Conversion of heavy metal-containing biowaste from phytoremediation site to value-added solid fuel through hydrothermal carbonization

Highlights

• Feasibility of linking phytoremediation to bioenergy production was evaluated.

• Heavy metal-contaminated biomass was hydrothermally carbonized.

• Heavy metals in hydrochar were reduced by hydrothermal carbonization.

• Produced hydrochar showed improved energy-related properties.

• Phytoremediated biomass can be a potential feedstock for value-added solid fuel.

Abstract

In this study, heavy metal-containing sunflower residues obtained from a phytoremediation site were hydrothermally carbonized at 160–260 °C. The properties of hydrochar thus produced were evaluated with respect to its potential as solid fuel. The results confirmed that hydrothermal carbonization (HTC) reduced the concentration of heavy metals in hydrochars, with the concentration lower than the maximum permissible level of domestic standards for bio-solid refuse fuel. Higher HTC temperatures resulted in improved energy-related properties of the hydrochar (i.e., coalification degree, fuel ratio, and higher heating value); however, HTC temperatures between 200 and 220 °C were deemed suitable for energy retention efficiency. Furthermore, as hydrochar contains low nitrogen and ash content, it can be considered as a clean energy source. The results of this study suggest a sustainable approach to the disposal and effective utilization of contaminant-containing biowastes. Moreover, this study suggests linking biomass cultivation for phytoremediation and converting the phytoremediated biomass into value-added solid fuel.

Introduction

The phytoremediation process has been accepted as a promising strategy as it is cost-effective, green, and effective in remediating contaminated soils (Lee et al., 2017). In the phytoremediation process, contaminants tend to accumulate in the crop biomass during cultivation (Attinti et al., 2017), leading to the generation of highly contaminated biowastes at the end of the harvest process (Gong et al., 2018). A successful phytoremediation process involves the production of large amounts of highly contaminated biowastes; thus, an appropriate disposal method is required. Until now, several disposal methods, based on thermochemical (pyrolysis, liquefaction, gasification, and combustion) or biochemical (anaerobic fermentation/digestion) reactions have been introduced to treat biowastes from phytoremediation sites (Lee et al., 2018b; Vocciante et al., 2019; Zhou et al., 2020). However, the technical maturities of these disposal methods have not been fully established, despite their merits (Van Slycken et al., 2013). Few studies have reported the applicability of specific disposal methods for contaminants containing biowastes; however, these do not account for their potential risks to the environment.

Due to increasing environmental concerns, efforts to reduce carbon emissions by substituting fossil fuels with bioenergy have attracted attention for several decades (Bhuiya et al., 2016). Bioenergy, which is a type of fuel derived from lignocellulosic biomass, has its merits in terms of renewability and abundance (Van Meerbeek et al., 2019). And the diverse potential application ranges of bioenergy owing to its multifarious forms in solid, liquid, or gaseous could be another merit of using (Valentine et al., 2012). However, due to limited availability of agricultural land, there is a conflict between choosing edible plants for consumption and selecting bioenergy feed stock (Dastyar et al., 2019). In this regard, a versatile approach that links biomass cultivation for phytoremediation with bioenergy recovery using phytoremediated biomass can aid in alleviating environmental concerns and help produce bioenergy.

Hydrothermal carbonization (HTC), a thermochemical process that involves sub-critical water at a moderate temperature range (160–260 °C), can be a viable method to convert biomass into useful carbon-rich material (i.e., hydrochar) (Lee et al., 2018a). The hydrochar thus produced can aid in a range of purposes, such as carbon sequestration, soil improvement, environmental remediation, and energy production (Ahmad et al., 2014; Melo et al., 2018). Recently, considerable research has focused on the use of hydrochar as an alternative solid fuel, as its properties are better than that of biomass in terms of material structure, heating value, and thermal stability (Cui et al., 2020). Due to the improvements in its properties during hydrothermal reaction, the produced hydrochar can be used on its own or combined with another fuel source in combustion facilities (Kim et al., 2015). However, endogenous contaminants in phytoremediated biomass may cause trouble when combusting the hydrochar resulting from HTC of contaminants containing biomass.

This study focused on the conversion of heavy metal-containing biomass obtained from a phytoremediation site into hydrochar. Moreover, it explored the cultivation of biomass for contaminated land remediation and the value added to the abandoned biowastes in terms of energy recovery. To this end, the harvested sunflower residues in the heavy metal-contaminated site were hydrothermally carbonized under various temperatures ranging from 160 to 260 °C. Additionally, the properties of the produced hydrochar (including heavy metal concentrations in it) and its feasibility as an alternative solid fuel were investigated.