Electron paramagnetic resonance (EPR) has been extensively used for the identification of free radicals that are generated from advanced oxidation processes (AOPs) so as to establish the reaction mechanism. However, some misinterpretations or controversies on the identity of detected EPR signals remain in the literature. This study, with Cu(II)-based AOPs as examples, comprehensively investigated the origin of 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adducts in Cu(II) alone, Cu(II)/H2O2, Cu(II)/peroxymonosulfate (PMS), and Cu(II)/peroxydisulfate (PDS) systems. In most Cu(II) systems, DMPO-OH signals can be detected even without any peroxygens, indicating the presence of other origins of this adduct in addition to the genuine spin trapping of •OH by DMPO. According to the formed secondary radical adducts (DMPO-OCH3 from a nonradical process or DMPO-CH2OH from a radical oxidation) derived from methanol quenching, we propose that CuO+, instead of free radicals, is involved in the Cu(II)/PMS system, while •OH is indeed generated in the Cu(II)/H2O2 and Cu(II)/PDS systems under neutral conditions. Notably, 17O-incorporation experiments demonstrate that -OH in the detected DMPO-OH adduct originates 100% from water in the Cu(II) alone system but the amount of -OH is over 99.8% from the oxidant while peroxygens are added. In addition, DMPO-O2- appears only in the Cu(II)/PDS system under highly alkaline conditions and H2O is not involved in superoxide formation.

EPR Evidence for Mechanistic Diversity of Cu(II)/Peroxygen Oxidation Systems by Tracing the Origin of DMPO Spin Adducts.

Nitrogen-doped hydrochar prepared by biomass and nitrogen-containing wastewater for dye adsorption: Effect of nitrogen source in wastewater on the adsorption performance of hydrochar.

Performance comparison of phosphorus recovery from different sludges in sewage treatment plants through pyrolysis

Co-hydrothermal carbonization of rape straw and microalgae: pH-enhanced carbonization process to obtain clean hydrochar

Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5∼6 €/kg P, while the cost of recovering P by thermochemical methods is about 2∼3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions. 

Phosphorus recovery from sewage sludge ash (SSA): An integrated technical, environmental and economic assessment of wet-chemical and thermochemical methods.

Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus.

Mechanistic insights into enhanced waste activated sludge dewaterability with Cu(II) and Cu(II)/H2O2 treatment: Radical and non-radical pathway.

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Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus.

Mechanistic insights into enhanced waste activated sludge dewaterability with Cu(II) and Cu(II)/H2O2 treatment: Radical and non-radical pathway.