Repository logo
 

Synthesis and Characterisation of Layered Double Hydroxides (LDHs) Using Red Mud and their Environmental Applications


Type

Thesis

Change log

Authors

Zhang, Yiyun 

Abstract

The disposal of industrial waste has posed many environmental challenges. Recently, transition to low-carbon economy has gained increasing awareness and is calling for new practices of waste reduction. Utilisation of waste materials therefore present twofold merits: (1) minimise waste disposal, (2) serve as alternative resources. Bauxite residue, also known as red mud (RM), the aluminium industry’s largest solid waste stream, is the by-product from the Bayer process of bauxite ore digestion. Worldwide, there are around 80 active Bauxite processing plants and over 150 million tonnes of red mud are generated every year, whereas the portion of RM having been reused is still negligible. This thesis explores the feasibility of using RM as a combined source of metal oxides for layered double hydroxides (LDHs) synthesis and evaluates the characteristics of the as-synthesised products and their potential in practical applications. LDHs have received great attention recently for their unique structural properties and diverse applications, such as catalysts and catalyst precursors, anion exchangers, flame retardants, electroactive and photoactive materials etc. In particular, LDHs are excellent anion clay materials that often have high removal capacity for organic and inorganic water pollutants as adsorbents.

Two RM samples were sourced from Aluminium-Oxide Stade GmbH (AOS) in Germany and Aughinish Limerick plant in Ireland, respectively. In the first part of this thesis, a thorough characterisation of RM raw materials was performed. Based on the review of RM surface chemistry and the conventional ways for preparing LDH, four different synthetic scenarios were proposed. The key operating parameters include different pre-treatments (heat treatment, dry grinding and wet grinding) and process variables (dispersion, MII/MIII ratio, liquid-to-solid ratio and aging condition). Subsequently, the physiochemical properties of the as-synthesised products were characterised to assess if they meet the ‘golden rules’, i.e., agree with a set of structure features of a typical LDH. The formation of LDH phases was most easily observed in Scenario III products, which were named as RM-LDHs and synthesised from Ireland RM and reactive MgO. The optimal experimental parameters were determined, which were CO32- solution environment, L/S ratio = 20, MII/MIII ratio = 3.0 and aging at 70°C for 72 h. RM-LDHs were recognised to be carbonates intercalated Mg2+/(Al3+, Fe3+)-LDH, with a higher substitution of Fe+3 than Al3+ in the hydroxide sheet.

The ‘role plays’, i.e., chloride binding study and dye adsorption study, were carried out to further understand the anion removal capacity, thermal stability and ‘memory effect’ of the synthesised products. A material screening test was firstly conducted, suggesting RM-w-CLDH (calcined RM-LDH with wet grinding pre-treatment) was the optimal product for chloride removal. While the raw materials for synthesising RM-LDHs exhibited significantly lower sorption capacities, indicating the removal of chloride was mostly contributed by the CLDH phase in RM-w-CLDH. Batch sorption studies found the chloride sorption capacities of RM-w-CLDH and commercial CLDH were 53.3 mg/g and 17.3 mg/g. Microstructural characterisations revealed that ion exchange and reformation of the initial layered structure via ‘memory effect’ were the main sorption mechanisms for RM-w-CLDH. This agrees well with the literature and further confirms the existence of LDH structure in RM-w-LDH.

In the last section, the selected RM-LDHs were investigated for Remazol Brilliant Blue R dye (RBB) immobilisation. RBB is among the reactive dyes, widely used in textile industry and however, it is a harmful dye and can damage aquatic and vegetal life. The non-biodegradable and chemically stable characteristics of RBB make it difficult to be removed from effluents using conventional treatment process. Adsorption appears to be a viable way to control the dye pollutant in wastewater. In the present research, sorption studies were also performed for raw materials and the commercial Mg/Al LDH as comparisons. All adsorbents showed certain sorption for RBB and the calculated sorption capacities were in the order of RM-w-CLDH > RM-w-LDH > RM-d-LDH > Commercial LDH > RM > CRM (i.e., calcined red mud). The sorption mechanisms were determined through batch sorption tests and microstructure characterisations. Reconstruction of LDH structure and partial intercalation of RBB ions were suggested to be the main sorption mechanisms for RM-w-CLDH. Regeneration study indicated that the contaminated sorbent RM-w-CLDH-R could be cleaned up by calcination treatment and reused in dye sorption. After the 3rd adsorption cycle (and the 2nd regeneration cycle), the removal rate maintained at 87.2% of the original RM-w-CLDH. In terms of RM-LDHs, ion exchange and external surface adsorption were the dominant mechanisms. For the raw material IRM and Cal-IRM, however, the adsorption rate constants were much lower, and the equilibrium took longer to be achieved. Physical adsorption was the main sorption mechanism. The physicochemical characteristics of RM-LDHs, RM-w-CLDH and commercial LDH, including the BET surface area, pore size, total pore volume and particle size, were not the controlling factors to the sorption performance.

In summary, this thesis proposed a novel strategy for RM reuse, working as a proof-of-concept. It confirmed that LDHs were successfully synthesised from RM, and RM-w-CLDH in particular has a great potential to serve as chloride inhibitor and dye amendment in wastewater. However, the effectiveness of this synthesis is sensitive to the origin of RM. Follow-up research is encouraged to consider more precise control of the MII/MIII ratio and reaction pH. The predominant drawback of RM-LDH products is the impurities, some inherited from RM and some formed during the synthesis. Further study is expected to identify the effects of these impurities in environmental applications.

Description

Date

2020-01-16

Advisors

Al-Tabbaa, Abir

Keywords

Red mud, Layered double hydroxides, LDH, Environmental applications, Synthesis

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge