Hydrothermal treatment of alkaline red mud and sewage sludge: formation of a soil-like matrix

ABSTRACT

Increasing attention has been focused on the comprehensive utilisation of alkaline red mud (RM) derived from the aluminium industry. Phytoremediation serves as an effective strategy, but it is limited by the drawbacks of red mud. This study proposed ‘co-hydrothermally treating red mud and sewage sludge (SS)’ for producing a soil-like matrix, and explored the impacts of SS addition on the characteristics of hydrothermal solid and liquid products of RM. The results showed that the introduction of SS could improve the characteristics of hydrothermal products, including pH, the particle aggregation, and organic components. During hydrothermal treatment, the acid components released from SS could neutralise the alkalinity of RM, reducing the pH of hydrothermal product from 10.1 (without SS) to and 8.2 (80% SS), respectively. With the increase of addition ratio of SS, the main range of particle size distribution in hydrothermal solid products changed from 0.1∼1 μm to 10∼100 μm, suggesting the positive role of SS in improving the particle aggregation. XRD analysis showed that the addition of SS hindered the mineral crystallization of RM during hydrothermal treatment, while FTIR and XPS analysis confirmed that SS could serve as a ‘supply source’ of organic components, which created favourable conditions for hydrothermal solid products as soil-like matrix. The addition ratio of SS presented the negative correlation with the pH value and positive relative with chemical oxygen demand of hydrothermal liquid products. The hydrothermal liquid product modified by SS was beneficial to further improve soil-like matrix. The strategy of co-hydrothermal treating RM and SS to produce the soil-like matrix could massively consume solid wastes, which is a prospective approach to deal with the trouble of the aluminium industry and sewage treatment plants.

GRAPHICAL ABSTRACT

KEYWORDS:

• red mud
• hydrothermal treatment
• sewage sludge
• soil-like matrix
• phytoremediation

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

All authors contributed to this study. Hongyan Nan and Xue Zhang contributed to the experimental studies and the draft of the manuscript. Jiapeng Yang and Kai Jia contributed to the data analysis. Yijun Cao and Chongqing Wang contributed to the data analysis and revision of the manuscript. All authors read and approved the final manuscript.

Data availability statement

Data are available on request from the authors.

Additional information

Funding

This work was supported by the National Key Research and Development Project [grant number 2020YFC1908802], the Key Technology Research and Development Programme of Henan Province [grant number 202102310216], the Open Foundation of State Key Laboratory of Mineral Processing [grant number BGRIMM-KJSKL-2020-08], and funding support from Zhengzhou University.