A review on dissolved organic matter derived from biochar in soil: Characterizations, interaction with pollutants, and transformation

Abstract

Biochar-derived dissolved organic matter (BCDOM), an essential component of biochar, plays a vital role in regulating the physicochemical and biological properties of soils during biochar application. However, the influence of BCDOM on soil organisms has not been clearly explained. Hence, this review aims to discuss the factors affecting BCDOM and its interaction with soil substances including organic pollutants, heavy metals, and microorganisms. Results displayed that the quantity of BCDOM ranges from 0.17 to 37.03 mg/g, which was influenced by feedstock, preparation methods of biochar, and extraction methods. With the decrease in lignin content of feedstocks, carbonization temperature, and acidity of extraction solution, the content of BCDOM increased. Through complexation and adsorption, protein-like components in BCDOM interact with heavy metals, promoting the adsorption and immobilization of heavy metals onto biochar. Furthermore, BCDOM enhances the adsorption of organic pollutants by biochar through π−π interactions, hydrogen bonding, and redox processes. More importantly, BCDOM promotes plant growth by enhancing microbial activities, providing nutrients, and improving soil properties. However, the transport and fate of BCDOM in soil have not been well studied, and more researches are needed to explore the interaction mechanisms between BCDOM and soil organisms.

1. Introduction

Environmental pollution is an increasingly serious problem that has attracted widespread attention. In particular, soil pollution caused by heavy metals and organic contaminants has increased dramatically over the last 30 years [1−5]. Therefore, to ensure sustainable agricultural development, it is urgent to search for a cost-effective and environmentally friendly approach to reduce soil pollution.

Biochar, also known as bio-coal, is generally produced by pyrolysis of natural and waste biomass under an inert atmosphere at a wide range of temperatures. Due to its unique physicochemical properties, biochar has received much attention over the past two decades for its ability to modify soil properties [6−8], clean fuel production [9, 10], waste management strategies [11−14], environmental remediation [15−17], and climate change mitigation [18, 19]. Previous research has indicated that biochar can immobilize heavy metals and organic contaminants, thereby reducing soil pollution [20−25]. One study indicated that after the application of multiple-modified biochar, the extractable Cu(II) and Cd(II) were decreased by 100% and 92.0% for farmland soil, and 100% and 90.3% for vegetable soil, respectively [26]. In addition, the application of biochar to soil could promote soil bacteria growth, supply nutrients, and modify soil structure and properties, including soil pH, organic carbon, available phosphorous and potassium, electrical conductivity, and water retention, thus increasing crop yields [27−29].

Due to the above advantages, biochar has been favored by a wide range of research in the field of soil remediation in recent years [25, 30]. When biochar is applied to the soil, it can release dissolved organic matter (BCDOM) into the soil environment under the conditions of irrigation or natural rainfall, thus changing the properties of the soil by binding soil particles together, leading to better water retention, lower erosion, and higher soil porosity [31, 32]. Generally, BCDOM is composed of organic molecules that can pass through a 0.45 μm filter, including humic biopolymers, hydrophobic organic carbon, and some acids with low molecular weight [33−36]. Although BCDOM has a small proportion in biochar (0.17 −37.03 (mg/g)), it has received noticeable attention as an essential part of biochar and plays an important role in the application of biochar for soil remediation [13, 37, 38]. Because of the abundance of functional groups, BCDOM can interact with some soil contaminants including heavy metals, organic pollutants, and biologicals, which can alter the transport and fate of BCDOM and soil pollutants [39−41]. In addition, BCDOM can serve as a food source for soil microorganisms, stimulating microbial activity and increasing soil fertility and nutrient cycling.