Red mud-based inorganic polymer spheres bulk-type adsorbents and pH regulators

Rui M. Novais, João Carvalheiras, Maria P. Seabra, Robert C. Pullar, João A. Labrincha

Red mud is a highly alkaline and hazardous waste generated during alumina production, but its recycling and reuse, despite decades of intensive research, still remains a huge challenge for alumina refining industries. The global red mud stockpile is estimated to reach four thousand million tonnes [1]. This distressing scenario is attributed to the wastes’ strong alkalinity, particle fineness, and toxicity, which hinders recycling. In fact, less than 2.7% of the annual red mud production (∼150 million tonnes) is reused [2], while most of it is disposed of in large lagoons or tailing dams. This means that the already significant red mud stockpile will inevitably increase by 144 million tonnes per year, unless innovative and high-volume applications are implemented. Catastrophic disasters in Hungary (2010) and China (2016), whose long-term impacts on the environment and human health are still being evaluated, have raised public awareness over current red mud management methodologies. This greater public perception may be the driving force to change the alumina industry paradigm to consider red mud as a resource – not as a waste. In this context, novel, low cost, and environmentally benign strategies to prevent/mitigate red mud disposal are imperative. One interesting approach to reuse red mud could be its alkaline activation to produce inorganic polymers. These are an exciting class of new binders which may be synthesized at ambient temperature by the alkaline activation of amorphous aluminosilicate precursors. They are also known as geopolymers – a term coined by Davidovits in the late 1970s [3]. One very stimulating feature of this technology is the possibility of using various industrial (hazardous and non-hazardous) waste streams as a source of reactive silica and alumina, instead of virgin raw materials. Inorganic polymers have been extensively considered as a lower carbon footprint alternative to Portland cement. Nevertheless, they present other interesting properties, such as their negatively charged aluminosilicate network and their leaching behavior, which opens their use in other less investigated, but high added-value, applications (e.g. heavy metals [4] and dyes adsorbents [5] and pH regulators [6]).

The extraction of heavy metals or dyes from wastewaters is usually performed using powdered adsorbents (e.g. activated carbon). However, despite the interesting performance, their recovery after use is challenging. Bulk-type inorganic polymers that could be easily retrieved after exhaustion may be an excellent alternative to the use of powdered adsorbents. However, this topic has been somewhat neglected. The authors have recently reported the synthesis of mm-size waste-based inorganic polymer spheres [7], and their subsequent use as pH regulators in anaerobic digesters [8] and as methylene blue adsorbents [9]. This exciting material can be directly used in wastewater treatment systems without the need for support materials (e.g. polymer foams), this being a safer and easier alternative to the use of powdered adsorbents in wastewater treatment systems.

The scanning electron microscope (SEM) micrograph of our porous red mud-based inorganic polymer (geopolymer) spheres that features on the cover of this issue of Materials Today have been synthesized by means of a simple, low-cost, and industrially scalable procedure. The micrograph was collected on a Hitachi SU-70 SEM.

The material shown here was produced using 100% waste aluminosilicate sources through a suspension-solidification approach, as described in detail in Refs. [1], [7]. Besides red mud, another unexplored waste was used to produce the spheres – biomass fly ash coming from the paper and pulp industry. The mixture design and the spheres’ porosity can be used to tailor the alkalis leaching from the spheres over time in order to ensure continuous and prolonged pH buffer capacity. This feature is particularly relevant in several industrial applications where a narrow pH fluctuation is crucial to ensure high efficiency levels. The red mud-based spheres showed very high alkalis leaching from their structure which suggests that they will be excellent pH regulators in anaerobic digestion and in wastewater treatment systems.

These highly porous ∼3-mm spheres are envisioned for environmental remediation applications such as industrial wastewater treatment (adsorbents) and anaerobic digestion of easily biodegradable substrates, for methane generation, where pH control is challenging. These applications may allow the valorization of significant amounts of red mud, due to the renewed interest in biogas production using anaerobic digestion, and to the alarming worldwide water scarcity that makes industrial wastewater treatment mandatory.

We are currently evaluating the red mud-based inorganic polymer spheres, and other waste-based inorganic polymers, for environmental remediation applications in the form of bulk-type materials, instead of powders, as an appealing and safer alternative. We are also developing green and sustainable geopolymeric foams and inorganic polymer composites for non-structural applications attempting to increase the global sustainability of the construction sector.

Acknowledgments

This work was developed in the scope of the project CICECO – Aveiro Institute of Materials UID/CTM/50011/2013 (Compete Reference: POCI-01-0145-FEDER-007679), Associated Laboratory of University of Aveiro, financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. R.C. Pullar wishes to thank FCT grant IF/00681/2015 for supporting this work.

Red mud-based inorganic polymer spheres bulk-type adsorbents and pH regulators

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