Biochar catalysts from animal manure: Production and application

Abstract

Biochar is a sustainable functional material rich in carbon, derived from renewable resources such as biowaste (e.g., animal manure). It has unique chemical structure, large surface area and porosity, and tailored surface functional groups via proper activation and/or functionalization, thereby having high potential as heterogeneous catalytic materials applied to different chemical processes. Therefore, using animal manure-derived biochar as catalytic materials could be key to sustainable agriculture and chemical industries. Recent recognition of animal manure-derived biochars as versatile media of catalytic applications has encouraged rudimentary studies on their catalytic capabilities; however, the use of animal manure-derived biochar as catalytic materials has not been systematically reviewed yet. This review gives an overview of recent achievements in producing biochar from animal manure and subsequent modification methods. The catalytic properties of the biochar with respect to its production/modification recipes are also discussed. Furthermore, the catalytic performances of animal manure-derived biochars for different catalytic applications, such as transesterification, hydrogen production, hydrolysis, C–C coupling reactions, and electrodes for oxygen reduction reaction, supercapacitor, and lithium-ion battery, are evaluated.

Keywords

biorefinery carbon material pyrolysis functional material organic waste sustainable material

Introduction

Improper handling of manure in livestock farming has negative impacts on the environment, associated with the pollution caused by releasing organic matter and nutrients and the emissions of harmful gases. When N and P-rich manure are spread over the land beyond its capacity, the surplus nitrogen and phosphorus are washed out by seepage and/or surface run-off, causing eutrophication in water resources.¹ In addition, the breakdown of animal manure at any stage of manure handling releases toxic gas (e.g., ammonia) and greenhouse gases. Animal manure is a major source of ammonia emissions to the atmosphere becoming a threat to human health by forming fine particles.² The livestock sector greatly contributes to global greenhouse gas emissions such as methane and nitrous oxide.³ The scale of these negative environmental impacts is more closely associated with animal manure management than other factors⁴ in terms of greenhouse gas emissions.⁵ These environmental challenges require new solutions. In the past decades, biochar made from animal manure has been proposed as a sustainable agriculture practice to address the above-mentioned environmental issues.

Biochar can be defined as the carbonaceous product that is obtained when biomass or organic waste (e.g., animal manure) is subjected to heat treatment via gasification,⁶ pyrolysis,⁷ or hydrothermal carbonization.⁸ Biomass waste-derived biochar is regarded as a renewable carbon material because it contributes to climate change mitigation in reducing greenhouse gas emissions.⁹ Renewable functional materials have gained great interest in order to achieve carbon neutrality and sustainable development goals. Biochar is considered a class of sustainable functional materials due to the abundance of raw materials,¹⁰ simple production process,¹¹ straightforward modification of surface properties and porosity,¹² and the ability in carbon sequestration.^13,14

Biochar has microstructure with numerous pores, leading to high surface area (8–132 m² g⁻¹) and total pore volume (0.016–0.083 cm³ g⁻¹).¹⁵ By using right precursor and controlling pyrolysis parameters, the surface area and total pore volume of biochar could increase to 490.8 m² g⁻¹ and 0.25 cm³ g⁻¹, respectively.^16,17 Post-treatments (e.g., KOH activation) further increased the surface area (3263 m² g⁻¹) and total pore volume (1.772 cm³ g⁻¹) of biochar.¹⁸ The activation leads to large surface area and multi-scale porous structure,^19,20 while the functionalization leads to abundant surface functional groups (e.g., carbon monoxide, hydroxyl, carbonyl, and carboxylic acid).^15,21 The modified biochars have been employed in many applications including soil amendment,²² CO₂ sequestration,²³ and the removal of organic and inorganic pollutants from air,²⁴ soil,²⁵ and water environments.²⁶

Other than the aforementioned applications, the modified biochars with large surface area and abundant surface functional groups²⁷ have held great promise as catalytic materials in many chemical reactions such as hydrogenation²⁸ and hydrogenolysis²⁹ because the characteristics can provide a number of catalytic active sites. For example, Lee research group prepared rice straw-derived biochars made under different atmospheric conditions (e.g., N₂ and CO₂) and used the biochars as catalyst support for biomass conversion reactions such as the production of tetrahydrofuran and 1,4-butanediol from furan.²⁹ The biochar-supported catalyst could be more active than conventional activated carbon-supported catalyst.³⁰ Furthermore, biochar-based catalysts have been used for catalyzing hydrolysis, dehydration, nitrogen oxide reduction, transesterification, and bio-oil upgrading reactions.^31,32

Nevertheless, there is still a gap between the current stage of the animal manure-derived biochar catalyst and its commercialization. This necessitates a systematic review on the utilization of animal manure-derived biochar as a catalytic material in order for encouraging more researchers to conduct research and development on using animal manure-derived biochars as catalytic materials for chemical/electrochemical reactions. Although several reviews overviewed the utilization of biochars made from biomass as catalytic materials,^31–35 specific catalytic applications of animal manure-derived biochars have not been systematically overviewed yet. Therefore, providing a comprehensive overview of recent studies on using animal manure-derived biochars as catalytic materials is still required. In this regard, this review first discusses the production of biochar from animal manure, the modification of the biochars, and how the modification methods enhance the biochar properties. After that, versatile applications of animal manure-derived catalytic materials are discussed, including transesterification, catalytic pyrolysis, hydrolysis, C–C coupling reaction, and electrodes for oxygen reduction reaction, supercapacitor, and lithium-ion battery. Catalytic degradation of organic and inorganic pollutants via adsorption,^36–39 Fenton-like reaction,⁴⁰ and persulfate activation^41–43 is intentionally excluded in this review because such topics have recently been reviewed several times.

Production of biochar catalysts from animal manure and their catalytic properties

The characteristics of feedstock and pyrolysis parameters strongly influence the features of biochar.⁴⁴ The physicochemical properties of biochar are also heavily dependent on the type of raw material (thermochemical and elemental compositions), carbonization process (gasification, pyrolysis, and hydrothermal conversion), and modification (activation and functionalization) methods.^45–48 For instance, biochars derived from animal manure typically have a lower content of carbon than those derived from lignocellulosic biomass, while animal manure-derived biochars contain much more inorganics (e.g., minerals and nutrients) than lignocellulosic biomass-derived biochars.⁴⁹

Pyrolysis is the most commonly used process to make biochar from animal manure. During pyrolysis, volatile matter is released from the raw material by thermal degradation in an absence of oxygen, resulting in only stable carbon remaining with the yield of 30–55 wt% (per the mass of raw material). For the production of biochar from animal manure, the raw materials were pyrolyzed at 300–700°C (heating rate of <10°C min⁻¹) for feed residence time ranging from 1 to 10 h under N₂ or Ar environment, as listed in Table 1. Table 1 also summarizes the production conditions and catalytic properties of various animal manure-derived biochars used for different catalytic applications discussed in Chemical reactions to which animal manure-derived biochars are applied and Animal manure-biochar as electrode materials.

Table 1.

Production conditions and catalytic properties of biochars made from animal manure.

Entry	Biochar raw material	Biochar production	Biochar yield (wt%)	Surface area (m² g⁻¹)	Total pore volume (cm³ g⁻¹)	Pore size (nm)	Acidity (mmol g⁻¹)	Basicity (mmol g⁻¹)	Ref.
1	Chicken manure	Calcination at 550 °C (2 °C min⁻¹) for 4 h	-	11.2	-	15.62	-	0.8	⁵⁰
		Calcination at 750 °C (2 °C min⁻¹) for 4 h	-	11.1	-	15.65	-	9.5
		Calcination at 850 °C (2 °C min⁻¹) for 4 h	-	10.8	-	15.48	-	12
		Calcination at 950 °C (2 °C min⁻¹) for 4 h	-	10	-	15.44	-	11.7
2	Chicken manure	Pyrolysis at 350 °C (10 °C min⁻¹) for 1 h in N₂	∼35	5.54	0.043	31.47	-	-	⁵¹
		Pyrolysis at 450 °C (10 °C min⁻¹) for 1 h in N₂	∼35	15.06	0.072	19.6	-	-
		Pyrolysis at 550 °C (10 °C min⁻¹) for 1 h in N₂	∼35	12.25	0.067	22.2	-	-
3	Chicken manure	① Pyrolysis at 350 °C (10 °C min⁻¹) for 1 h in N₂ (600 mL min⁻¹) ② Oxidation at 500 °C for 2 h	33.4	5.54	0.043	31.47	-	-	⁵²
		① Pyrolysis at 450 °C (10 °C min⁻¹) for 1 h in N₂ (600 mL min⁻¹) ② Oxidation at 500 °C for 2 h	33.4	15.06	0.072	19.6	-	-
		① Pyrolysis at 550 °C (10 °C min⁻¹) for 1 h in N₂ (600 mL min⁻¹) ② Oxidation at 500 °C for 2 h	33.4	12.25	0.067	22.2	-	-
		① Pyrolysis at 660 °C (10 °C min⁻¹) for 1 h in N₂ (600 mL min⁻¹) ② Oxidation at 500 °C for 2 h	33.4	15.27	0.095	25.21	-	-
4	Chicken manure	Pyrolysis at 650 °C in N₂ (600 mL min⁻¹)	-	4.81	-	-	-	-	⁵³
4	Chicken manure	Pyrolysis at 650 °C in CO₂ (600 mL min⁻¹)	-	139.7	-	-	-	-	⁵³
5	Swine manure	Pyrolysis at 500 °C (10 °C min⁻¹) for 2 h in N₂ (100 mL min⁻¹)	37.8	15.42	0.032	8.28	-	-	⁵⁴
5	Swine manure	Pyrolysis at 650 °C (10 °C min⁻¹) for 2 h in N₂ (100 mL min⁻¹)	34.8	25.96	0.038	5.97	-	-	⁵⁴
6	Lipid-extracted swine manure	Pyrolysis	34.5	-	-	-	-	-	⁵⁵
7	Cow dung	Pyrolysis at 600 °C for 1.5 h in N₂ (100 mL min⁻¹)	-	111	0.166	7.31	-	-	⁵⁶
8	Chicken manure	① Calcination at 800 °C (10 °C min⁻¹) for 4 h	-	38.74	0.0085	-	-	-	⁵⁷
9	Cow dung	② Pyrolysis at 500 °C for 2 h in N₂ ③ H₂SO₄ treatment at 180 °C for 10 h in N₂	-	-	-	-	16.7	-	⁵⁸
10	Hen manure	T = 200–720 °C (10 °C min⁻¹) in N₂ or CO₂ (100 mL min⁻¹)	Up to 44 wt%	-	-	-	-	-	⁵⁹
11	Cow manure	Pyrolysis at 500 °C for 4 h in Ar (60 mL min⁻¹)	-	13	0.021	6.76	-	-	⁶⁰
		① Impregnation with CuSO₄ ② Pyrolysis at 500 °C for 4 h in Ar (60 mL min⁻¹)	-	26	0.031	4.85	-	-
		① Impregnation with PbSO₄ ② Pyrolysis at 500 °C for 4 h in Ar (60 mL min⁻¹)	-	15	0.025	6.65	-	-
		① Impregnation with ZnSO₄ ② Pyrolysis at 500 °C for 4 h in Ar (60 mL min⁻¹)	-	8	0.02	9.75	-	-
12	Poultry litter	Slow pyrolysis at 400 °C for 8 h	-	2.3	-	-	-	-	⁶¹
13	Pig manure	Pyrolysis at 350 °C for 2 h	-	23.8	0.053	8.83	-	-	⁶²
13	Pig manure	Pyrolysis at 700 °C for 2 h	-	32.6	0.035	4.26	-	-	⁶²
14	Chicken manure	① Pyrolysis at 400–800 °C in N₂ ② Modification with CPTMS and then TBA^a ③ Immobilization with Pd(II) acetate ④ NaBH₄ addition for 2 h	-	-	-	-	-	-	⁶³
15	Cattle manure	① Pyrolysis at 40° C (2.5 °C min⁻¹) for 2 h in N₂ (100 mL min⁻¹) ② KOH (6 M) treatment ③ Thermal treatment at 400 °C for 0.5 h & 700 °C for 1 h	53.8	875.98	0.42	5.26	-	-	⁶⁴
16	Insect (Tenebrio molitor) feces	① HCl (1 M) treatment for 12 h ② Pyrolysis at 50° C (10 °C min⁻¹) for 1 h in N₂	73.8	232.1	0.13	2.3	-	-	⁶⁵
		① HCl (1 M) treatment for 12 h ② Pyrolysis at 500 °C (10 °C min⁻¹) for 1 h in N₂ ③ KOH treatment ④ Thermal treatment at 600 °C for 1 h in N₂	75.2	1267.9	0.63	2	-	-
		① HCl (1 M) treatment for 12 h ② Pyrolysis at 500 °C (10 °C min⁻¹) for 1 h in N₂ ③ KOH treatment ④ Thermal treatment at 700 °C for 1 h in N₂	63.7	2081.8	1.07	2.1	-	-
		① HCl (1 M) treatment for 12 h ② Pyrolysis at 500 °C (10 °C min⁻¹) for 1 h in N₂ ③ KOH treatment ④ Thermal treatment at 800 °C for 1 h in N₂	46.6	1959.3	1.17	2.4	-	-
17	Dairy manure	Pyrolysis at 600 °C for 4 h	-	37.4	0.059	-	-	-	⁶⁶
		① Pyrolysis at 600 °C for 4 h ② Impregnated with Ni(NO₃)₂ for 24 h	-	36.1	0.050	-	-	-
		① Pyrolysis at 600 °C for 4 h ② Impregnated with Ni(NO₃)₂ for 24 h ③ Microwave treatment (1000 W) for 0.5 h	-	29.8	0.046	-	-	-
18	Poultry litter	① Pyrolysis at 550–570 °C for 45–60 min ② KOH activation at 200 °C (5 °C min⁻¹) for 1 h & at 800 °C (5 °C min⁻¹) for 1 h	-	3035	1.73	-	-	-	⁶⁷
19	Swine manure	① Pyrolysis at 500 °C for 0.5 h in N₂ ② HF pickling at 25 °C for 1 h ③ KOH treatment for 24 h	-	2321	1.60	>2	-	-	⁶⁸
20	Chicken litter	① Pyrolysis at 40° C (2.5 °C min⁻¹) for 2 h in N₂ (100 mL min⁻¹) ② KOH (6 M) treatment ③ Thermal treatment at 400 °C for 0.5 h & 700 °C for 1 h	∼45	821.5	0.6	4.1	-	-	⁶⁹
20	Chicken litter	① HCl (0.25 M) treatment ② Pyrolysis at 40° C (2.5 °C min⁻¹) for 2 h in N₂ (100 mL min⁻¹) ③ KOH (6 M) treatment ④ Thermal treatment at 400 °C for 0.5 h & 700 °C for 1 h	∼39	678.9	0.52	4.12	-	-	⁶⁹
21	Dairy manure	① Pyrolysis at 500 °C for 2 h in N₂ ② Ball milling for 2 h	-	37.6	0.028	2.98	-	-	⁷⁰
		① Pyrolysis at 600 °C for 2 h in N₂ ② Ball milling for 2 h	-	42.1	0.031	2.95	-	-
		① Pyrolysis at 700 °C for 2 h in N₂ ② Ball milling for 2 h	-	47	0.034	2.9	-	-

CPTMS = (3-chloropropyl)trimethoxysilane; TBA = 2-(thiophen-2-yl)-1H-benzo[d]imidazole.

Several studies explored the effect of pyrolysis temperature on the biochar properties, indicating that pyrolysis temperature plays a prominent role in changing the yield and properties of biochar.^71,72 An increase in pyrolysis temperature decreases the yield of biochar from animal manure.⁷³ This results from higher temperatures favoring a release of more volatile matter from the feedstock.⁷⁴ Furthermore, rising pyrolysis temperature also affected the specific surface area and porosity of biochar. For example, Jung et al. observed an increase in the Brunauer–Emmett–Teller (BET) surface area of a chicken manure-derived biochar from 5.5 to 15.3 m² g⁻¹ with increasing the pyrolysis temperature from 350 to 660°C (Entry 3 of Table 1).⁵² A similar trend was also observed with dairy manure-derived biochar (Entry 18 of Table 1).⁷⁰ This is because the removal of more volatile species from animal manure at high temperatures results in increasing surface area and micropore volume.⁷⁵ In general, the specific surface areas of biochars derived from animal manure (Table 1) are lower than biochars derived from crop biomass (e.g., rice husk).⁷⁶ This is most likely due to high molar H/C and O/C ratios of crop biomass forming extensive cross-linkage during carbonization.⁷⁷ The activation of animal manure-derived biochar involving KOH treatment and subsequent thermal treatment could considerably enhance its surface area to reach >800 m² g⁻¹ (Entries 15–16 of Table 1). Another approach to increase the surface area of animal manure-derived biochar is to use CO₂ as a pyrolysis medium instead of common inert gas like N₂ and Ar. For instance, Jung et al. pyrolyzed chicken manure under a CO₂ environment, and the resultant biochar had 28-fold higher BET surface area than the chicken manure-derived biochar made under a N₂ environment (Entry 4 of Table 1).⁵³ The large surface area may be attributed to CO₂ enhancing the release of volatile matter from chicken manure during pyrolysis.⁷⁸ Treatment of chicken manure with strong acid (e.g., hydrochloric acid) prior to pyrolysis decreased surface area and pore volume of the chicken manure-biochar (Entry 17 of Table 1),⁶⁹ as the acid pretreatment changed the feedstock characteristics, such as molar H/C and O/C ratios and the contents of carbon and ash.⁷⁹

Biochars made from different kinds of animal manure contain different inorganic species with different compositions. Poultry litter and chicken manure typically contain calcium and potassium^80,81; thus, the catalysts derived from poultry litter and chicken manure often have basicity. The basicity of poultry litter or chicken manure-derived biochar catalyst is affected by the temperature at which it is made. For a biochar catalyst made from chicken manure, increasing calcination temperature increased the number of base sites on the catalyst surface, associated with the formation of more calcium oxide phase at higher temperatures (Entry 1 of Table 1).⁵⁰ Treatment of a cow dung-derived biochar with highly concentrated sulfuric acid led to providing the Brønsted acid sites on the catalyst surface associated with the formation of carbonyl sulfide resulting from sulfonic functional groups attaching to the cow dung-derived biochar (Entry 9 of Table 1).⁵⁸

Chemical reactions to which animal manure-derived biochars are applied

Transesterification and esterification reactions

Fatty acid methyl esters (FAMEs) are employed to produce biodiesel (a renewable substitute for petrodiesel⁸²) as they are primary molecules in biodiesel.⁸³ FAMEs are typically produced by transesterification of fats with methanol. However, conventional transesterification reactions suffer from the inevitable generation of acidic or basic waste stream because of using strong acid (e.g., sulfuric acid) or alkali (e.g., sodium hydroxide and potassium hydroxide) as a homogeneous catalyst.^84,85 To both resolve this issue and seek a sustainable alternative for expensive non-renewable catalysts, animal manure biochar has been proposed as a renewable heterogeneous catalyst.

In Table 2 (Entries 1–6), the experimental conditions and results associated with the production of FAMEs via transesterification over animal manure-derived biochar catalysts are summarized. Waste cooking oil and lipid fraction of swine manure were used as the feedstocks for FAMEs. The highest yield of FAMEs (>99.2%) from waste cooking oil was achieved with a swine manure-derived biochar made at 500 °C (Entry 5 of Table 2).⁵⁴ It was suggested that the large pore size of the swine manure-derived biochar catalyst contributes to its high activity for the transesterification reaction. Cho et al. suggested as a way to fully utilize swine manure as a biofuel resource involving (1) the extraction of lipid fraction from swine manure, (2) the production of biochar catalyst from the lipid-extracted swine manure via pyrolysis, and (3) the production of FAMEs from the lipid fraction extracted from swine manure using the catalyst derived from the lipid-extracted swine manure (Entry 6 of Table 2).⁵⁵ Through the process, 24.4 wt% of swine manure could be recovered as FAMEs.

Table 2.

Catalytic performances of various animal manure-derived biochars for different chemical reactions.

Entry^a	Biochar raw material	Reaction	Reactant	Product	Reaction condition	Catalytic performance^b	Ref.
1	Chicken manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 65 °C; t = 6 h; methanol/waste cooking oil = 15; catalyst loading = 7.5 wt%	Y = 10.7%	⁵⁰
						Y = 60.4%
						Y = 90.9%
						Y = 88.6%
2	Chicken manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 350 °C; t = 10 s; methanol	Y = 95.6%	⁵¹
						Y = ∼45%
						Y = ∼30%
3	Chicken manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 350 °C; methanol/waste cooking oil = 20	Y = 95%	⁵²
						Y = ∼42%
						Y = ∼33%
						Y = 0%
4	Chicken manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 170 °C; methanol/waste cooking oil = 20	Y = ∼45%	⁵³
4	Chicken manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 170 °C; methanol/waste cooking oil = 20	Y = ∼95%	⁵³
5	Swine manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 300 °C; methanol/waste cooking oil = 20	Y = 99.2%	⁵⁴
5	Swine manure	Transesterification	Waste cooking oil	Fatty acid methyl esters	T = 210 °C; methanol/waste cooking oil = 20	Y = 96.7%	⁵⁴
6	Lipid-extracted swine manure	Transesterification	Lipid fraction of swine manure	Fatty acid methyl esters	T = 220 °C; methanol/waste cooking oil = 20	Y = 24.4% (per dried swine manure)	⁵⁵
7	Cow dung	Transesterification	Glycerol	Glycerol carbonate	T = 85 °C; t = 1.5 h; dimethyl carbonate/glycerol = 2; catalyst loading = 3 wt%	Y = 97.1%	⁵⁶
8	Chicken manure	Transesterification	Algal oil extracted from oceanic algae (E. compressa)	Biodiesel	T = 65 °C; t = 3 h; methanol/reactant = 9; catalyst loading = 5 wt%	Y = 85%	⁵⁷
9	Cow dung	Esterification	Palm fatty acid distillate	Fatty acid methyl esters	T = 90 °C; t = 1 h; methanol/palm fatty acid distillate = 18; catalyst loading = 4 wt%	Y = 96.5%	⁵⁸
10	Hen manure	Catalytic pyrolysis	Hen manure	Combustible gas	T = 200–720 °C (10 °C min⁻¹) in N₂ or CO₂ (100 mL min⁻¹)	Inferior to Ni/SiO₂ catalyst	⁵⁹
11	Cow manure	Catalytic pyrolysis	Wheat straw	Hydrogen	T = 800 °C; t = 10 min; feed/catalyst = 2	C_product = 185 mL g⁻¹	⁶⁰
						C_product = 197 mL g⁻¹
						C_product = 177 mL g⁻¹
						C_product = 185 mL g⁻¹
12	Poultry litter	Cyclization	Trifluralin	2-Ethyl-7-nitro-1-propyl-5-(trifluoromethyl)-1H-benzo[d]imidazole	Ambient condition; biochar/dithiothreitol = 8.33; 20 mM phosphate buffer	-	⁶¹
12	Poultry litter	Reduction	Hexahydro-13,5-trinitro-13,5-triazine	Formaldehyde		C_product = 0.012 mM	⁶¹
13	Pig manure	Hydrolysis	Carbaryl	-	T = 20–24 °C; t = 12 h; 150 rpm; catalyst loading = 1.25 g L⁻¹	C = 71.8%	⁶²
13	Pig manure	Hydrolysis	Atrazine	-		C = 27.9%	⁶²
14	Chicken manure	C–C coupling reactions	Benzene derivatives	Biphenyl derivatives	T = 80 °C; t = 1–25 h; Pd loading = 0.715 mol%	Y = 64–96% TON = 89–135	⁶³

Same as Entry number of Table 1.

Y = product yield; C = reactant conversion; C_product = product concentration; TON = turnover number.

Other than the catalyst originating from swine manure, chicken manure-derived biochars were found to be effective at transestrifying waste cooking oil to produce FAMEs with high yields of >90% (Entries 1–4 of Table 2). Chicken manure-derived biochars could have basicity (Entry 1 of Table 1). Given that transesterification requires a base catalyst,⁸⁶ those biochar catalysts can be active for FAME production. Using a chicken manure-derived biochar catalyst, up to >90% FAME yield was achieved, as shown in Table 2 (Entries 1–4).

Shikhaliyev et al. reported a transesterification reaction between glycerol (a byproduct of biodiesel production⁸⁷) and dimethyl carbonate to obtain glycerol carbonate.⁵⁶ Note that glycerol carbonate is a value-added chemical serving as a source of polycarbonates and polyurethanes.⁸⁸ A cow dung-derived biochar (made by pyrolysis at 600 °C) having a specific surface area of 110 m² g⁻¹ and total pore volume of 0.17 cm³ g⁻¹ was used as the catalyst for the reaction, which achieved a high glycerol carbonate yield of 97.1% (Entry 7 of Table 2).⁵⁶ An increase in the pyrolysis temperature over 600°C led to decreasing the glycerol carbonate yield. This is most likely due to self-activation and consequent formation of more rigid carbon structure at high temperatures.⁸⁹ Sangar et al. prepared another cow dung-derived biochar catalyst and used the catalyst for esterification of palm fatty acid distillate as a low-cost feedstock.⁵⁸ The biochar catalyst was treated with a highly concentrated sulfuric acid in order to give surface acidity, leading to a high FAME yield of 96.5% at 90°C (Entry 9 of Table 2). The high esterification activity was ascribed to sulfonic acid groups strongly attached on the catalyst surface.⁵⁸

Catalytic pyrolysis

Catalytic pyrolysis is a thermochemical process that can produce hydrogen-rich gas from biomass feedstocks.⁹⁰ For example, Lee et al. reported the experimental data of catalytic pyrolysis of hen manure using a hen manure-derived biochar catalyst.⁵⁹ It was shown that the pyrolysis medium (N₂ or CO₂) did not markedly affect the catalytic activity of the hen manure-derived biochar for the catalytic hen manure pyrolysis, which was inferior to a typical silica-supported nickel catalyst in terms of H₂-rich gas production (Entry 10 of Table 2).

Zeng et al. developed a process involving catalytic pyrolysis of agricultural biomass (e.g., wheat straw) using cow manure-derived biochar catalysts impregnated with different heavy metals (e.g., copper, lead, and zinc; Entry 11 of Table 1) to enhance hydrogen production.⁶⁰ Among the different catalysts originating from cow manure, the highest yield of gaseous product (∼51.6 wt%) was achieved with the cow manure-derived biochar catalyst containing copper (prepared at 500 °C), equivalent to 550 mL g_{wheat straw}⁻¹. The gaseous product contained ∼35.9 vol% hydrogen corresponds to 197 mL g_{wheat straw}⁻¹ (Entry 11 of Table 2). The cow manure-derived biochar catalyst contained a plenty of hydroxyl, carbonyl, and other carbon–oxygen functional groups that act as proton donors⁹¹ that enhance catalytic reforming reactions.⁹² The proton donors given by the surface functional groups and the metal active sites could promote thermal decomposition of the lignocellulose-derived pyrolytic volatiles into non-condensable gases such as hydrogen.⁶⁰

Other reactions

The chemical reactions other than transesterification, esterification, and catalytic pyrolysis that are catalyzed by animal manure-derived biochars (e.g., hydrolysis and C–C coupling reactions) are listed in Table 2 (Entries 12–14). For instance, pig manure-derived biochars prepared at two different temperatures (350 or 700°C) were used to catalytically hydrolyze carbaryl and atrazine.⁶² Hydrolysis of carbaryl and atrazine occurs faster when the pig manure-derived biochar catalyst made at 700 °C was employed, reaching their conversions of 71.8% and 27.9% (within 12 h), respectively (Entry 13 of Table 2). The enhanced hydrolysis performance was mainly due to the increased solution pH by adding the biochar, minerals present on the catalyst surface, and dissolved metal ions released from the biochar.

Moradi et al. used a chicken manure-derived biochar as catalyst support with which palladium nanoparticles are dispersed.⁶³ Prior to immobilization with palladium nanoparticles, the chicken manure-derived biochar was modified with (3-chloropropyl)trimethoxysilane and 2-(thiophen-2-yl)-1H-benzo[d]imidazole. After that, the catalyst support was impregnated with palladium(II) acetate. The resultant catalyst was applied to C–C coupling reactions such as Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions. High turnover numbers (>110) were achieved for all tested reactions (Entry 14 of Table 2) most likely due to the impregnation with Pd species (Entry 14 of Table 1). In addition, the biochar-based catalyst was applicable to green solvent like polyethylene glycol. The chicken manure-derived palladium catalyst was also reusable for at least seven cycles, maintaining >90% yield.

Animal manure-biochar as electrode materials

Abundant minerals (e.g., transition metal elements) in biochar make its catalytic activity for redox reactions.⁹³ Some other minerals (e.g., calcium, potassium, sodium, phosphorus, and silicon) are present as either soluble or insoluble salts, which form the type of ionic conductive. This features biochar having the properties of extrinsic semiconductor along with high electrical conductivity. Therefore, other than chemical reactions discussed in Chemical reactions to which animal manure-derived biochars are applied Section, animal manure-derived biochars have been used as electrode materials for different processes including oxygen reduction reaction, supercapacitor, and lithium-ion battery, as summarized in Table 3.

Table 3.

Electrode performances of various animal manure-derived biochars for different processes.

Entry^a	Biochar raw material	Application	Electrode preparation	System	Performance	Ref.
15	Cattle manure	Supercapacitor	① Biochar/Super P/PVDF (8/1/1) in NMP ② Loaded on Ni form ③ Vacuum drying at 100 °C for 3 h ④ Pressed under 5 MPa for 30 min	• Biochar (working electrode) • Pt wire (counter electrode) • Hg/HgO electrode (reference electrode) • 6 M KOH (electrolyte)	• Specific capacitance = 161 F g⁻¹ (at 0.4 A g⁻¹) • High rate-performance (62% of low-capacitance) • Charging-discharging cycles = 10,000 (at 2.7 A g⁻¹)	⁶⁴
16	Insect (Tenebrio molitor) feces	Supercapacitor	① Biochar/acetylene black/PTFE (7.5/1.5/1) in isopropanol ② Loaded on Ni form ③ Drying at 75 °C for 2 h ④ Pressed under 10 MPa	• Biochar (working electrode) • Pt wire (counter electrode) • Saturated calomel (reference electrode) • 6 M KOH (electrolyte)	• Specific capacitance = 335.8 F g⁻¹ (at 0.5 A g⁻¹) • Energy density = 33.97 Wh kg⁻¹ (at 0.5 kW kg⁻¹) • Capacitance retention = 91.47% (after 10,000 charging-discharging cycles)	⁶⁵
17	Dairy manure	Supercapacitor	① Biochar/PTFE (8.5/1.5) slurry ② Loaded on stainless steel wire	• Biochar (working electrode) • CR2032 coin cell case • 0.5 M KOH (electrolyte)	• Specific capacitance = 39.1–123 F g⁻¹ • Resistance value = 610–140 Ω • Specific capacitance loss = 3.1–1.6%	⁶⁶
18	Poultry litter	Supercapacitor	① Biochar/water/ethanol suspension ② Loaded on Ni form ③ Vacuum drying at 60 °C ④ Pressed under 5 MPa	• Biochar (working electrode) • CR2032 coin cell case • 7 M KOH or 1 M Na₂SO₄ (electrolyte)	• Specific capacity = 229 F g⁻¹ • Current density = 10 A g⁻¹	⁶⁷
19	Swine manure	Supercapacitor	① Biochar/water/ethanol suspension ② Loaded on Ni form ③ Vacuum drying at 60 °C ④ Pressed under 5 MPa	• Biochar (working electrode) • Ag/AgCl electrode (counter & reference electrodes) • 6 M KOH (electrolyte)	• Specific capacity = 278 F g⁻¹ (at 0.5 A g⁻¹) • Capacitance retention = 93% (after 10,000 charging-discharging cycles) at 5 A g⁻¹	⁶⁸
20	Chicken litter	Lithium-ion battery	① Biochar/Super P/PVDF (8/1/1) in NMP ② Loaded on Cu foil ③ Vacuum drying at 80 °C for 3 h ④ Final active material loading = ∼2.4 mg cm⁻²	• Biochar (working electrode) • Coin-type CR2032 cells with Li metal (counter & reference electrodes) • Polypropylene separator • 1 M LiPF₆ in a 5:5 mixture of ethylene carbonate/dimethyl carbonate (electrolyte)	• Battery capacity (at 50 mA g⁻¹) = 279.8 mAh g⁻¹ • Battery capacity (at 1000 mA g⁻¹) = 106 mAh g⁻¹	⁶⁹
21	Dairy manure	Oxygen reduction reaction	Biochar layer on stainless steel mesh via PTFE binder	• Biochar (working electrode) • Pt wire (counter electrode) • Saturated calomel (reference electrode) • 0.5 M Na₂SO₄ (electrolyte)	• Current density (vs. RHE) = 0.26–1.88 mA cm⁻² (at 0.15 V) • Half-wave potential (vs. RHE) = 0.24–0.3 V • Electron transfer number = 2.1 (at 0.25 V) • Charge transfer resistance = 25–53.9 Ω • Double layer capacitance = 5.6 × 10⁻⁵–9.2 × 10⁻⁵ F	⁷⁰

Same as Entry number of Table 1.

Abbreviation: PVDF = polyvinylidene fluoride; NMP = 1-methyl-2-pyrrolidinone; PTFE = polytetrafluorethylene; RHE = reversible hydrogen electrode.

Various animal manure feedstocks were tried to prepare supercapacitor electrodes, including cattle manure,⁶⁴ insect feces,⁶⁵ dairy manure,⁶⁶ poultry litter,⁶⁷ and swine manure,⁶⁸ as summarized in Entries 15–19 of Table 3. For instance, a cattle manure-derived biochar was employed as a supercapacitor electrode. The porous carbon residue after pyrolysis was further activated make a supercapacitor electrode.⁶⁴ The electrode derived from cattle manure was a hierarchical porous carbon having bicontinuous structure that provides pathways for ion and electron transfer, thereby enabling fast charging-discharging performance. In a basic solution electrolyte, the cattle manure-derived biochar electrode showed a high specific capacitance of 161 F g⁻¹ at 0.4 A g⁻¹ with high rate-performance equivalent to 62% of low capacitance (Entry 15 of Table 3). The cattle manure-derived electrode also allowed at least 10,000 charging-discharging cycles at 2.7 A g⁻¹.

In a more recent study, chicken litter was used as a raw material for the synthesis of lithium-ion battery electrodes.⁶⁹ The effect of pretreatment of chicken litter with hydrochloric acid on the electrode capacity was explored (Entry 20 of Table 3). The hydrochloric acid treatment of chicken litter prior to pyrolysis led to a high yield of pyrolytic gas and a low yield of biochar, compared to the chicken litter-derived biochar without pretreatment with hydrochloric acid. The biochar made from untreated chicken litter had a large capacity of 280 mAh g⁻¹ at 50 mA g⁻¹, closely related to its graphitic morphology and high surface area (>800 m² g⁻¹; Entry 20 of Table 1). The biochar made from chicken litter treated with hydrochloric acid exhibited enhanced cycle stability. This results from the hydrochloric acid treatment removing impurities contained in chicken litter that potentially cause cycling decay.

Zhang et al. prepared three different dairy manure-derived biochars made at 500, 600, and 700°C and investigated their oxygen reduction reaction activity.⁷⁰ The experimental results proved that all the three biochar electrodes originating from dairy manure exhibit electrocatalytic activity to a certain degree. For instance, their current density ranged from 0.26 to 1.88 mA cm⁻² at 015 V, half-wave potential ranged from 0.24 to 0.3 V, electron transfer number was 2.1 at 0.25 V, charge transfer resistance ranged from 25 to 53.9 Ω, and double layer capacitance ranged from 5.6 × 10⁻⁵–9.2 × 10⁻⁵ F in a sodium sulfate solution (Entry 21 of Table 3). The increase in pyrolysis temperature at which the biochar is made enhanced the current density and half-wave potential, attributed to increasing aromatization degree of the biochar (i.e., H/C ratio).

Economic and environmental aspects of producing animal manure-derived biochar catalysts

Biochar production from animal manure is a key step for the practical use of animal manure-derived biochar as catalytic materials. Thus, several studies have conducted techno-economic and environmental impact assessments for the production of biochar from animal manure. For example, a model was developed to analyze the return on investment (ROI) of poultry litter valorization processes, indicating that pyrolysis has a higher ROI than hydrothermal conversion and gasification at an annual production capacity of 150 kt.⁹⁴ However, drying and pyrolysis of animal manure are somewhat energy-intensive; thus, they often incur high operation costs.⁹⁵ Furthermore, malodors can be emitted from animal manure transportation, potentially having negative impacts on the environment and human health. Despite such challenges, 0.3–2 Gt CO₂ can be sequestrated every year by producing, using, and storing livestock manure-derived biochar,⁹⁶ which greatly contribute to reducing the environmental burden.

Struhs and co-workers investigated the market opportunity and sustainability benefits of onsite transformation of cattle manure into biochar using a portable facility.⁹⁷ They also carried out not only technoeconomic assessment to calculate total cost for the production of biochar from cattle manure but also life cycle analysis to evaluate global warming potential of the cattle manure biochar production and distribution, considering on a case in Twin Falls, Idaho, USA. When the biochar is produced near feedlots, the total cattle manure biochar production cost and total greenhouse gas emission were estimated to be about $237 and 950 kg CO₂ eq. per metric ton, respectively. The moisture content of cattle manure and the portable refinery unit operation were found to be two most important factors that contribute to further decreasing biochar production cost and carbon footprint of cattle manure management.

In addition, using biochar in chemical and material production may support the SDGs (in particular, SDG 7 and 12). For SDG 7 (Affordable & Clean Energy), the byproduct of biochar production (i.e., syngas and bio-oil) can be used to produce heat and electricity.⁹⁸ Moreover, the conversion of biomass waste (e.g., animal manure) into biochar that displaces non-renewable expensive materials will lead us to a more circular economy, associated with SDG 12 (Responsible Consumption & Production).

Summary and outlook

Here in this review, various approaches to prepare animal manure-derived biochars used as catalytic materials were introduced and discussed. The physicochemical properties of the biochar-based catalysts were compared in line with their preparation methods. The morphology, porosity, and surface functionality of animal manure-derived biochars can be modified through various activation and functionalization methods; hence, they have the high potential as a substitute for non-renewable conventional catalytic materials. In addition to the biochar catalyst production from animal manure, the performances of the animal manure-derived biochar-based catalysts were evaluated for diverse reactions. The potent role of the catalysts based on the animal manure-derived biochars is demonstrated to be successful in chemical reactions such as transesterification, hydrogen production, hydrolysis, and C–C coupling. Furthermore, animal manure-derived biochars have shown promise as sustainable electrodes of supercapacitor and Li-ion battery systems.

Catalytic properties of animal manure-derived biochars (e.g., specific surface area, porosity, functionality, and catalytic site density) closely depend upon the characteristics of animal manure (e.g., proximate and ultimate compositions), the type of biochar production process and its operation conditions, and activation/functionalization methods. Hence, maximizing catalytic properties of animal manure-derived biochar required for an intended reaction needs more efforts to control of the combined effects of key variables such as animal manure type, biochar production conditions (e.g., temperature, residence time, and pyrolysis medium), and activation/functionalization conditions (e.g., acid or base treatment and metal impregnation). Nevertheless, it is available little information about precise control on the catalytic properties of animal manure-derived biochars for targeted applications. Therefore, further studies on optimizing the catalytic properties of animal manure-derived biochars would be crucial for designing highly active, selective, and stable catalysts originating from animal manure.

In order to make animal manure-derived catalytic materials more viable substitute for conventional industrial catalysts and electrodes, it is necessary to develop biochar catalyst production systems at industrial scales. The steady supply of animal manure must be secured to ensure the continuous production of biochar catalysts having constant properties in the industrial-scale systems. In addition, the combination of the biochar production from animal manure with chemical reaction process has not received much attention yet. The co-production of a biochar-based catalytic material (from animal manure) and a target chemical product (via an intended chemical reaction) would be beneficial to fully utilize animal manure-derived biochars as catalytic materials that can be applied to different chemical industries. When all the challenges are overcome, real-world applications of animal manure-derived catalytic materials will be stimulated, and it will ultimately facilitate the replacement of conventional catalysts with more sustainable catalysts.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (Ministry of Science and ICT; MSIT) (No. RS-2023-00209044 and 2020M1A2A2079801).

ORCID iDs

Young-Kwon Park

Jechan Lee

Wooyoung Yang is a master's degree student at present in the Department of Global Smart City, Sungkyunkwan University, Suwon, South Korea. Her research interests include waste recycling/upcycling, monomer recovery, and value-added product production from renewable resources.

Young-Kwon Park is a Professor of School of Environmental Engineering at University of Seoul. He received his BS, MS and PhD from the Chemical Engineering of Korea Advanced Institute of Science and Technology.

Jechan Lee is received his PhD in chemical engineering from University of Wisconsin-Madison in 2015 and MS in environmental engineering from Columbia University in 2010. After his PhD, he worked as a postdoctoral researcher at the Catalysis Center for Energy Innovation at the University of Delaware (2015-2016) and Sejong University (2016-2018). He was a faculty member of Ajou University from 2018 to 2022. He is now an associate professor in Sungkyunkwan University (SKKU), South Korea. His research interests are in the areas of waste upcycling, waste-to-energy, biorefinery, and green chemistry. He has currently authored more than 250 peer-reviewed SCI(E) papers. Furthermore, he is a recipient of Highly Cited Researcher (HCR) by Clarivate in 2022-2023.

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