Special issue on "advanced technology of waste treatment" [Editorial]

The protection of human health and the environment (representing the main reason for waste management), as well as the sustainable use of natural resources, requires chemical, biological, physical and thermal treatment of wastes [...

Mineral carbonation of basic oxygen furnace slags

Mineral carbonation is a method in carbon capture and utilization (CCU) in which carbon dioxide reacts with natural or synthetic mineral phases to form carbonates. In this study, BOF slag consisting of alite, Ca3SiO5, belite, Ca2SiO4, melilite, (Ca,Na)2(Mg,Al)[4][Si2O7], brownmillerite, Ca(Fe,Al)2O5, calcium ferrite, Ca2FeO4 and Ca-, Mg- and Mn-bearing wuestite, (Mg,Ca,Mn,Fe)O, was crushed into different particle size fractions and exposed over various durations (1 d, 3 d, 9 d, 14 d, 24 d) at a grate to 120 °C hot off-gas with a CO2 content of 25%. However, the total inorganic carbon (TIC) content never increases above the detection limit of 0.5% throughout the experimental duration. The determination of the carbonation depth using phenolphthalein does not reveal a homogeneous carbonation front, but an irregular carbonation. This observation was confirmed by microprobe analyses using elemental mapping. The solubility of the slag increases with increasing carbonation, e.g., the leachability of sulfate increases from 7.8 to 8270 mg/kg dry matter (DM), and of calcium from 940 to 3860 mg/kg DM. The leaching of environmentally relevant element varies: the leachable concentration of molybdenum increases from 0.017 mg/kg DM to 0.089 mg/kg DM, that of chromium remains constant (ca. 0.05 mg/kg DM) whereas that of vanadium decreases from 1.1 to 0.45 mg/kg. In summary, the chosen carbonation technology must be improved to enhance carbonate yield

Thermal delamination of end-of-life crystalline silicon photovoltaic modules

Thermal delamination – meaning the removal of polymers from the module structure by a thermal process – as a first step in the recycling of crystalline silicon (c-Si) photovoltaic (PV) modules in order to enable the subsequent recovery of secondary raw materials was investigated. A correlation between treatment temperature and duration was established by an iterative process. Furthermore, chemical characterization of the resulting solid outputs (glass, cell, ribbons and residues) was performed in order to assess their further processing options. Additionally, the effect of removing the backsheet as a pre-treatment before the actual delamination process was investigated in relation to the aforementioned aspects of treatment duration and output quality. Results show that increased temperatures reduce the necessary treatment duration (65 minutes at 500°C, 33 minutes at 600°C) while generating the same output quality. The backsheet removal leads to an additional duration decrease of more than 45% at each considered temperature, while also having positive effects relating to fewer solid residues and easier flue gas handling. In regard to the main output specifications no significant influence of the pre-treatment is observed. Overall thermal delamination can be seen as a feasible method in order to obtain high value secondary raw materials from c-Si PV modules, while backsheet removal as pre-treatment should be considered as advantageous from multiple standpoints

Application and development of zero-valent iron (ZVI) for groundwater and wastewater treatment

Zero-valent iron has been used for more than 130 years for water treatment. It is based on redox reactions as well as on sorption to the corrosion products of iron. It is successfully applied for the removal of metals and organic pollutants from groundwater and wastewater. There are different variations how zero-valent iron can be used, especially (i) permeable reactive barriers, (ii) fluidized bed reactors and (iii) nanoscale zero-valent iron. Permeable reactive barriers are used for in situ treatment of groundwater in trench-like constructions or in a funnel and gate system. Their advantages are low maintenance cost, inexpensive construction and prevention of excavation wastes, and their disadvantages are surface passivation and clogging of pores by corrosion products. Zero-valent iron nanoparticles are injected directly in contaminated soil or groundwater. Their advantages are a higher reactivity than coarse-grained zero-valent iron and their mobility in the subsurface to reach the contaminated areas. However, they also have some major disadvantages like fast ageing in the system, phytotoxicity, agglomeration during migration and high costs. The latest development is a fluidized bed process (“ferrodecont process”) which avoids the passivation and clogging observed in permeable reactive barriers as well as the high costs and toxicity issues of nanoscale zero-valent iron. First results of this technology for Cr(VI) and organically contaminated groundwaters and metal removal from industrial wastewaters are highly promising