The thermochemical conversion of heavy-metal (HM)–contaminated biomass offers a pathway to produce syngas and functional biochar while simultaneously remediating the environment. Here, Zn/Pb-contaminated birch biomass from a phytoremediation site was carbonized and the resulting char was gasified under two oxidizing atmospheres (100 vol% CO2 and 50/50 vol% H2O/CO2) at 700–900 °C. Gas composition, char conversion, biochar properties (SEM, Raman, N2 adsorption), and solid-phase metal retention (ICP-OES) were evaluated together with thermodynamic equilibrium calculations (FactSage 8.4/FactFlow) based on measured elemental inputs (C, H, O, N, S, ash-forming elements, and trace metals). Gasification in an H2O/CO2 atmosphere markedly increased reactivity, achieving > 60 % conversion at 700 °C compared with < 20 % under CO2 alone. The product gas was dominated by CO and CO2, with enhanced H2 under H2O/CO2. Zn retention decreased from 52.1 % at 700 °C to < 2 % at 900 °C, while Pb retention decreased from 86.1 % to 13.1 % under H2O/CO2. Activation produced biochars with BET surface areas up to ∼ 673 m2 g−1 and average pore diameters up to ∼ 1.50 nm. Equilibrium calculations indicated increased Zn volatilization above ∼ 800 °C and predicted Pb stabilization as condensed PbO/PbS at lower temperatures, while K, Ca and Al were predicted to form stable condensed silicates/oxides. Overall, the combined experimental and equilibrium analysis quantifies trade-offs between conversion/activation performance and HM retention during CO2 and H2O/CO2 gasification of contaminated biomass.

Municipal sewage sludge (MSS) has been identified as a promising feedstock for producing biochar with potential applications as a soil conditioner and animal feed. However, the high heavy metal content and limited availability of nutrients, such as phosphorus (P), pose significant challenges. This study aimed to improve the quality of MSS-derived biochar through copyrolysis with wheat straw (rich in K and Si) and bakery waste husks (rich in K) at temperatures of 500, 650, and 900 °C. Thermodynamic equilibrium calculations (TEC) were performed using FactSage and HSC Chemistry to predict the stability of P-bearing compounds and the fate of heavy metals in the biochars. The morphology and physicochemical properties of the biochars were examined by using SEM and Brunauer-Emmett-Teller (BET) analyses. The results indicate that increasing the proportions of wheat straw and bakery waste husks, along with higher pyrolysis temperatures, reduced the biochar yield. TEC demonstrated that these blends enhanced the formation of plant-available phosphates compared with pure MSS biochar. This improvement was primarily because of the formation of K/Mg-bearing phosphates in different amorphous and crystalline phases, such as K2P2O7, CaK2P2O7, KPMgO4, and KZnPO4, instead of Fe/Al-based phosphates. Additionally, copyrolysis reduced the concentrations of heavy metals such as cadmium (Cd), lead (Pb), and zinc (Zn) in the biochars compared to MSS pyrolysis alone. However, it had no significant effect on the copper (Cu), chromium (Cr), and nickel (Ni) levels. In conclusion, copyrolysis with wheat straw and bakery waste husks not only improved the nutrient profile and physicochemical properties of MSS-derived biochar but also mitigated heavy metal contamination. Additionally, this method reduced the presence of heavy metals, making it a more suitable alternative to biochar produced through monopyrolysis for use in agricultural applications.

Phosphorus (P) is essential for life and vital for agricultural fertilizers, with growing demand driving interest in recovering P from secondary resources like municipal sewage sludge (MSS). MSS contains nutrients such as P, potassium, calcium, sulfur, and magnesium, making it a potential agricultural resource. However, contaminants, such as heavy metals, organic pollutants, and pathogens, along with variability in MSS composition limit their direct use. Pyrolysis offers a sustainable solution, converting MSS into nutrient-rich biochar, while stabilizing organic matter and reducing contaminants. Factors such as temperature, feedstock composition, and reactor design play crucial roles in optimizing phosphorus recovery and heavy metal removal. This thesis investigates the optimization of the pyrolysis process for phosphorus recovery and heavy metal removal from MSS through biochar production. In Papers I and II, three types of MSS – digested and undigested samples – were pyrolyzed at 500-900 °C using a fixed bed. Chemical composition analysis of the biochars was combined with thermodynamic equilibrium calculations (TECs) to model phosphorus speciation. The results showed that over 90% of phosphorus was retained in the biochar, with temperature having a minimal effect on phosphorus release. Furthermore, copper (Cu), zinc (Zn), and cadmium (Cd) concentrations in the biochar were reduced to meet Swedish agricultural standards. Paper III explored the behavior of trace elements during pyrolysis in a rotary pyrolyzer and identified temperature-dependent patterns in their speciation. The optimal pyrolysis temperature for pollutant removal and carbon preservation was found to be between 600-700 °C. Paper IV expanded these findings by engineering biochar to enhance nutrient recovery and minimize environmental risks. Co-pyrolysis of MSS with wheat straw and bakery waste improved the nutrient profile of biochar while immobalizing heavy metal concentrations compared with MSS biochar alone. This engineering approach facilitated the formation of plant-available phosphorus compounds such as KPM_gO₄ and enhanced the suitability of biochar for agricultural applications.

Municipal sewage sludge (MSS) contains significant amounts of trace elements including zinc, copper, cadmium, and lead. This study investigated the behavior of these trace elements in municipal sewage sludge biochar during the pyrolysis of both anaerobically digested and undigested sludge at temperatures ranging from 500 °C to 900 °C using a rotary pyrolyzer. Microwave plasma-atomic emission spectroscopy (MP-AES) was used to measure the concentrations of trace elements in the biochars. Additional analyses included Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis to determine the activation energies, and Brunauer–Emmett–Teller (BET) analysis coupled with scanning electron microscopy to assess the porosity, specific surface area, and morphology at different temperatures. Thermodynamic equilibrium calculations (TECs) were utilized to understand the speciation of trace elements under varying thermal conditions. The results demonstrated that as the pyrolysis temperature increased, both the undigested and digested biochars exhibited higher surface areas and total porosity volumes, along with a decrease in the average pore size. The digested biochar showed a higher surface area and pore volume at 500 °C than the undigested biochar. However, both types processed at 900 °C had similar average pore diameters of approximately 7.5 nm, according to BET analysis. All biochars had H/C ratios below and around 0.2, indicating strong degradation resistance. TECs results indicated that, at 500 °C, the thermal volatility sequence of the trace elements was established as Cr < Ni < Cu < Mn < Zn < Pb < Cd. Consequently, higher temperatures led to reduced concentrations of Cd, Pb, and Zn in all biochars while Cr, Ni, and Cu were largely retained in the biochar. Phosphorous (P) was enriched in the biochars at all temperatures for both MSS, however anaerobically digested MSS biochar exhibiting higher concentration of nutrient such as P, higher porosity and a finer pore structure compared to undigested one. Biochar yield decreases as pyrolysis temperature rises (500–900 °C) for both digested (BSS) and undigested (LSS) sludge, with BSS consistently yielding more than LSS.

To achieve a closed nutrient cycle and more sustainable food production, enhanced nutrient recycling in the agri-food system is a necessity. Pyrolysis is an emerging technology to recycle the nutrient content of sewage sludge. The produced biochar can be used to reduce the need for mineral fertilizers; in addition, pyrolysis can also handle potential pollutants such as microplastics and pathogens present in sewage sludge. In this research, a life cycle assessment (LCA) was carried out to determine the environmental impact of sewage sludge pyrolysis as an alternative to current practices of two different cases of sewage sludge treatment in two municipalities in Sweden. The results indicated that avoiding mineral fertilizer production by using biochar has a significant influence on the environmental benefits. Furthermore, it showed that an integrated system of anaerobic digestion followed by pyrolysis could perform as the most environmental-friendly option for sewage sludge treatment with a lower risk of transferring pollution to the soil.

The present study investigates how the original sewage sludge characteristics influence the composition of sewage sludge-based chars for land applications. Sewage sludge from two different wastewater treatment plants in Sweden was pyrolyzed at 500, 700, and 900 °C, and the resulting chars were analyzed. Thermodynamic equilibrium calculations (TEC), together with chemical fractionation, were implemented to simulate the char after the pyrolysis process at different temperatures. The results showed that, in general, for both the municipal sewage sludge (MSS), phosphorus (P) was significantly retained in the char at various temperatures. However, no specific correlation could be found between the pyrolysis temperature and the amount of P remaining. With regard to the heavy metals removed from the char after the pyrolysis reaction, the concentrations of copper, chromium, lead, nickel, zinc, and cadmium were below the limits of the Swedish regulations for farmland application.

Sewage sludge is regarded as a potential source for soil fertilizer However, the direct utilization of sewage sludge in agricultural land is restricted since it also contains heavy metals, pathogens, and toxic compounds. Pyrolysis of the sewage sludge destroys the organic pollutants and partly volatilizes the heavy metals. In this study, pyrolysis of sewage sludge was carried out in order to determine the optimum residence time and temperature to recover the phosphorous and remove heavy metals from the resultant sewage sludge char (SSC). Pyrolysis was conducted on dried sewage sludge (DSS) by means of thermogravimetric analysis (TGA) and high-temperature oven with an N2-atmosphere. Microwave Plasma-Atomic Emission Spectroscopy (MP-AES) was used to determine the concentration of P and trace elements in the resulting solid char fraction. A combination of chemical fractionation (step-by-step leaching) of the DSS and thermodynamic equilibrium calculations were utilized to estimate the availability of phosphorous and removal of heavy metals in the SSC fraction at different temperatures. The results from the thermodynamics calculation were in line with the measured chemical composition of the SSC. Furthermore, the energy contents of the SSC obtained at different temperatures were measured. The pyrolysis evaluation results indicate that phosphorous was enriched in the char, while lead, zinc, and cadmium were significantly removed.