PAHs, PCBs and Environmental Contamination in Char Products

Abstract

Biochar can have unique benefits to terrestrial and aquatic ecosystems. Investigations of biochar effectiveness within these environments often come from homogenous feedstocks, such as plant biomass, which have simple thermochemical processing methods and produce physically and chemically stable biochar. Current methods to increase biochar production include the addition of oil-derived products such as plastics, which produces a more heterogenous feedstock. This feedstock is similar to materials from waste recycling streams. The adoption of more heterogenous feedstocks produces additional challenges to biochar production and use. This can result in pollution contained within the feedstock being transferred to the biochar or the creation of pollutants during the processing. With the current climate emergency, it is essential to eliminate environmental contamination arising from biochar production. It is critical to understand the physiochemical composition of biochar, where detailed analysis of contaminants is often overlooked. Contamination is common from heterogenous feedstocks but on commercial scales, even homogeneous biochar will contain organic pollutants. This chapter investigates biochar produced from various waste feedstocks and the challenges faced in thermochemical processing. Using Automotive Shredder Residue (ASR) as an example of a heterogeneous feedstock, the levels of contamination are explored. Potential solutions are reviewed while assessing the environmental and economic benefits of using biochar from mixed sources.

1. Introduction

Biochar has been promoted as a solution to enhance soils as a conditioner and as an additive to enhance contaminated land remediation. For many of these proposed applications the positive properties of the biochar in the environment are championed, however, there is little investigation into their negative impacts on the environment. The main area of concern is the presence of persistent organic pollutants (POP), polyaromatic hydrocarbons (PAH) and polychlorinated biphenyl (PCB) within the biochar itself. Much of the research on the sources of material for biochar is carried out on small scale laboratory test rigs with carefully chosen homogenous feed sources. This does not represent the potential commercial application where a more heterogeneous feed would be present. There is also a drive to enhance and improve the production of biochar by the combination of organic and plastics. This again can give rise to contamination with undesirable by-products.

It is well known that soils already contain POPS however, there are concerns over these levels [1]. The addition of biochar containing POPs would increase the concentration. The main barrier to analysis of POP in soil is the variability of the soil and a methodology is complex and there is no specified guidelines and corresponding legislation [2]. Consequently, there is no incentive to analyze for organic contamination and the main analysis reports metal levels. For the threshold levels of POP in soil under UK legislation a risk assessment-based approach is required [3].

Many research projects investigating biochar from plant biomass assess the chemical status via the evaluation of organic elemental composition and the biochar porosity only [4, 5]. Although this is a useful method to understand how the char will develop in soil and its potential to absorb nutrients, further analysis of the inorganic metal concentration and the organic pollutants (such as PAHs) contained within the product may provide further information on the environmental contamination from the feedstock that is being added to the soil. Currently, there are limited regulations surrounding biochar reuse from organic products such as biomass, as it is assumed that plants are inert. However, bio-uptake from energy crops contaminated land sources, such as miscanthus, could be a result in a significant amount of pollution retained within biochar following thermal processing. Advanced chemical analysis of biochar is required to ensure that pollution from initial feedstock sources do not cause further pollution.

Biochar is the solid residue obtained during the thermochemical conversion of biomass in an oxygen limiting environment. Unlike combustible ash residues, biochar is a stable solid, rich in pyrogenic carbon. Biochar resides from biomass feedstock, of which there are 6 main sources: agricultural waste, forestry waste, animal waste, industrial residues, and municipal solid waste. Re-use of biochar from waste materials could have many positive environmental and economic effects for the waste recycling industry, including a reduction in waste to landfill and the provision of a circular economy from waste recycling. However, the chemical consistency of the feedstock can have significant implications on the quality of biochar and its potential re-use in certain applications. The use of non-homogeneous feed stocks such automotive shredder residue (ASR) is a good example of a mixture of organic material with oil derived plastic.

There are many applications of biochar, however it is commonly applied to agricultural systems as a soil improver. Addition of biochar to agroecosystems can have significant benefits to soil properties and plant health [6], where carbon sequestration, water retention, microbial activity and herbicide suppression is increased [7, 8, 9, 10], whilst nutrient leaching is decreased [11]. It has been calculated that biochar addition can increase soil organic carbon (SOC) stocks by 29% (13 Mg ha¯¹) [6]. Biochar can be added to soil via different methods, it can be mixed directly into soil, or used as an additive to other processes, such as compost, manures and fertilizers, where the biochar acts as a carrier for the nutrients. Through biochar applications increasing carbon storage within soil, the carbon footprint caused by thermal processing of biomass waste for energy is reduced [12, 13]. Little investigation has been carried out on the level of POP that come from the processing process and different feedstocks. This omission means that we do not have the full picture on what we are depositing onto the land.

Alongside the addition of biochar to agroecosystems to increase soil fertility and improve crop growth, the adsorbent properties of char make it a useful product for removal of contaminants in remediated soil sites and in aquatic environments. In soil, biochar can be used to immobilize contaminants such as lead, cadmium, arsenic and atrazine [14, 15, 16]. In water, biochar can be used to adsorb and remove metal ions such as cadmium, copper and zinc [17] and phenolic compounds [18]. Biochar can also be used to depollute wastewater, removing ammonia [19], dyes such as methylene blue [20] and toxic heavy metals [21, 22]. The chemical consistency and physical structure of biochar determines the pollutants that it can adsorb, where high aromaticity and porosity increase the sorption of organic contaminants and oxygen-containing functional groups increase the sorption of metals [23]. Feedstock type and pyrolysis conditions can alter the char chemical and physical consistency, which has an impact on its use for depollution. Outside of environmental applications, biochar is often used as an additive in the construction sector; where the porous structure acts as a micro-filler within concrete composites [24, 25]. Processing of heterogenous feedstocks to make char as a filler in concrete could increase carbon sequestration and reduce the carbon footprint of the concrete [26]. Biochar can also be added as an asphalt binder, increasing its high temperature performance and its resistance to aging [24]. Biochar from waste can also be added to epoxy resins, used in microelectronic, automotive and aircraft industries [27, 28, 29, 30, 31]. The adoption of biochar from more complex heterogenous sources such as municipal solid waste (MSW), contaminated wood and ASR could become a more viable option for the recycling industry. The caveat being that these types of products would retain any hazardous chemicals contained in the char and could pose problems at the end of life.

To use solid waste residues as a biochar for soil modification either depollution of feedstock may be required or that of the produced biochar. This will be required in some cases to meet the environmental requirements set by different governmental organizations [32, 33]. There are three main types of regulated contaminants that concern biochar these are: (i) PAHs, (ii) PCBs and (iii) heavy metals. PAHs is a term used for a large group of compounds which have multiple benzene rings in their chemical structure. PAHs are large compounds which are difficult to degrade in the environment. Many PAHs are non-toxic, yet some PAHs with specific chemical structures are carcinogenic and human exposure should be avoided [34, 35]. Current exposure limits to PAHs set by the UK government are 0.25ng/m³ in air and < 0.2 ppb in water [36]. PCBs is a term used for a group of compounds which have two or more chlorine bonds within their hydrocarbon structure. PCBs are highly toxic and are banned in the UK and Europe [37]. Heavy metals that are regulated include lead, mercury and arsenic [37, 38]. There are different exposure limits set dependent on the location (inhalation, ingestion, skin contact). An example of the more common organic pollutants found within biochars is presented in Figure 1.

Figure 1.

The chemical structures of some common (A) PAHs, and (B) PCBs detected in solid residue products.

Char samples are typically analyzed for PAHs and PCB by chemical solvent extraction followed by GC–MS (Gas Chromatography—Mass Spectrometer) were extracted from cone and quartered samples of the ASR and pyrolysis solid residues. A common sample preparation method is ultrasonic-enhanced solvent extraction, based on the EPA 3550 method [39]. An example method for PAH analysis is shown in Table 1, where anhydrous sodium sulphate is added to a 5g biochar sample, which is extracted using ultrasonic extraction with a 50:50 mixture of hexane/acetone. In this example (Table 1), PAHs, PCBs, TPHs and BTXs were detected using Agilent 7890 and 6890 gas chromatographs, in various configurations.

Pollutants	Agilent instrument	Injection volume μl	Detector	Column	Temperature program	Carrier gas
PAH	7890	2.0	GC/MSa	DB-5 ms	40°C for 1 min to 120°C at 25°C min^−1, then 160°C at 10°C min^−1 and finally to 300°C at 5°C min^−1, final hold time of 15 min.	He
PCB	7890	2.0	GC/ECDb	HP-5 ms	75°C for 3 min, to 150°C at 15°C min^−1, then to 260°C at 6°C min^−1, finally to 300°C at 20°C min^−1 rate held for 5 min	N2
TPH	6890	1.0	GC/FIDc	DB-5 ms	40°C for 1 min to 320°C at 10°C min^−1, final hold of 40 min.	He
BTEX	6890	1.0	GC/FID	DB-642	30°C for 1 min, to 100°C at 5 °C min^−1 to 220°C at 8°C min^−1, final hold of 5 min.	He

Table 1.

Organic analysis operating conditions. Sourced from ref. [40].

^a

GC/MS: gas chromatography equipped with high resolution mass spectrometry.

^b

GC/ECD: gas chromatography equipped with electron capture detector.

^c

GC/FID: gas chromatography equipped with flame ionization detector.

Biochar produced from the thermal processing of organic solid residues is a growing technology which may be used to enhance processing a depollution of waste. However, waste streams are often a complex heterogenous mixture of material, making thermal processing methodology more complex. This chapter will define thermal processing methods and the effect on production of biochar in complex heterogenous waste streams.

2. Processes

Solid residues can be processed to produce biochar using two thermal processing methods: pyrolysis and gasification. Pyrolysis is the thermal processing of a material at an elevated temperature (400–1000°C) in the absence of oxygen. Pyrolysis produces three main products: syngas, oil, and biochar. Pyrolysis instruments vary in design with the main differences being in the type of kiln used to heat the feedstock and whether post-pyrolysis the gas is being distilled to remove any oil. Pyrolyser designs are often tailored by the feedstock, industry and components such as condensers and distillation systems can be added. Common pyrolyser designs are presented in Figure 2.

Figure 2.

Schematic diagrams of pyrolysis reactors used in waste processing. A = bubbling fluidised bed; B = circulating fluidised bed; C = screw reactor; and D = rotating cone reactor. Adapted from Khodier [41]; original source: [42].

In addition to pyrolyser design, its operating parameters (temperature, residence time) can have a significant impact on the end products. The temperature and time that the waste is exposed to heat influences the breakdown of compounds and the development and chemical consistency of the end-products. Often a higher temperature (800–1000°C) can increase char and syngas production, where lower temperatures (400–800°C) increase oil production [41]. Lower pyrolysis temperature does result in a reduction in contamination due to lower activation energy for larger compounds. Biochar is often produced as a byproduct, with the energy produced from pyrolysis of feedstock influencing the methodology. Often, this results in higher pyrolysis temperatures, causing pollutants to be contained within the biochar products, which requires clean-up.

Pyrolysis operating parameters can have significant impacts on the quality and yield of biochar. It is widely acknowledged that increased pyrolysis temperature and residence time can reduce the reactivity of the char produced [43, 44]. The effect of pyrolysis temperature (range 500–900°C) on char chemical structure was analyzed by Zhao et al. [45] where the pyrolysis temperature is greater than 700°C there was a significant reduction in the carbonyl groups within the aromatic structure. As there was a corresponding increase in oxygen within quinine compounds. Benzene ring condensation increased at 900°C with char having >6 benzene rings within the carbon structures. This was seen to be lower within the higher temperature chars. However, this reduced the char’s chemical volatility and contributed to a larger pore size within the particles. Therefore, biochar produced under lower pyrolysis temperature had increased oxygen content and lower particle size, with the higher temperature biochar had increased chemical stability. This has an impact on what market the biochar can be utilized in and as we will see later the types of organic compounds present within the biochar structure.

Biochars that are lower in chemical reactivity may not be suitable for products within the depollution sector (chemical absorbent in water and air depollution) or as a feedstock for gasification. The chemical structure changes in char with pyrolysis temperature (explained above) has significant effects on the adsorption capability in water systems [46]. Pyrolysis temperatures above 500°C an increase the hydrophobicity of biochar, increasing the sorption of organic pollutants [47]. The reduction in biochar pore size and increase in oxygen content within hydrocarbon compounds in lower pyrolysis temperatures (<500°C) can encourage the sorption of inorganic pollutants from water systems, such as heavy metals [46]. It will also influence the retention of POP within the structure. Optimizing pyrolysis methodology to improve biochar utilization in the environment is crucial to meeting environmental targets. Types of biochar feedstocks and their products from different processing routes is presented in Table 2.

Feedstock	Catalyst	Pyrolysis Temperature (°C)	Experimental Scale	Biochar type	Commercial Application
Lignin biomass	FeSO4 [48] Char and metal oxide [49] Metal loaded Zeolite [50]	300–800	Laboratory	Biochar	Magnetic biochar [48] Biofuel [49, 50]
Cellulose Biomass	Synthetic Zeolites [51, 52, 53] Pyrolysis char [54]	600–800	Laboratory	Biochar	Hydrogen gas [51] Biofuel [52, 53, 54]
Wood Biomass	Biochar with metal catalysts [55, 56] Synthetic Zeolite [57]	550–600	Laboratory	Charcoal	Heating and Syngas [55, 57]; Activated carbon [56]
Pine Sawdust	Biochar with steam activation [58, 59, 60]; Treated steel slag [61]	850–1200	Laboratory	Charcoal	Heating [54, 112, 135] Fuel [61]
Straw Residues	CO2 [62]	800	Laboratory	Biochar	Energy and Fuel [62]
Seaweed	CO [63]	800	Laboratory	Biochar	Energy and Fuel [63]
Papermill sludge	Fe3O4 [64]; CaCO3 and Fe3O4 [65]	800	Laboratory	Magnetic biochar	Activated carbon [64, 65]
Municipal Sludge	Metal loaded zeolite with char [66]; Bentonite [67] HCl and Na2CO3 pretreatment [68]; biochar [69]	800–900	Laboratory	Pyrolysis char	Energy [66, 67] Activated carbon and hydrogen production [69, 68]
Cattle Manure	Biochar with metal oxides [54]	800	Laboratory	Charcoal	Energy production [54]
Food waste	Pyrolysis Char [70, 71]	700	Laboratory	Activated carbon	Air depollution [70, 71]
Waste Plastic	Synthetic Zeolite [72, 73, 74] with CaO [75] Dolomite [76] Modified pillared clays (MPCs) with Fe [77]	325–880	Laboratory	Pyrolysis char	Hydrogen production [72, 73, 74, 75, 76, 77] Biofuel [73, 77]
Plastics & Biomass	HZSM-5 [146, 147]	500–600	Laboratory	Biochar	Biofuel production [78, 79]
Plastic from ELVs	Metal loaded Zeolites [80, 81, 82]	325–485	Laboratory	Pyrolysis char	Heating and Hydrogen production [80, 81, 82]
Biochar	Pyrolysis Char [83];	800	Laboratory	Pyrolysis char	Hydrogen production [83]

Table 2.

Types of feedstock that can be used in pyrolysis and the various products they can make under specific catalysts.

Investigations of biochar physical structure is focused on effects of homogenous biochar from biomass [84, 85, 86, 87] or plastic feedstocks [28, 29, 30, 88]. The influence of co-feedstocks is currently being explored to increase syngas quality (CO:H₂ ratio), utilize waste, and improve byproducts, where research is still developing. Current findings suggest that co-feeding biomass with plastic feedstocks could have a synergistic effect on the quality of pyrolysis byproducts, where the lower oxygen concentration in plastic feedstocks lower oxygen concentrations and increase hydrogen and carbon concentration [58, 89, 90, 91, 92]. However, most studies focus on the production of bio-oil and there is limited research on the impact on biochar structure and any corresponding update of POPS. Biochar from plastics requires pyrolysis at higher temperatures (900°C) [93] to fully decompose, therefore future research should investigate the effects of co-feeding at higher temperatures to determine the impact on solid residue products. This may reduce the potential for organic pollutants within the biochar structure.

Biochar itself is currently being reused as a co-feed back into pyrolysis and gasification systems. Gasification is often conducted at a higher temperature than pyrolysis (800–1200°C) with controlled amounts of oxygen or steam to increase the rate of reaction [94]. Gasification does not produce bio-oil as one of the products. In some gasification systems, biochar is used as the feedstock, so the different techniques can complement one another [95]. Biochar produced at lower pyrolysis temperatures can be re-used back within the gasification system and reduce the activation energy required in syngas production. Biochar has been used as a co-feed for many pyrolysis and gasification feedstocks including biomass [49, 54, 57, 94, 95], sewage sludge and municipal waste [44, 66, 70, 71] and coal [61, 96, 97, 98]. Addition of biochar as a co-feed could also enhance the quality of the secondary biochar, causing a reduction in the inorganic components within the material. Recycling biochar back into the system will reduce pyrolysis impacts on waste to landfill. Addition of renewable biomass could improve secondary biochar quality and its effectiveness as a product. However, the impact towards the production of POPs is less understood due to limited research in this area.

A significant challenge for the waste industry is the complexities in processing material, which often results in heterogenous biomass feedstocks and the challenge to produce a usable biochar. This results in material often ending up in landfill. A heterogenous feedstock whose use as a biochar is being explored is ASR. The following section will discuss some of the hurdles of heterogeneous feedstocks and will use ASR as one of the worst-case materials.

3. Heterogeneous biochar feedstock: automotive shredder residue

To highlight the potential contamination within bio-chars a particular example has been chosen. This example will address some of the worst case for mono-source and mixed source feedstocks, ASR is a heterogenous organic waste produced at the end of the waste recycling process of ELVs (End-of-Life Vehicles). ASR makes up approximately 25% of the components of an ELV and is a mixture of organic biomass (textiles, wood) mixed with other waste (consisting of foams, plastics, fibers, glass and residual metals) [40, 93] (Figure 3). Recent ELV legislative targets in the UK and Europe require 95% of an ELV is required to be recycled or recovered by 2030 [99]. Currently ASR is sent to landfill; to meet legislative requirements further recovery or re-use is required therefore, the renewed interest in its conversion into a biochar as a potential processing route.

Figure 3.

Image of ASR from waste recycling plant. Sourced from ref. [93].

At present, there are only a small number of investigations into pyrolysis of ASR and its suitability for biochar production. As the trend increases to pyrolysis more heterogenous waste streams there will be an increase in the amounts of biochar which will require an end market. Studies indicated that carbon concentration within heterogeneous feedstocks such ASR char were not affected by temperature. This contrasts with crop-based feedstocks as mentioned in the earlier section [93, 100, 101, 102, 103], however, the calorific value of the char did decrease with temperature [93, 103]. This could be caused by chemical structure changes within the char, previously seen in pyrolysis of other feedstocks. Further chemical analysis of carbon molecular structure of ASR pyrolyzed at different temperatures would be required to confirm this. The challenge being the heterogeneous nature of material and sampling errors. With governmental pressure to improve recycling activities and the environmental emergency requiring the elimination of fossil fuel energy production, research into this area is expected to expand over the next decade as more product types of biochar emerge.

Some of the key findings from research of ASR pyrolysis suggest that there is a significant effect of processing temperature on char particle size and chemical consistency. The biochar from ASR was produced in a 60 kg per hour pilot scale plant by Khodier and Williams [93]. The chars produced were subject to physio-chemically analysis under different temperature conditions (800–1000°C). Findings indicated that finer char was developed at higher pyrolysis temperature (1000°C), with a higher calorific value and lower oxygen content. The biochar produced at both 800 and 1000°C were separated into ‘coarse’ and ‘fine’ particle size fractions (coarse: > 0.1 mm diameter; fine = < 0.1 mm) see Figure 4. Therefore, allowing the biochar from different particle sizes having different applications depending on their characteristics. The larger particle sizes could be used in iron sintering [104] and to make H₂ through steam activation [69, 83, 91] Lower particle sized char, with its more irregular shape [100], which along with an increased microporosity has higher absorbent properties and would be more useful in environmental applications such as water storage in soils and water purification [105, 106, 107].

Figure 4.

Optical images of coarse (a) and fine (b) char. Source taken from ref. [92].

Biochar produced through pyrolysis of ASR and other heterogenous materials may have similar positive effects on soil properties and water purification as traditional homogenous feedstocks, however this still has to be proved as research into this area is limited. Recent laboratory studies indicated that coal residue biochar can increase SOC (Soil Organic Carbon) and TN (Total Nitrogen) concentration, when compared with maize biomass biochar, fresh residues and control soil [107], which could enhance crop growth. A supposition could be put forward that this would be true for heterogeneous feedstocks. However, it should be noted that with coal residue biochar no toxic contaminants within the chars (e.g., heavy metal concentration, PAH and PCB concentrations) were not studied or any impacts of leaching. As we will see later there are potential restrictions on the use of biochar produced from pyrolysis of heterogenous materials (such as ASR and waste sources such as contaminated wood etc.), due to the high concentrations of PAHs and dioxins in the char being over governmental limits for agricultural processes [36]. Increased chemical depollution of biochar from ASR and other heterogenous feedstocks will be required before use on land [108, 109].

It was found through more detailed analysis of the coarse and fine char fractions of ASR that there was a clear difference in organic pollutants. The fine particle sized fractions (<0.1 mm) had increased concentrations of PAHs and PCBs, which altered with temperature [40]. In contrast to the coarse char (>0.1 mm) which was determined to be inert with low contamination, (levels reported in Tables 3 and 4). There were significant effects of pyrolysis temperature on the PAH and PCB levels within the fine char component, where PAHs decreased with higher pyrolysis temperature (Table 3) and concentrations of PCBs increased (Table 4). Further investigations on the effect of temperature on the formation and recreation of compounds from heterogenous mixtures is required to determine the best method to reduce environmental contaminants held within the feedstock through process control. It should be noted that the heterogeneous nature of the feedstock makes process control as the sole solution questionable. A more resilient solution would be secondary processing as an effective method to upgrade the biochar and reduce organic pollutants. The next section will define and evaluate current secondary processing of biochar from heterogenous waste sources.

Target Compounds	CAS	R.T. (min)	Char 800°C (mg kg^−1)	Fit (%)	Char 1000°C (mg kg^−1)	Fit (%)
Naphthalene	91–20-3	3.23	5010.00	99	46.60	99
Acenaphthylene	208–96-8	4.36	2040.00	99	91.00	99
Acenaphthene	83–32-9	4.48	56.80	73	<8.00	—
Fluorene	86–73-7	4.87	192.00	99	9.63	97
Phenanthrene	85–01–8	5.72	3980.00	99	429.00	99
Anthracene	120–12-7	5.77	724.00	97	101.00	98
Fluoranthene	206–44-0	7.07	2470.00	89	879.00	90
Pyrene	129–00-0	7.36	2870.00	87	1250.00	88
Benzo[a]anthracene	56–55-3	9.05	401.00	96	93.70	94
Chrysene	218–01–9	9.11	504.00	99	124.00	97
Benzo[b]fluoranthene	205–99-2	10.58	583.00	97	268.00	90
Benzo[k]fluoranthene	207–08-9	10.62	211.00	98	70.30	90
Benzo[a]pyrene	50–32-8	11.01	609.00	97	336.00	96
Indo[1,2,3-cd]pyrene	193–39-5	12.38	496.00	89	451.00	91
Dibenzo[a,h]anthracene	53–70-3	12.41	42.10	85	13.10	72
Benzo[g,h,i]perylene	191–24-2	12.68	524.00	93	627.00	95
Coronene	191–07-1	14.88	136.00	52	285.00	68
Total (USEPA16) PAHs			20712.90		<4797.33

Table 3.

Concentrations of PAHs in fine char (at 800 and 1000 C) from ASR feedstock. Data sourced from ref. [40].

Compounds	Char 800 °C (mg kg^−1)	Char 1000 °C (mg kg^−1)
PCB28	<25.0	59.1
PCB52	<25.0	87.2
PCB101	<25.0	53.9
PCB118	<25.0	<25.0
PCB153	<25.0	<25.0
PCB138	<25.0	210.9
PCB180	<25.0	47.6
Benzene	13,100	420
Toluene	1220	<25
Ethylbenzene	167	<25
Xylenes	855	<75
m/p-xylenes	679	<50
o-xylene	176	<25
MTBE	<50	<50

Table 4.

Concentrations of PCBs (7 congeners) and BTEX in produced fine char fraction (at 800 and 1000°C). Data sourced from ref. [98].

4. Secondary processes to reduce organic pollutants in biochar from heterogeneous sources

Environmental contamination within biochar from heterogenous sources limits its use in other applications, therefore is often sent to landfill as hazardous waste [93]. Secondary processing of contaminated biochar could reduce the amount of waste to landfill and enable biochar from heterogenous sources to be used as de-pollutants in contaminated land and water systems. There are many methods to reduce pollutants held within biochar. Based on the example in Section 3 [40], a simple reduction in contamination would be size segregation by sieving. If the biochar was sieved to <0.1 mm particle size, the contaminated fine char could be segregated, and the coarse fraction could be re-used. Size segregation of chars would not fully eliminate waste to landfill, so further secondary processing to clean up finer fractions of biochar would be required.

A common secondary process of biochar is carbonization and activation [110]. Carbonization is where volatile and inorganic components of feedstock are removed through thermal treatment, such as a secondary pyrolysis or calcination. The carbon contained in biochar from the pyrolysis process has a disorganized physical structure. Activation is the upgrading of the carbon porosity to regulate the structure. This is conducted though steam or CO₂ activation and the addition or impregnation of a catalyst (such as ZnCl₂, H₃PO₄ or KOH) [68, 111]. Activated carbon is chemically stable, with good conductivity due to its’ high surface area and can be used to generate EDLCs (Electrical Double Layer Capacitators) and used in depollution of water due to its high adsorption capacity [112]. However, organic waste containing heterogenous components, such as those from ASR, still produce substantial amounts of pollutants following activation [113, 114], so further post treatment is required. Nitric acid addition can be used to remove inorganic metals, followed by a base to neutralize. Studies suggest that this can significantly improve the conductivity of the EDLC without altering the porosity and char texture [115]. However, acid treatment results in excessive amounts of waste which then requires depolluting [116, 117], so may not be a cost-effective solution. Current research has explored molten salt post treatment as an alternative to acid treatment which removes the metal impurities [118]. Cleaner alternatives to activated carbon production for heterogenous feedstocks is required if this is to be economically viable.

In addition to activation and carbonization, another secondary processing method applied to biochar is magnetic synthesis, which can enhance its use as a water decontamination agent, due to the easy removal from the system post-adsorption [119, 120]. The use of Fe₃O₄ as a catalyst under CO₂ can encourage the formation of magnetic biochar (magnetite Fe₃O₄; saturation magnetization 28.4 emu g⁻¹), which has a high heavy metal adsorption [64]. Magnetization could increase removal of heavy metals from aquatic environments and improve water quality in polluted areas [120]. Impregnation of metal composites such as FeSO₄ into heterogeneous feedstock pre-pyrolysis can produce magnetic biochar. It has been found that an iron loading of 8% in the feedstock also enhanced biochar production [48]. Impregnation of iron composites within pyrolysis systems with heterogenous feedstocks, such as ASR, could enhance the utilization of the biochar as a magnetic activated carbon product.

Within heterogenous waste streams, further sorting of material pre-treatment could have significant effects on the contamination found within the biochar product. Using ASR as an example, the elimination of PVC from plastics within the material could significantly reduce the number of PCBs in the final product [93]. In addition to this, improved sorting could reduce the number of contaminants within the biochar, making secondary treatment more effective. Further sorting of the biomass (wood) and polymer (plastics, foams) materials of ASR may improve secondary depollution of biochar [108] and improve production of activated carbon [121]. Certain types of plastic removal from ASR would increasing the homogeneity of the feedstock to be pyrolyzed [122]. Development of feedstock sorting practices is possible; however, this would require significant changes to waste management practices which may not be practical.

If feedstock sorting is not a viable homogenization option, pre-treatment of feedstock by calcination could increase homogenization of the feedstock by reducing the particle size without causing depolymerization of hydrocarbons and devolatilization of plastic components [123]. It should be noted that typical feedstocks have not been tested at larger pilot scales, so it is difficult to evaluate the impact of scaling on the outputs. Torrefaction may be another suitable method of homogenizing feedstocks without fine metal sorting [124]. The process of calcination is a thermal pre-treatment conducted under limited oxygen, whereas torrefaction is conducted in the absence of oxygen. Further tests are required to determine the differences between torrefaction and calcination on the chemical consistency of the improved heterogenous feedstock to provide information on the optimum conditions. The economic impacts of an extra thermal pre-treatment step on the overall pyrolysis process requires careful evaluation to determine if increase product quality and yield are enough to promote investment.

Another method of reducing POP contamination of biochar is the reprocessing of it back into a thermal process. Biochar can be utilized back within the pyrolysis system to upgrade and clean the syngas, where the absorbent properties of char can increase H₂S removal [70] and can improve production of other byproducts such as ethylbenzene [71], where its catalytic properties can crack hydrocarbon chains. Alongside directly altering syngas properties, addition of char as a co-catalyst can increase regeneration of catalysts, improving production costs [57]. Utilization of char in other pyrolysis systems where the feedstock is more oxygenated, such as plant biomass, can have a deoxygenation effect, improving the quality of bio-oil products and increasing the syngas value [125]. Added to syngas systems, biochar can be used to clean up combustion systems by adsorbing CO2 emissions, reducing the negative industrial impact on global warming. Upgrading biochar through addition of metal composites such as Fe₂O₃ and Al₂O₃ can increase the adsorption through increasing char surface area and sorption capacity [57]. This is a low-cost CO₂ adsorption method, where catalyst desorption and regeneration temperature occurs at 120°C. Reprocessing biochar developed from heterogeneous feedstocks could be a viable option of creating more homogenous products which can be more effectively utilized. If using this approach, it is essential that contaminants within biochar are monitored to ensure that the addition of a catalyst in the gasification process can increase H₂ production [126] and limit PAH formation within the biochar [127]. Using Ca/Na compounds as a catalyst reduces the production of aromatic structures and increase the formation of more active intermediates beneficial to gasification [127]. However, research into the effects of biochar as a feedstock for gasification and H₂ production is focused on char derived from homogenous feedstocks [59, 96, 97, 126, 127, 128, 129]. ASR derived biochar has a more volatile carbonaceous structure than homogenous chars [130], meaning it could be more effective in gasification processes under steam activation. However, it could also be more difficult to select the correct catalyst with a wide range of pollutants present in the char (Tables 3 and 4). Further information on the physiochemical structure of ASR-derived biochars and effects of catalyst addition under steam activation is required ensure that depollution of char in this process is effective.

Alternative methods to processing contaminated biochar from heterogenous waste sources is to look at the sequestration of this product in composite materials. This will be in areas where the physical structure of the biochar improves the physical composition of the material and the pollutants are contained; reducing their effects. Containing polluted char within concrete may be a sustainable method for their use whilst at the same time reducing natural resource depletion from production concrete materials. Although, there is significant potential to utilize char in concrete materials, where an increased cement hydration and the immobilization of contaminants has been determined, there is still significant research required before commercial products can be manufactured and sold. The effect of char particle size, feedstock type and dosing amount can influence the tensile strength of the concrete, where if not correct, micro cracking can be caused [131]. Caeteno et al., [132] highlighted how finer fractions of heterogeneous bio char (ASR) when added to concrete had a beneficial effect. From the earlier sections highlighting that POPs were associated with certain size fractions for biochar from ASR sieved finer char could be used as a concrete agent and the inert coarser biochar for other applications. However, size separation of bulk material could be a time-consuming and expensive process.

A significant limiting factor of research into biochar production and its’ secondary processing is the lack of pilot scale projects. Many initial pyrolysis trials of biochar production and applications were conducted at a laboratory-scale (Table 2). Upscaling of laboratory scale to pilot scale systems is required to increase accuracy in the effectiveness of a catalyst on product yield and quality. To gain accurate results from a laboratory scale experiment a large amount of replication is required due to the small sample size (often 1-10 g feedstock), where upscaling to a pilot reactor can process 1000× more material, providing more realistic results. This would benefit heterogenous feedstocks by reducing error in sampling due to missing of potential contamination. This is also true of homogeneous feedstocks with added plastic material to improve yield. The next step in the development of products from heterogeneous feedstocks such as ASR will be to test effective catalysts on a pilot scale reactor. This would provide more accurate information on the effects of catalysis on a commercial scale, improving depollution of complex feedstocks. Due to the heterogeneity of the material, more than one catalyst may be required to target specific components of the feedstock. Two-stage catalysis of plastics has been investigated [133, 134], which might be a viable option for heterogenous biochar production.

5. Economic and environmental impacts driving biochar depollution

Producing biochar from heterogenous feedstocks and the potential contamination from POPs will be decided by two conflicting economical drivers: (i) whether biochar is being developed to stop feedstock going to landfill, or (ii) whether biochar is being produced for a specific application (such as activated carbon). If reducing material to landfill was the business focus, then biochar production from heterogenous sources (such as ASR) will be the driver for secondary processing development. Although secondary processing of biochar will reduce environmental contamination, the energy and resources required to implement these changes may outweigh other costs, such as landfill. If it is to produce biochar for specific applications then pre-processing technology would be the focus. In future, plastic components within waste streams including ASR will be classed as hazardous [40]. Many plastics already contain significant amounts of POPs, increasing the cost of landfill tax and the expense of disposal [3, 135], (EU Regulation on persistent organic pollutants (2019/1021) was adopted on 20 June 2019). This may lead to unforeseen consequences where businesses are making biochars that cannot be used because of elevated levels of contamination. The economics of processes will lead to a trade-off between reduction in waste to landfill and the creation of contaminated char with or without secondary processing. An analogous situation also arises with the use of crops being used to depollute contaminated soil systems and then used for energy. The biochar produced will contain pollutants which is then spreading contaminated biochar as a conditioner. This section will assess the economic and environmental costs to a waste recycling business when introducing thermal processing systems and the challenges and opportunities faced during commercialization.

Waste biochar produced from heterogenous sources such as ASR and MSW can produce a circular economy from waste streams [136, 137, 138]. In addition to the environmental incentive of reducing waste to landfill, a reduction in landfill tax is a significant economic opportunity, where current UK rates are £98.60 per tonne [139]. However, the chemical contamination within biochar (Tables 3 and 4) means that biochar from certain waste streams (such as ASR) could be classified as hazardous [40] which would increase landfilling costs. This could deter waste recycling industries from investing in biochar production, where a large financial investment is already required upon the purchase of a pyrolysis plant (Table 5) Upgraded biochar from secondary processing methods could produce a viable product that would promote a circular economy. However, the addition in business costs from development and maintenance of a secondary processing system might outweigh the costs of landfilling contaminated biochars. Long-term lifecycle assessment studies are required to investigate the payback and carbon/energy balances of these systems, which will determine whether secondary processing is an appropriate method in the future. There is no simple solution, and we are potentially creating legacy problems for the future.

Plant size (t d^−1)	Feedstock	Capital investment (M $)	Annual operating costs (M $)	Feed costs ($/t)	Production costs ($/gal)
2000	Forest residues	427	154	69	6.25
2205	Woody biomass	546	25.41	80	3.46
2205	Woody biomass	700	37.66	80	3.39
2000	Corn stover	200	12.3	83	0.26
1650	Wood pellet	180	12	—	0.24
1000	Dry wood	68	10.6	44	0.41
1000	Wet wood	72	11.3	30	0.60
1000	Peat	76	10.2	20	0.61
1000	Straw	82	10.2	42.5	0.64
900	Wet wood	46	9.9	34	0.50
550	Dry wood	48.2	9.6	45	0.71
400	Wet wood	14.3	8.8	36	1.02
250	Dry wood	14	8.92	44	0.55
200	Wet wood	8.8	4.84	36	1.11
100	Wet wood	6.6	2.84	36	1.48
24	Rice husk	3.89	0.170	22	0.82
2.4	Rice husk	0.97	0.34	22	1.73

Table 5.

Summary of reported pyrolysis plant cost. Sourced from ref. [39].

6. Conclusion

Production of biochar from heterogenous materials is likely to increase over the next decade as governments attempt to reach environmental targets for 2030 following COP26. The use of waste biomass for energy sources will be a driver in future energy production as the world resorts towards cleaner energy and away from fossil fuels. As highlighted in this chapter, utilization of biochar produced from thermal recycling of heterogenous waste feedstocks pose many challenges due to prominent levels of POPs and heavy metals within the feedstock. The amounts and types of persistent organic present is discussed. Secondary processing is a potential solution to remove contamination from biochars but the economics and readiness for the market are currently the limiting factors. Future opportunities to upgrade biochar through secondary processing are being adopted within the sector but are yet to be commercially available.

Acknowledgments

The authors gratefully acknowledge the following financial support: Higher Education Innovation Fund UKRI, Innovate UK’s support through the Knowledge Transfer Partnership (KTP). As well as both the University of Central Lancashire and Recycling Lives Limited, Preston, UK for their financial support and access to their facilities.

Conflict of interest

The authors declare no conflict of interest.