Polluted lignocellulose-bearing sediments as a resource for marketable goods—a review of potential technologies for biochemical and thermochemical processing and remediation

The remediation of pollutants in the lignocellulose-bearing sediments, employing physical and/or chemical procedures to extract toxic elements, commonly known as soil washing or soil extraction, could be a potential pre-treatment to prepare the material as a feedstock for further treatments (Dermont et al. 2008; Ferraro et al. 2016). For instance, pressurized liquid extraction (PLE), supercritical fluid extraction (SFE) and subcritical water extraction are well-established extraction technologies that have applications in the food science and technology, environmental contaminant analysis, fuel industry, etc. (Henry & Yonker 2006; Herrero et al. 2006). With various degree of selectivity and recoveries/yields, these treatments are capable to extract a wide range of organic pollutants, including organochlorine pesticides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls and dioxins (Islam et al. 2019; Tagliabue et al. 2020). In the case of subcritical water extraction, heavy metals and metalloids such as As, Cd and Cu can be removed from the matrix without the need of complexing agents (Islam et al. 2019). PLE, SFE and subcritical water extraction are typically not considered for large-scale applications due to their high cost. Costs for ex situ soil washing technology have been estimated to $187 per m³ for large sites and $70 per m³ for small sites (FRTR 2021). Other physicochemical cleanup methods include autoclaving, electrokinetic remediation, precipitation and of reducing agents, flotation (Malaviya & Singh 2011; Wen et al. 2020; Zhang et al. 2019). The high costs associated with many physicochemical cleanup methods make them unsuitable for remediation of large volumes of lignocellulose-bearing sediments, but for smaller batches when high-value end products are envisaged, the higher cost may be justified to ensure an efficient and fast remediation.

Some physicochemical pretreatment methods intended to remove or degrade pollutants may have the additional benefit to alter the chemical composition of the material and make it more suitable for posterior biorefining steps. Increased biodegradability and sugar yield are typical chemical changes associated with physicochemical pretreatment that facilitates microbial fermentation, for instance. Various psychochemical pre-treatment methods have been proved to remove lignin and hemicellulose, release sugars from lignocellulose and decrease the crystallinity of cellulose (Baruah et al. 2018; Tayyab et al. 2018). However, harsh pre-treatment conditions (high temperature, high pressure, high acid/ base concentration) typically enhance the sugar degradation and therefore form inhibitory compounds such as furfural and hydroxymethylfurfural (HMF) that affect the microbial growth in posterior fermentation processes (Chiaramonti et al. 2012; Singh et al. 2015).

The term phytotechnology implies the use of plants to provide environmental solutions, including phytoremediation options using plants and associated microbes to extract, degrade, contain or immobilize both trace toxic elements and organic pollutants in a polluted medium (Mench et al. 2009). Phytoremediation is a low-cost, safe technology, with high probability of public acceptance that can be adopted to a number of substrates (Lakshman 2016; V. C. Pandey and Bajpai 2019) and may thus be a promising alternative for the cleanup of polluted lignocellulosic waste. As far as we know, there is no reference in the scientific literature to the use of plants for the remediation of polluted lignocellulose-bearing sediments.

The physical and chemical properties of this waste material are different from polluted sediments and soils in which phytoremediation is habitually used. When dealing with the treatment of waste materials with unknown characteristics, a stepwise treatability study may aid in reducing failure possibilities and develop the maximum potential of the selected strategy. With the objective of finding appropriate plants for remediation of material from lignocellulose-bearing sediments, the authors of this article developed a stepwise plant screening study, consisting of a profuse literature survey for plants selection, an acute phytotoxicity test of the material on selected plant seeds and a greenhouse bench-scale experiment to assess plants growth and development in potting media prepared with the polluted material. Figure 1 illustrates the pathway followed by our research group for the evaluation of the phytoremediation approach.

The bioavailability and concentration of the pollutants, as well as biological properties, affects the phytotoxicity of the material for selected phytoremediation plant species. In an ongoing experiment, among other shortlisted species, Brassica juncea, Brassica napus, Horderum vulgare and Poa annua are multi-element hyperaccumulators plants that showed high tolerance to a multi-contaminated (persistent organic pollutants (POP) and inorganic pollutants) and were able to grow with satisfactory biomass yield in soilless potting media prepared with up to 90% polluted material from lignocellulose-bearing sediments (Paladino et al. 2021). The removal efficiency of toxic elements (in terms of translocation and bioaccumulation factors) and rhizo- or phytodegradation of organic pollutants during plant growth are being quantified at the time of writing. The post-remediation treatment protocol is decisive for the sustainability of phytoremediation technologies (Haller & Jonsson 2020). An interesting option for the polluted biomass is to use it as a feedstock for the thermochemical methods described in 3.4. Phytoremediation technologies typically need to be tailor-made for each particular combination of pollutants. The lack of one-fits-all solutions may be perceived as a barrier for its implementation.

Composting processes have been optimized for varied sources of feedstock, but to date, no composting protocol has been developed for lignocellulose-bearing sediments. Due to its high lignin content, material from lignocellulose-bearing sediments is more resistant to decomposition compared to the typical compost sources and longer composting periods are needed than for most agriculture waste or MSW. Experiences from composting of similar lignocellulosic waste such as fibrous forestry residues indicate that composting time can be shortened by thermophilic conditions and N-rich additions of manure, etc. (Raviv 2005; Raviv et al. 1986). Trials are needed to test and fine-tune the optimum mix of feedstock and ideal composting procedure for material from lignocellulose-bearing sediments. Encouraging studies claim that composted organic waste material with similar physical properties such as tree bark, sawdust and sludge produced growing substrates with physical and chemical properties that are equal or even superior to peat-based growing substrate (Fascella 2015; Sánchez-Monedero et al. 2004). The presence of salinity and pollutants in the lignocellulose-bearing sediments needs to be monitored to develop a composting protocol that addresses the specific composition of each batch. If salinity or phytotoxic ions are present, those may be leached by flushing prior to composting. The presence of heavy metals however is much more problematic since their leachability is limited. Composting is capable of complexing, absorbing and (co)precipitating heavy metals (Burgos et al. 2010; Park et al. 2011), but if high concentrations of inorganic pollutants are present, other methods are necessary and composting (if used) should primarily be considered a pre-treatment step to reduce the volume prior to further treatments (Haller & Jonsson 2020). Organic pollutants, on the other hand, can be degraded with specially designed composting processes (Adams et al. 2015; M. Chen et al. 2015). Composts are typically rich sources of pollutant-degrading microorganisms that can be used to mineralize organic pollutants or transform these into less toxic substances. Composting is reportedly capable of degrading even the most persistent pollutants present in lignocellulose-bearing sediments such as chlorophenols (Laine & Jorgensen 1996), PAHs (M. Chen et al. 2015) and dichlorodiphenyltrichloroethane (DDT) (Purnomo et al. 2011). Additions of whey are known to speed up the degradation rate of petroleum hydrocarbons (Haller et al. 2020; Jonsson & Östberg 2011), and other lignocellulosic waste materials, such as corn cobs, sugarcane bagasse and sawdust, have been shown to enhance degradation of many contaminants, including PAHs and organochlorines (Mohee & Mudhoo 2012). In-depth monitoring programmes of the composting of lignocellulose-bearing sediments are necessary to make sure that the final product is safe to be marketed. If the final compost product only contains permissible levels of pollutants, it may be commercialized as growing substrate. Compost of lower quality may be used on landfills, etc. Significantly less is known about the potential of anaerobic composting of lignin-rich substrates such as material from lignocellulose-bearing sediments, but such methods may prove to have some advantages over aerobic composting (T. Meyer & Edwards 2014). For instance, to degrade some of the organochlorine pollutants, an initial anaerobic step may be necessary (Allen et al. 2002; Lacayo et al. 2004).

The role of fungi in the detoxification of the environment is well known. Fungi have properties such as mycelia structure and secretion of enzymes and other compounds that makes it interesting for degradation and/or accumulation of pollutants (Gadd 2001; Prasad 2018). Fungal biotechnologies also have the ability to produce a wide range of marketable products including food, feed, chemicals, fuels and textiles (V. Meyer et al. 2020; Yadav 2019). Fungi can thrive in numerous habitats, and white-rot (wood-decaying) and soil-living fungi such as Thermothelomyces thermophila and Fusarium solani can be cultivated directly in the material from lignocellulose-bearing sediments at ambient temperature to assist the degradation of organic pollutants (Perigon, Massier et al. 2019). Smaller batches of lignocellulose-bearing sediments can also be used in bioreactors where oxygen levels, temperature, etc., can be controlled.

Fungi are widely used in fermentative industries for the production of ethanol, organic acids and enzymes (Yadav 2019). The high lignin content of lignocellulose-bearing sediments is an obstacle for the saccharification of cellulose and hemicellulose towards the fermentable sugar production (Chiaramonti et al. 2012; Tayyab et al. 2018). Therefore, lignin removal is essential for the production of monomeric sugars. Some fungi present extracellular lignin-degrading enzymes and cellulase enzyme complex that can be used for delignification and saccharification, respectively (Gupta et al. 2011; R. K. Pandey & Tewari 2018). This can also be achieved by different pretreatments that are discussed in Sect. 3.2. The main monomeric sugars that can be obtained from lignocellulosic biomass are glucose, xylose, arabinose and mannose, and these can be converted into biofuel such as biodiesel, bioethanol or butanol by further microbial fermentation (Baruah et al. 2018; Tayyab et al. 2018).

White-rot fungi have the ability to biodegrade POPs, absorb metals/metalloids and use lignocellulosic material as a carbon source (Dao et al. 2019; Sharma, Giri, & Sharma, 2020). In an ongoing experiment conducted by the authors of this article, twenty species of white-rot fungi naturally growing in Sweden are tested for their biodegradation ability of POPs, and bioaccumulation ability of metals/metalloids on material from lignocellulose-bearing sediments taken from Sundsvall Bay (physical and chemical properties are displayed in Table 2). The fungi are growing on hagem-agar discs with material from lignocellulose-bearing sediments. The growth rate of the fungi is recorded with a camera to quantify circular growth, and the POPs degradation will be analysed by gas chromatography-mass spectrometry (GC-MS) and metal uptake by high-resolution scanning electron microscopy (HI-SEM) (data not published). If the remediation is successful, selective lignin-degrading white-rot fungi such Phanerochaete chrysosporium may be used to provide an unprotected carbohydrate for subsequent use as animal feed and/or biofuel substrate (Dashtban et al. 2010).

Pyrolysis is a thermochemical technology for converting biomass into energy and marketable products such as liquid bio-oil, solid biochar and pyrolytic gas. The optimum temperature for maximized mixed liquid and solid product yields is in the range of 400–550 °C. Pyrolysis seems to be the thermochemical method where the lowest concentration of pollutants ends up in the marketable goods (Dastyar et al. 2019). Higher temperatures enhance the thermal degradation of organic pollutants and produce higher yield and quality of biogas (Kan et al. 2016). Less biochar is produced with high-temperature pyrolysis, but the biogas production should not be neglected since it can be further converted to hydrogen-rich synthetic gas to be used as fuel directly or as an intermediate resource for production of hydrogen, ammonia, methanol and synthetic hydrocarbon fuels. For lignocellulose-bearing sediments with predominantly inorganic pollutants, lower pyrolysis temperatures may be more suitable. Experiments suggest that in the presence of Cd, it is necessary to operate at temperature lower than 430 °C to obtain a heavy metals free vapour phase fuel. In the presence of one or more metals among Pb, Cu and Zn, on the other hand, it is possible to conduct a pyrolytic treatment at higher temperatures obtaining a char with high surface area and lower metals mobility (Grottola et al. 2019).

Gasification is a technology to convert biomass (or fossil fuel-based feedstock) into gases at elevated temperatures (700–1000 °C) via partial oxidation of compounds using steam, air or oxygen. The principal product is the gas mixture typically denominated syngas (Molino et al. 2018). The high temperatures and pressure used in gasification may lead to volatilization of metals present in the feedstock. The distribution of metals between solid and gaseous state and the quality of syngas produced from gasification are influenced by factors such as i) the chemical speciation of metals and dynamics of fluidization, ii) operating conditions (temperature, pressure, etc.), iii) type of gasifier system (fluidized bed, fixed bed, entrained bed reactors, etc.), iv) impact of fluidized bed materials and v) type of gasification agent (Dastyar et al. 2019). In an experiment conducted by Jiang et al. (2016) As, Cd, Zn and Pb were rapidly volatized at > 600 °C, and Ni, Cu, Mn and Co thoroughly transferred into gaseous phase in the temperature range of 1000–1200 °C, whereas Cr, Al, Fe and Mg remained immobile in the solid phase at temperatures > 1200 °C. Although experiments suggest that with the exception of Cd, most heavy metals (i.e. Cu, Ni, Pb, and Zn) typically remain in the solid phase during gasification, in some experiments, high concentrations of metals (such as Cr, Ni, Cu, Fe, and Mo) have been observed in the syngas (Cui et al. 2013; Tafur-Marinos et al. 2014). Trials are thus needed to optimize those parameters in order to obtain clean metal-free products from lignocellulose-bearing sediments. Electrostatic precipitators (ESP) may be used for the collection of solid particles from the gaseous phase in gasification chambers. In an experiment with ESP on contaminated feedstock, at least partial collection of Cu, Zn, Cr and Ni was reported (Poškas et al. 2018). Organic pollutants are likely to be thermally degraded in high-temperature gasification, but trials are needed to develop protocols for gasification of material from lignocellulose-bearing sediments. In particular, the formation of dioxins must be monitored and temperatures above 850 °C should be used (McKay 2002; Zhou et al. 2015).

In hydrothermal liquefaction (HTL), wet biomass slurries are heated to temperatures of 200–500 °C under high pressure (5–20 Mpa) to produce a mixture of products including bio-oil, gaseous products and a small amount of solids (Gollakota et al. 2018). Thanks to the high content of both nutrients and organics in liquefied products, the resulting products are suitable for further industrial processing into polyol suitable to be used in the formulations of sprayable polyurethane foam, etc. (dos Santos et al. 2018). The HTL process has under optimized conditions removed Zn, Pb, Cu from the substrate at an efficiency of > 99%. The metals were found in the water phase, whereas the bio-oil, the commercial product, was free of the analysed metals (Yang 2010). The fate of organic pollutants present in lignocellulose-bearing sediments is yet to be determined. A relative advantage with HTL compared to pyrolysis and gasification is that considerably less dewatering is necessary before it can be used as feedstock.

Hydrothermal carbonization (HTC) is a technology, appropriate for materials with a high water content, that is performed in a temperature range of 180–350 °C during which the biomass is submerged in water (and sometimes acids) and heated under pressure (2–6 MPa) for 5–240 min. Similar to HTL, expenses for dewatering can be cut with HTC since the feedstock can have a high water content. The main product of HTC is solid (sometimes referred to a hydrochar), but liquid and gaseous (mainly CO2) by-products are also formed (Antero, Alves, de Oliveira, Ojala, & Brum, 2020; Heidari et al. 2019). HTC is not likely to be suitable for lignocellulose-bearing sediments with high concentration of metals since inorganic pollutants tend to accumulate in the hydrochar (Liu et al. 2018). Organic pollutants (including POPs such as β-HCH) on the other hand seem to be efficiently degraded in the HTC process (Weiner, Baskyr et al. 2013). The formation of dioxins, however, was not assessed in that study and that may be a serious limitation for sediments polluted with organochlorines.

Torrefaction is a thermochemical process that involves the interaction of drying and incomplete pyrolysis at temperatures between 200 and 300 °C. The final products from torrefaction are similar to pyrolysis biochar, but one advantage is that the carbon efficiency (the amount of carbon from the original product that remains in the final product) may be as high as 80–100% compared to 50% for pyrolysis (Jorge Miguel Carneiro, Radu, João Carlos de Oliveira, & Leonel Jorge Ribeiro, 2018; Shankar Tumuluru et al. 2011). The parameters influencing the torrefaction process include (a) reaction temperature, (b) heating rate, (c) absence of oxygen, (d) residence time, (e) ambient pressure, (d) flexible feedstock, (e) feedstock moisture and (f) feedstock particle size (Shankar Tumuluru et al. 2011). Little is known about the fate of pollutants in the torrefaction process, but Edo et al. (2017) report that chlorinated compounds migrated from the feedstock to the gas phase, whereas trace metals remained in the char. Torrefaction of biomass is still to a large extent experimental (Jorge Miguel Carneiro 2018). To date, torrefaction has mainly been assessed for fuel production, but ongoing research is determining its potential to produce an alternative to peat for growing substrate. The hydrophobic behaviour, typical of torrefied biomass however, is a limiting factor for such uses (Rolfsson 2019).