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Surface -functionalised silica networks are advanced adsorbents. They have been given much attention for treating wastewater using the adsorption technique due to the silanol reactivity, resulting in strong binding affinities towards many pollutants. This review discusses the removal of anionic azo dyes utilising various functional groups such as amines, surfactants, polymers, macrocyclic, and other chelating groups functionalised on silica's surface. This review also reveals the steadily increasing interest in surface-functionalised silicas as adsorbents, emphasising the scholarly advancements in this field as a platform for future research. For that, adsorption capacities with different experimental conditions have been compared. The possible adsorption mechanisms, rate-limiting step, and factors affecting the anionic azo dye adsorption process have been comprehensively discussed. This review discloses that adsorbent characteristics such as porosity and functional groups, besides structural properties of an anionic azo dye, significantly affect adsorption. The adsorption process followed the Langmuir isotherm and pseudo-second-order models, with a predominantly spontaneous and endothermic nature. Multiple interactions, including electrostatic interaction, π-π interactions, and hydrogen bonding, are observed between dyes and functionalised silicas, indicating the adsorption process's complexity. Regeneration and cost-economic analysis are also presented to provide a roadmap for sustainable improvements. Chemical and biological regeneration techniques restore > 80% of the spent functionalised silicas. There is a significant opportunity to improve their efficiencies and regenerability, resulting in surface-functionalised silicas being used commercially instead of only in the laboratory. Finally, future research has been proposed by identifying current research gaps, particularly concerning the application of functionalised silicas in wastewater treatment. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mono-amine modified silica (MAMS) was used for the removal of azo dye. [•] Bioregeneration duration increased in decreasing dye-loaded MAMS particle size. [•] Bioregeneration of AO7-loaded MAMS decreased in decreasing particle size. [•] Acclimation without co-substrate was preferred for bioregeneration of spent MAMS. [Copyright &y& Elsevier]

Highlights: [•] Longer anoxic-REACT period resulted in higher dye decolorization efficiency. [•] Higher decolorization rate observed for Acid Red 14 (naphthol type) azo dye. [•] Higher COD removal rate found for Acid Yellow 9 (non-naphthol type) azo dye. [•] Dye molecular structures affected performance of mixed bacterial culture in SBR. [•] Better performance of SBR operated without co-substrate in color and COD removal. [Copyright &y& Elsevier]

Abstract: In this study, the operational factors affecting the bioregeneration of AO7-loaded MAMS particles in batch system, namely redox condition, initial acclimated biomass concentration, shaking speed and type of acclimated biomass were investigated. The results revealed that with the use of mixed culture acclimated to AO7 under anoxic/aerobic conditions, enhancement of the bioregeneration efficiency of AO7-loaded MAMS and the total removal efficiency of COD could be achieved when the bio-decolorization and bio-mineralization stages were fully aerated with dissolved oxygen above 7 mg/L. Shorter duration of bioregeneration was achieved by using relatively higher initial biomass concentration and lower shaking speed, respectively, whereas variations of biomass concentration and shaking speed did not have a pronounced effect on the bioregeneration efficiency. The duration and efficiency of bioregeneration process were greatly affected by the chemical structures of mono-azo dyes to which the biomasses were acclimated. [Copyright &y& Elsevier]