Review: Biotic and abiotic approaches to artificial humic acids production

Agricultural practices such as successive cropping and irrational fertilization have led to a steady decline in soil fertility in many regions. Humic acids (HAs) are a type of effective additives for enhancing soil fertility, but natural HAs reserves are limited. Artificial HAs can be produced from biomass by biotic composting and abiotic hydrothermal reaction (HTR). The principles, influencing factors, and product characteristics of these methods have been comprehensively summarized in previous reports, but less attention has been paid to issues linked to scaling up production, such as facilities, equipment, process control, and energy consumption. This paper presents a systematic review on the potential engineering issues involved in artificial HAs production to serve as a reference for actual project designs. Biotic method was summarized as mainly based on the type, scale, configuration and control of the reactor. The multifunctional compost reactor showed better HAs production performance, with yields ranging from 35 to 82 g kg−1 and 139–248 g kg−1 at lab- and pilot-scale, respectively. The abiotic method was categorized based on the HTR reaction steps. The two-step and hydrochar methods have high potential for HAs production from recalcitrant biomass, with maximum reported yields of 283 and 435 g kg−1, respectively. A method for evaluating the energy consumption of HTR is presented, estimating the energy input of HTR at 1825–31987 kJ kg−1 HAs, which is mainly influenced by the HAs yield and reaction duration.