Potassium recycling from organic waste and urine for the production of engineered biochars to use as fertilizers

Abstract

Potassium (K) is one of the most essential nutrients required by plants, playing a key role in processes such as enzyme activation, stomatal regulation, and carbohydrate transport. In highly weathered soils, such as those found in Brazil, the naturally low availability of K demands high applications of potassium fertilizers, especially KCl, which presents high solubility, a high salt index, and elevated chloride content. These characteristics can negatively affect crop development, highlighting the need for more sustainable alternatives. Organic residues have emerged as potential sources of potassium, provided their use is technically evaluated to prevent environmental risks. Among these alternatives, biochar (BC) has gained prominence over the past decades for its potential as a nutrient source in agricultural systems. This thesis aimed to investigate strategies for potassium recovery and utilization from different waste materials, using biochar-based technologies and chemical precipitation. The following research questions were evaluated: (i) What is the potential of different K-rich biomasses (banana peel, coffee husk, and chicken manure), processed at two pyrolysis temperatures (300 and 650 °C), and their co-pyrolysis with KCl, in producing potassium fertilizers with high agronomic value? (ii) What is the potassium release kinetics from these fertilizers under controlled conditions and real corn cultivation scenarios? (iii) Are these materials agronomically effective in partially or fully replacing KCl for corn cultivation in soils with different textures and organic matter contents? (iv) What are the main chemical and physical factors affecting potassium recovery from synthetic urine in the form of K-struvite? (v) Are thermodynamic models such as PHREEQC and Visual MINTEQ effective in predicting K-struvite precipitation under different chemical conditions? (vi) Can biochar be used as an effective seeding material to enhance K recovery as K-struvite? (vii) Does the addition of biochar alter the factors influencing the precipitation of pure K-struvite? (viii) Does the type of biochar affect K-struvite formation? (ix) Do mixing and resting times influence the crystallization of K-struvite? The results indicated that biomass type, pyrolysis temperature, and co-pyrolysis with KCl significantly affect the agronomic efficiency of biochar-based fertilizers. Banana peel pyrolyzed at 650 °C was the most promising material, allowing for an 82% reduction in KCl use. In sandy soils, split application or granulation of the composites was recommended to reduce potassium leaching. In the context of synthetic human urine, potassium recovery as K-struvite was maximized at pH 10 and a Mg:K:P molar ratio of 2:1:2, with efficiency exceeding 78%. PHREEQC proved more accurate than Visual MINTEQ in predicting K-struvite formation. Biochars derived from wood pyrolyzed at 500 °C enhanced nucleation and K-struvite recovery, while manure-derived biochars released K into the solution, hindering precipitation. Kinetic tests showed that biochar + K-struvite composites act as slow-release potassium carriers, with strong potential as agricultural fertilizers. Thus, this thesis proposes sustainable and integrated approaches for potassium recycling from waste materials, with applications in circular agriculture and decentralized sanitation systems.