Integrating Biochar, Bacteria, and Plants for Sustainable Remediation of Soils Contaminated with Organic Pollutants

Abstract

The contamination of soil with organic pollutants has been accelerated by agricultural and industrial development and poses a major threat to global ecosystems and human health. Various chemical and physical techniques have been developed to remediate soils contaminated with organic pollutants, but challenges related to cost, efficacy, and toxic byproducts often limit their sustainability. Fortunately, phytoremediation, achieved through the use of plants and associated microbiomes, has shown great promise for tackling environmental pollution; this technology has been tested both in the laboratory and in the field. Plant-microbe interactions further promote the efficacy of phytoremediation, with plant growth-promoting bacteria (PGPB) often used to assist the remediation of organic pollutants. However, the efficiency of microbe-assisted phytoremediation can be impeded by (i) high concentrations of secondary toxins, (ii) the absence of a suitable sink for these toxins, (iii) nutrient limitations, (iv) the lack of continued release of microbial inocula, and (v) the lack of shelter or porous habitats for planktonic organisms. In this regard, biochar affords unparalleled positive attributes that make it a suitable bacterial carrier and soil health enhancer. We propose that several barriers can be overcome by integrating plants, PGPB, and biochar for the remediation of organic pollutants in soil. Here, we explore the mechanisms by which biochar and PGPB can assist plants in the remediation of organic pollutants in soils, and thereby improve soil health. We analyze the cost-effectiveness, feasibility, life cycle, and practicality of this integration for sustainable restoration and management of soil.

Keywords: biochar; organic pollutants; phytoremediation; plant growth-promoting bacteria; soil pollution.

Figure 1

Dependence of biochar properties on feedstock type and pyrolysis temperature: (a) specific surface area (SSA), (b) volatile matter, (c) (oxygen + nitrogen)/carbon ratio, and (d) pH. We analyzed data for normality. If the data did not conform to the normal distribution, we used a natural logarithm (ln) or square root (SQRT) to transform the data. Different colored circles indicate different feedstock types, and the black line is a general linear regression between biochar properties and pyrolysis temperatures. Panels a and d show an increasing pyrolysis temperature is linked to an increasing specific surface area and pH, while panels b and c show a negative relationship between the (oxygen + nitrogen)/carbon ratio and volatile matter content with pyrolysis temperature. Data are from Table S1.

Figure 2

Proposed mechanisms by which biochar mediates the remediation of organic pollutants in soil. (a) Free radicals from biochar can react with O₂ to produce •OH and/or activate S₂O₈^2– or H₂O₂ to produce reactive oxygen species (^•OH, SO₄^•–, and O₂^•–), which facilitate the oxidation/degradation of organic pollutants. (b) Organic pollutants can be immobilized in the soil through several interactions with biochar, including pore filling, electrostatic interaction, partition, electron donor and acceptor interaction, hydrophobic interaction, and π–π electron donor–acceptor interaction.^, The carbonized fraction of biochar mediates adsorption (electrostatic attraction and nonpolar biochar–pollutant interactions), while the uncarbonized faction of biochar mediates partition. (c) With its porous surface, biochar can accommodate soil microbial communities, immobilize and release enzymes, improve soil processes and functions, and transfer electrons to microorganisms and pollutants, hence influencing plant and microbial metabolism of organic pollutants in soil.

Figure 3

Bioaugmentation by PGPB and biostimulation by biochar are the two main strategies to complement phytoremediation and improve soil qualities. PGPB can (1) fix nitrogen, solubilize phosphate and potassium, and oxidize sulfur (ruby arrows); (2) stimulate mycorrhiza formation, which can be beneficial for both plant protection and nutrient availability; and (3) produce allelochemicals (e.g., siderophores, lytic and detoxifying enzymes, indole-3-acetic acid, and ACC deaminase) (gold arrows), regulate plant growth and development, and protect plants from pathogens. (4) With its porous structure, biochar can serve as a habitat for PGPB and other soil microbiomes, contributing to their growth, survival, and activity. (5) Due to its ability to retain and release nutrients slowly, biochar may act as a source of nutrients for both microbes and plants for an extended period of time in soil. (6) Biochar can also improve soil aeration, pH, and water and carbon content and thus alter the growth of soil microbiomes and plants and their interactions. (7) By reducing sodium uptake and increasing potassium uptake by roots, biochar can maintain ion homeostasis in plants.^,

Figure 4

Proposed mechanisms of synergistic contributions of biochar, PGPB, and plants for remediation of organic pollutants in soils. (1) Various functional groups, a porous structure, and a large surface area of biochar promote the sorption of organic pollutants, thereby reducing the toxicity of organic pollutants to microbes and plants in the soil. (2) Biochar-resident PGPB may further degrade the adsorbed organic pollutants, while plants provide C as root exudates facilitating microbial co-metabolism. (3) PGPB and biochar can improve root growth and activity, which is beneficial for the absorption of pollutants by plants. (4) Once in the plant endosphere, pollutants can accumulate in various plant tissues, be degraded by plant enzymes and endophytic microorganisms, or be released into the atmosphere through volatilization.

Figure 5

Example of a sustainable model that maximizes the cyclicality and profitability of biochar–bacterium–plant systems for organic pollutant remediation and climate change mitigation.