Pyrolysis is a promising thermochemical process for managing sewage sludge while simultaneously producing biochar, a valuable co-product. This study systematically investigated the effects of the pyrolysis parameters of temperature (200-800 °C), residence time (5-60 min), and inert gas flow rate (0.25-1.0 dm³/min) on the properties of biochars obtained from sewage sludge. Comprehensive characterization of the biochars was conducted, including composition analyses (ultimate, proximate, elemental, and molecular), acidity, specific surface area and pore size, and the assessment of eight heavy metals (As, Cd, Pb, Cr, Zn, Mn, Ni and Cu) in the biochars and sewage sludge. The results showed pyrolysis temperature and residence time were the most critical parameters affecting biochar quality, with negligible influence of inert gas flow rate. Higher pyrolysis temperatures (>500 °C) increased biochar pH to alkaline values (>10), ash content, and nutrient concentrations (Ca, K, Mg, P). Temperatures above 600 °C significantly increased biochar surface area, reduced pore size, and yielded H/C ratios below 0.57, improving suitability for soil remediation. Pyrolysis also facilitated volatilization of heavy metals, particularly As and Cd, which were reduced to safe levels, with Cd removal exceeding 90% at 700 °C. Metal analysis confirmed the immobilization of heavy metals in biochar, significantly reducing the environmental risk, from high (PERI=1158) in the sewage sludge feedstock to low (PERI<50) in biochar obtained at temperatures above 600 °C. Most heavy metals in biochar at these temperatures were concentrated in oxidizable and residual fractions. The results provide valuable new data to guide development of pyrolysis for the sustainable management of sewage sludge.

High energy ball milling is used to make first the quaternary sulfide Cu₂ZnSnS₄ raw nanopowders from two different precursor systems. The mechanochemical reactions in this step afford cubic pre-kesterite with defunct semiconducting properties and showing no solid-state ⁶⁵Cu and ¹¹⁹Sn MAS NMR spectra. In the second step, each of the milled raw materials is annealed at 500 and 550 °C under argon to result in tetragonal kesterite nanopowders with the anticipated UV-Vis-determined energy band gap and qualitatively correct NMR characteristics. The magnetic properties of all materials are measured with SQUID magnetometer and confirm the pre-kesterite samples to show typical paramagnetism with a weak ferromagnetic component whereas all the kesterite samples to exhibit only paramagnetism of relatively decreased magnitude. Upon conditioning in ambient air for 3 months, a pronounced increase of paramagnetism is observed in all materials. Correlations between the magnetic and spectroscopic properties of the nanopowders including impact of oxidation are discussed. The magnetic measurements coupled with NMR spectroscopy appear to be indispensable for comprehensive kesterite evaluation.

Thermogravimetry coupled with thermal analysis and quadrupole mass spectroscopy TGA/DTA-QMS were primarily used to assess the oxidation susceptibility of a pool of nanocrystalline powders of the semiconductor kesterite Cu₂ZnSnS₄ for prospective photovoltaic applications, which were prepared via the mechanochemically assisted synthesis route from two different precursor systems. Each system, as confirmed by XRD patterns, yielded first the cubic polytype of kesterite with defunct semiconductor properties, which, after thermal annealing at 500 °C under neutral gas atmosphere, was converted to the tetragonal semiconductor polytype. The TGA/DTA-QMS determinations up to 1000 °C were carried out under a neutral argon Ar atmosphere and under a dry, oxygen-containing gas mixture of O₂:Ar = 1:4 (vol.). The mass spectroscopy data confirmed that under each of the gas atmospheres, a distinctly different, multistep evolution of such oxygen-bearing gaseous compounds as sulfur oxides SO₂/SO₃, carbon dioxide CO₂, and water vapor H₂O was taking place. The TGA/DTA changes in correlation with the nature of evolving gases helped in the elucidation of the plausible chemistry linked to kesterite oxidation, both in the stage of nanopowder synthesis/storage at ambient air conditions and during forced oxidation up to 1000 °C in the dry, oxygen-containing gas mixture.