Characterisation of biochar from manure and sewage sludge using XRD and FTIR

Abstract

The characterisation of biochars produced from diverse feedstocks, including dry manure, digestate manure, biologically treated dry sewage sludge, and chemically treated digestate sewage sludge, was conducted using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). These analyses revealed significant structural and chemical transformations during pyrolysis at 400°C and 600°C, influencing the biochars' suitability for applications in soil improvement, nutrient retention, and carbon sequestration.

FTIR analysis indicated distinct functional group changes in all biochars, particularly reductions in hydroxyl (-OH) and carbonyl (C=O) groups, reflecting dehydration and the breakdown of oxygenated compounds. Aliphatic C-H peaks diminished, signifying increased aromaticity and the formation of more stable carbon structures. Nitrogen-related functional groups decreased across all feedstocks, suggesting nitrogen stabilisation into recalcitrant forms. Persistent phosphate (P-O) and metal oxide peaks confirmed the retention of essential nutrients such as phosphorus, calcium, and magnesium, making biochars valuable for soil fertility enhancement.

XRD analysis provided further insights into the mineralogical transformations in biochars. Dry manure biochars showed the formation of stable crystalline phases such as berlinite (AlPO₄) and sylvite (KCl), which contribute to phosphorus and potassium retention, respectively. Digestate manure biochars exhibited additional silicate and aluminosilicate phases, enhancing their cation exchange capacity (CEC) and nutrient-holding potential.

Biologically treated dry sewage sludge biochars demonstrated significant phosphorus stabilisation in berlinite and chlorapatite and retaining silicates like quartz and mica. These mineralogical changes enhance biochar's potential as a slow-release fertiliser while improving soil structure and aeration. Similarly, biologically treated digestate sewage sludge biochar retained essential nutrients like sulfur and silicon, with increased crystallinity ensuring better structural stability and moisture retention. The presence of barite (BaSO₄) and gypsum (CaSO₄·2H₂O) highlighted the biochar's ability to retain sulfur, an essential nutrient for plant growth and metabolic processes.

Chemically treated sewage sludge biochars exhibited stronger silicate and phosphate peaks, reflecting enhanced inorganic content and long-term stability. Variscite (AlPO₄·2H₂O) transformed into berlinite during pyrolysis, stabilising phosphorus for sustained nutrient availability. Sylvite (KCl) emerged as a significant phase, providing a readily available potassium source vital for crop yield and stress tolerance. The presence of calcite and gypsum further supported the biochar’s liming capacity, which is helpful for pH regulation in acidic soils.

The findings underline the importance of feedstock selection and pyrolysis conditions in tailoring biochar properties for specific environmental and agricultural applications. Pyrolysis stabilises organic matter and enhances biochar’s structural integrity, nutrient retention, and pollutant adsorption capacity. This study highlights the potential of biochar to address soil degradation, improve crop productivity, and support sustainable waste management in agriculture.