralization of red mud with the HCl solution results in the release of Na+ from the red mud particles into the leachate and can effectively decrease the pH value of the filtrate. The neutralization process can result in a significant decrease in the liquid limit, plastic limit and plasticity index, whereas the opposite was observed for the different parameters after the addition of the NaOH solution. In this sense, acid neutralization can significantly improve the cementation property of the red mud. This result will increase the water permeability of the acid-treated soil layer and improve the growth ability of plants. The specific surface area of red mud immersed in the NaOH solution decreased, whereas the specific surface area of red mud immersed in the HCl solution increased. This study contributes to our understanding of red mud properties after the red mud has been subjected to acidic and alkaline treatments, and the results can provide insights into the safe disposal of red mud.

Chemical Aspects of Scandium carbonate leaching from Red Muds

Red Muds (R.M.) – waste of bauxite reprocessing by the Bayer's method include many rare elements, such as Gallium, Titanium, Zirconium, Rare Earth Elements and Scandium, concentration of which in its reached 130 g/t. In the problem solving of Scandium extraction from R.M. get a two directions: immediate leaching of Scandium from R.M. and in passing Scandium recovery under complexing processing of R.M. The method of carbonate leaching of Scandium from R.M., which was work out in Institute of Solid State Chemistry Russian Academy of Sciences Urals Branch, include saturation by carbon dioxide of alkaline solutions, obtained after water process of R.M. and allow up to 20 % Scandium recovery in solutions. It is not effective for work out of technological scheme of R.M. processing. In recent paper, the chemical aspects of carbonate leaching of Scandium from R.M. in heterogeneous systems solid – liquid – gas carbon dioxide were investigated to determine of conditions for more Scandium recovery from R.M. It was shown, that Scandium leaching by aqueous sodium solutions without carbon dioxide gives small recovery yield and accompanied by alkaline hydrolysis under high pH in concentrated Na2CO3 solutions. Under saturation by carbon dioxide of carbonate R.M. pulp, the hydrolytic polymerization of hydroxocarbonates of Scandium and Aluminum in presence of carbonic acid proton take place. In the both process secondary precipitates are settle down and reduce of Scandium recovery yield. For increase of Scandium recovery from R.M. it is necessary to exclude the secondary precipitates under carbonate leaching with saturation by carbon dioxide.