Valorization of heavy metal contaminated biomass: Recycling and expanding to functional materials

Highlights

• Thermochemical technology can effectively immobilize heavy metals in biochar.

• Metals in contaminated biomass serve as active components and loading sources.

• Contaminated biomass can convert to environmentally functional materials.

• Recycling and valorization of contaminated biomass is a win-win strategy.

Abstract

Due to the increasing global severe soil pollution, a large number of heavy metal elements are enriched in biomass grown in polluted areas and hyperaccumulators used for phytoremediation. Disposing of above contaminated biomass through traditional in-situ combustion, return to the field, and composting strategies can result in heavy metals re-entering the environment and causing secondary pollution, which poses a huge challenge to the safe disposal of waste biomass. Pursuing more efficient strategies is necessary to achieve green, clean utilization of contaminated biomass and enhance the value. Retaining heavy metals in biomass serves them as functional materials through effective measures can be a sustainable idea. This review described the characteristics of widely used traditional biomass disposal strategies and discussed commonly used thermochemical technologies for heavy metal immobilization. The progress and application status of value-added materials such as chemicals, soil additives, catalysts, absorbents, capacitors, and antibacterial materials prepared from contaminated biomass were systematic overviewed, and the future development direction was prospected. It can be emphasized from the study that the conversion of contaminated biomass into high-value materials is significant and provides a new idea for the green treatment of contaminated biomass and are beneficial to the improvement of phytoremediation technology. Contaminated biomass deserves further exploration and attention as a resource with infinite potential.

Introduction

With the rapid development of industry, the damage of heavy metals to the environment has become more serious. These harmful heavy metal elements will enter the soil and water bodies with atmospheric deposition and surface runoff, thereby staying, accumulating, and migrating in organisms (Zwolak et al., 2019). It would pose a considerable risk to food safety and human health when heavy metals enter crops and accumulate (Huang et al., 2019). Phytoremediation of heavy metal contaminated sites through super-accumulative biomass is currently an emerging treatment method. By utilizing the unique adsorption performance of specific biomass for heavy metals, in the in-situ soil or water polluted environment, heavy metal elements are absorbed and accumulated in the biomass to achieve environmental purification (Zhou et al., 2020a). The safe disposal of the above two types of heavy metal-rich biomass from different sources has always posed significant challenges.

Disposal strategies such as return to the field, burning, and composting of biomass have been widely adopted in many countries due to their low cost and convenience advantages (Kovacs and Szemmelveisz, 2017). Heavy metals present in biomass may be released back into the surrounding environment due to leaching, temperature changes, or soil properties, causing secondary pollution (Chai et al., 2022a). The safe and proper treatment and upgrading utilization of heavy metal polluted biomass has become an essential issue in the utilization of biomass resources. Recycling the contaminated biomass in the form of value-added products is an ideal disposal strategy (Oustriere et al., 2019).

Some studies have tried to convert heavy metal-contaminated biomass into energy products such as bio-oil, biochar, and biogas through thermochemical treatment and achieved good results (Chen et al., 2019a; He et al., 2019). The contaminated biomass has been found to have the potential to produce organic acids (Guan et al., 2020) and furan compounds (Chai et al., 2022b, Hu et al., 2022), which shows excellent economic feasibility. It is worth noting that the contaminated biomass also has great potential to be prepared as high-value materials. One of the simplest is to separate and recover the metals or active ingredients in the contaminated biomass through different methods, also called plant metallurgy (Yang et al., 2009). The conversion of heavy metal-contaminated biomass into metal-loaded biochar composites through pyrolysis is another promising way. Previous high-performance metal-modified functional materials require the introduction of exogenous metals (Zheng et al., 2022) and excellent carriers with high specific surface areas (Guo et al., 2019), which may bring high-cost burdens and possible environmental risks. Functional metal modification and carbon material support can be simultaneously obtained by thermal conversion of contaminated biomass, which is significant in sustainable aspects, and avoids possible risks caused by heavy metals contaminated biomass. By using the transfer and transformation of heavy metals in the thermochemical process, the target metal element is retained in the carbon materials and exists in a specific form such as an oxide (Zong et al., 2021), which shows different functions such as catalysis (Chen et al., 2019b), adsorption (Wang et al., 2017a), energy storage (Zhang et al., 2021), or antibacterial (Yang et al., 2020).

Transforming contaminated biomass into various value-added materials considers the advantages of reducing carbon emissions, curbing heavy metal pollution, and improving secondary value (Mian et al., 2022). Meanwhile, the sustainable goal of treating waste with waste and turning waste into treasure has been realized, which is undoubtedly a win-win strategy (Qu et al., 2013). Current research on the behavior and role of heavy metals in the biomass conversion has received attention (Liu et al., 2017), but there is still a gap in the systematic about the application advances of contaminated biomass. A comprehensive review and prospect of the risks, recovery, and valorization of heavy metal-contaminated biomass are necessary to light this research gap.

Biomass rich in heavy metals should be regarded as a valuable resource rather than waste, which can provide a new direction for achieving cleaner production. Sorting out a clear context for the causes and consequences of the transformation of heavy metal contaminated biomass into functional materials is significant for the safe and sustainable disposal of contaminated biomass and the improvement of phytoremediation technology. The traditional disposal strategies of contaminated biomass were introduced in this review, and the common technologies for the immobilization of heavy metals in contaminated biomass were described, which is a crucial prerequisite for preparing different value-added materials. Next, the research advances of converting contaminated biomass into various value-added materials were comprehensive reviewed, and the future research direction has also prospected. The recycling and utilization of heavy metal contaminated biomass play a vital role in environmental protection and sustainable development. This review aims to broaden the value-added pathways of contaminated biomass and access to functional materials, and provide a feasible basis for the sustainable disposal of phytoremediation biomass.