Electrochemical wastewater treatment is a promising technique to remove recalcitrant pollutants from wastewater. However, the complexity of elucidating the underlying degradation mechanisms hinders its optimisation not only from a techno-economic perspective, as it is desirable to maximise removal efficiencies at low energy and chemical requirements, but also in environmental terms, as the generation of toxic by-products is an ongoing challenge. In this work, we propose a novel combined experimental and computational approach to (i) estimate the contribution of radical and non-radical mechanisms as well as their synergistic effects during electrochemical oxidation and (ii) identify the optimal conditions that promote specific degradation pathways. As a case study, the distribution of the degradation mechanisms involved in the removal of benzoic acid (BA) via boron-doped diamond (BDD) anodes was elucidated and analysed as a function of several operating parameters, i.e., the initial sulfate and nitrate content of the wastewater and the current applied. Subsequently, a multivariate optimisation study was conducted, where the influence of the electrode nature was investigated for two commercial BDD electrodes and a customised silver-decorated BDD electrode. Optimal conditions were identified for each degradation mechanism as well as for the overall BA degradation rate constant. BDD selection was found to be the most influential factor favouring any mechanism (i.e., 52-85% contribution), given that properties such as its boron doping and the presence of electrodeposited silver could dramatically affect the reactions taking place. In particular, decorating the BDD surface with silver microparticles significantly enhanced BA degradation via sulfate radicals, whereas direct oxidation, reactive oxygen species and radical synergistic effects were promoted when using a commercial BDD material with higher boron content and on a silicon substrate. Consequently, by simplifying the identification and quantification of underlying mechanisms, our approach facilitates the elucidation of the most suitable degradation route for a given electrochemical wastewater treatment together with its optimal operating conditions.

The ongoing challenge of water pollution by contaminants of emerging concern calls for more effective wastewater treatment to prevent harmful side effects to the environment and human health. To this end, this study explored for the first time the implementation of single-crystal boron-doped diamond (BDD) anodes in electrochemical wastewater treatment, which stand out from the conventional polycrystalline BDD morphologies widely reported in the literature. The single-crystal BDD presented a pure diamond (sp³) content, whereas the three other investigated polycrystalline BDD electrodes displayed various properties in terms of boron doping, sp³/sp² content, microstructure, and roughness. The effects of other process conditions, such as applied current density and anolyte concentration, were simultaneously investigated using carbamazepine (CBZ) as a representative target pollutant. The Taguchi method was applied to elucidate the optimal operating conditions that maximised either (i) the CBZ degradation rate constant (enhanced through hydroxyl radicals (^•OH)) or (ii) the proportion of sulfate radicals (SO₄^•−) with respect to ^•OH. The results showed that the single-crystal BDD significantly promoted ^•OH formation but also that the interactions between boron doping, current density and anolyte concentration determined the underlying degradation mechanisms. Therefore, this study demonstrated that characterising the BDD material and understanding its interactions with other process operating conditions prior to degradation experiments is a crucial step to attain the optimisation of any wastewater treatment application.

Natural fibre-based materials offer various advantages compared to synthetic fibres, however their applications are limited mainly due to their hygroscopic properties, which are affected by their chemical composition, microstructure and the porosity of the plant cells of which the fibre is composed. Therefore, this work investigates the hygroscopic behavior of natural fibres to obtain a better understanding of the relation of the chemical composition of the fibres, their crystallinity, and their equilibrium moisture content. The crystallinity index was determined to include amorphous cellulose into the developed models. Nine biomass samples were selected (flax, hemp, jute, spruce, bamboo, corn stalks, palm leaves, rice husk and wheat straw) to construct models via linear regression to predict the moisture sorption behavior of natural fibres. Thorough statistical (ANOVA, RMSE) analysis showed that the developed models are relevant and descriptive. From all major plant cell wall constituents (lignin, crystalline cellulose, amorphous cellulose and hemicellulose), it is hemicellulose's hygroscopicity that is largely responsible for the moisture uptake of the fibres, with (amorphous) cellulose and lignin playing a (much) smaller role. This study has improved the understanding of the hygroscopic behavior of natural fibres, and is important for optimal application of these fibres in composite materials.