Hydrochar for Potential Wastewater Treatment Applications: Comparison
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In today’s world, due to population increase, there are many alarming and potential catastrophic problems like climate change, environmental pollution and an enormous mass of wastes constantly produced by humankind to find innovative solutions for the management, recycling, and valorization of biowaste from agricultural production, food processing, and organic household residues. The search for sustainable and efficient wastewater treatment technologies has gained scientific interest recently; particular focus is on using biowaste to produce hydrochars (HCs) via the hydrothermal carbonization (HTC) process used as adsorbent materials for dye, heavy metal, and emerging pollutant removal. HTC materials derived from renewable resources are an environmentally friendly and adequate way to adsorb pollutants such as organic and inorganic molecules from wastewaters. 

  • HTC
  • hydrochar
  • water decontamination
  • biowaste

1. Introduction

Due to some alarming and potential catastrophic problems, such as climate change and the enormous mass of waste produced constantly, humankind has been forced to find innovative solutions for the management, recycling, and valorization of biomass from by-products of agricultural production, food processing, and organic household residues.
Knowledge based on the conversion of vegetal side streams into bio-based products and services is part of a global view called the bioeconomy concept [1], whereas aims to leverage the innovations in (life) sciences and bio-industries by moving from fossil to renewable resources [2]. In 2015, by reconsidering the issues of bioeconomy in direct relation to the circular economy, these two concepts were merged into the term circular bioeconomy, with a significant impact on biowaste management [3,4][3][4].
One of the main focuses of the circular bioeconomy is the valorization of biomass in integrated production, as a sustainable, efficient resource, using waste and optimizing its value over time. Such optimization emphasizes the environmental, economic, and/or social aspects, considering all three pillars of sustainability.
Biomass represents the raw material of bioeconomy; visionaries consider that the future will bring many biomass-derived products, such as biofuels, “green chemicals”, and biomaterials. The proportions of cellulose, hemicellulose, and lignin in biomass range from 40% to 50%, 25% to 30%, and 15% to 20%, respectively, but depending on the type of biomass, the concentrations of these components vary. Through the thermal treatment of biomass, biochar is produced; its properties depend on the reaction conditions (temperature, reaction time, the medium of the reaction), in addition to the composition of the biomass used.
Some of the most common technologies used for the management and treatment of biomass waste, together with the potential to produce bioenergy, biofuels, polymers and value-added chemicals in the context of circular bioeconomy, are: land fields, composting, thermochemical conversion processes (such as gasification, liquefaction, and pyrolysis) and biochemical conversion methods (such as fermentation, anaerobic digestion, and microbial fuel cells) [6][5]. Some disadvantages of these technologies require long reaction times, constant and costly maintenance of the equipment used, high energy consumption, huge carbon footprint, and expensive transport and storage costs [7][6]. The hydrothermal carbonization (HTC) process can overcome some of these problems, having benefits of lowering and/or eliminating the volume of organic waste which is often landfilled due to lengthy transportation routes, lower energy consumption during the process, lower cost because it uses water as a solvent, and other factors thus making it a viable technology which can be integrated in the circular bioeconomy flux [8][7].
Due to its rigorous treatment at high temperatures and, in certain cases, high pressure, thermochemical conversion is regarded as one of those methods that can treat any sort of feedstock [9][8]. The end products of this method can be broken down into three categories: solid, liquid, and gaseous. The ratios of the three fractions can be changed by adjusting the processing parameters (temperature, pressure, etc.), the reaction medium (O2, N2, and CO2 gases, water/liquid solvent, etc.), and the catalysts. In general, the temperature of the process ranges from 200 °C to 1600 °C and the pressure can range from atmospheric to as high as 400 bar [10][9], thus demonstrating the wide range of the thermochemical conversion process.
The treatment and management of solid waste, including biomass, are a global environmental and social challenge. However, these wastes can become raw materials for energy and value-added products. In addition to the conventional approaches of directly applying a thermal treatment method to biomass for energy production, the emerging approaches applied today include: (i) thermal treatment system modelling, (ii) the conversion of non-wood lignocellulosic biomass into value-added products, (iii) solid waste management, (iv) computational approaches for application in biotechnologies, and (v) carbon-based value-added materials [11][10].
Recently, there has been an increase in scientific interest in the search for a sustainable and affordable wastewater treatment technology. Due to their abundance and simplicity of availability, particular focus is on the use of sustainable and inexpensive adsorbents [12][11]. However, one common issue with crude adsorbents is the leakage of organic compounds, which can lead to secondary pollution and water oxygen depletion. Techniques to overcome these issues have been suggested, such as carbonization or charring [13][12]. Pyrolysis has long been the most popular conventional thermal process for carbonizing biowastes and natural resources [14][13]. Biochar is produced by this process at high temperatures (>400 °C) in an oxygen-free environment, and activated carbon is created by changing the physical and chemical properties of the organic matter at temperatures above 800 °C [15][14].
For many years, activated carbon has demonstrated its high efficiency in removing pollutants from wastewater treatment facilities and the food sector. Additionally, it is used to discolor syrups, vinegar, wine, refine sugar, and remove methylene blue (MB) and other dyes [16][15]. However, the high cost of manufacture restricts the use of activated carbon, attributable in part to the pyrolysis method and its application [17][16]. This limitation has driven the pursuit of a simpler and more lucrative technique for the carbonization of biowastes called hydrothermal carbonization (HTC) [18][17].
The HTC process usually occurs at temperatures ranging from 180 °C to 260 °C, time of residence between 0.5 h to 24 h, in the absence of air, leading to high autogenous pressures of 25–60 bar, using water as a reaction medium. Exothermic reactions require little heat energy, are very flexible, and can utilize a wide range of feedstock regardless of moisture content or heterogeneity. The HTC process is very complex, consisting mostly of reactions such as decarbonylation, decarboxylation, dehydration, hydrolysis polymerization, and re-condensation [19][18]. These reactions lead to a solid phase containing hydrochar (HC) with properties similar to fossil coal, a liquid phase where valuable chemicals such as furfural (FF) and hydroxymethyl furfural (HMF) can be found, and a gaseous phase consisting mainly of CO, with CO2 occurring mostly from the decarboxylation reactions. Additionally, the use of an appropriate catalyst can greatly influence the yield in HC, as well as other valuable chemicals present in the liquid and gaseous phases. The HCs are attractive materials which can be used for many purposes such as fuel, supercapacitor, soil amendment processes, adsorbents, catalysts, and nanostructured materials [20,21,22][19][20][21].
Dairy manure and wastewater sludge digestate [23][22], wet biodegradable residues [24][23], sewage sludge [25][24], activated sludge [26[25][26],27], hazelnut shells [28][27], and cigarette butts [29][28] are examples of biowastes investigated by the HTC process. Lignocellulosic biomass (LCB), frequently called simple lignocellulose, represents the plant dry matter, which is the Earth’s most plentiful raw mineral. Structurally, LCB is a complex mixture of carbohydrate polymers (cellulose, hemicellulose) interconnected with an aromatic polymer (lignin). In addition to these biopolymers, LCB contains many other types of bio-compounds such as polyphenols, alcohols, phenols, and phyto-acids.
The use of HC from biowaste recovered through the HTC process to remove environmental pollutants, such as persistent organic pollutants and heavy metals from industrial wastewater, has recently developed into a technology with enormous research potential [30][29].
It has been found that because of the speed and the modularity of HTC processes, scaling up can be achieved by increasing the number of reactors installed close to the waste source, thus eliminating the odor problems [8][7]. In this case, the CO2, CH4, and C emissions are reduced due to shorter distance transport of waste. Following IPCC (Intergovernmental Panel on Climate Change) recommendations for calculating CO2 emissions, it was estimated that the total CO2-eq (CO2-equivalent) emissions avoided were between 6.5 and 8.4 tons of CO2-eq/ton of hydrothermal coal [8][7].

2. Characteristics of the Hydrothermal Carbonization (HTC) Process

HC presents different properties in comparison to pyrochar (PC), the biochar obtained from pyrolysis of biomass, due to the reaction mechanism involved: in the course of HTC, the carbohydrates are hydrolyzed and the intermediates form the HC by polymerization, but during the pyrolysis process, the structural components of biomass decompose, yielding volatiles and the PC. Some advantages that the HC and PC present over fossil charcoal are their stable products, chemical purity, because they do not contain mercury or sulfur, a low content of nitrogen and ash, and a lower degree of aromatization. Therefore, HC and PC have a higher reactivity and porosity than fossil charcoal [32][30]. Among all biochar, due to the similarities with coal [33[31][32][33],34,35], HC can be applied for energy production, soil amelioration, and as an adsorbent in water treatment processes [36][34]. The HTC process reduces the hydrogen and oxygen contents in the biomass, due to the decarboxylation and dehydration reactions occurring at selected temperatures in the water medium. Water has a key role in the HTC process due to its adjustable physical and chemical properties, such as density, hydrogen bonding, ionization constant (Kw), and dielectric constant (k). Depending on the temperature and pressure, these properties are essential in the activity of the biomass to break the C–O or C–C bounds. Water acts as a catalyst and solvent in the HTC process; therefore, very few studies have been performed with additional catalysts. In most cases, weak organic acids, such as citric acid and acetic acid, are used to enhance the HTC process and to obtain HC with improved properties. In their studies, Amin Ghaziaskar et al. [37][35] enhanced the HTC process by recycling the process liquid, which exhibited catalytic activity and enhanced the energy yield and elemental composition of the HC; additionally, a higher heating value was observed for the HC from the catalyzed process when compared with the HC obtained from the uncatalyzed reaction. The catalytic activity of the process liquid was strongly related to the organic acids that form during the process, which led to a lowering of the pH, enhancing the dehydration and condensation reactions in the acid-catalyzed process and the HC production. The biomass processed by the HTC process involves several complex reaction pathways. There is a wide range of information available in the literature on possible chemical reactions that occur in the HTC process. Unfortunately, few studies have conducted in-depth analyses of the chemical pathways; some examples refer to the hydrolysis of cellulose. Decarboxylation and dehydration reactions combine to produce this reaction [38[36][37][38],39,40], which is recognized to be exothermic [40,41,42][38][39][40]. However, as of now, the complex reaction mechanisms are still not fully known. Reaction pathways documented in the literature originate from analogous networks of other reaction pathways. However, they are recognized for offering crucial knowledge regarding reaction mechanisms and are divided as follows: (i) hydrolysis (decomposition); (ii) decarboxylation and dehydration; (iii) condensation polymerization (recombination); and (iv) aromatization. Although the mechanisms of these reaction pathways rely on the structure and type of biomass waste utilized [43][41], achieving an understanding of the general reaction mechanisms of the HTC process in biowaste is important for further research. Parameters influencing the HTC process include the biomass solids content, temperature reaction, reaction medium and catalyst, reaction time, pressure, and pH used. They are discussed as follows.

2.1. Effect of Solid Raw Material Content

Due to their reactivity, furfural and 5-(hydroxymethyl) furfural (5-HMF) obtained by the dehydration of pentoses and hexoses present in the different biowastes are crucial in the synthesis of HCs. So long as these chemicals are present in the reaction medium, it is expected to obtain HC materials with comparable morphology and structure, despite the precursor used. In their study, Titirici et al. [44][42] showed that all carbon spheres derived from hexose and 5-HMF have similar morphology. Additionally, carbon spheres derived from furfural and xylose via the HTC process are indistinguishable as shape. Falco et al. [45][43], who studied the morphology of HCs from cellulose, glucose, and rye straw, provide more support. Carbon spheres made of glucose and cellulose appear to have a similar shape; however, it is believed that long cellulose fibers are destroyed during HTC processes, causing the development of shorter chains that take on a spherical shape to diminish contact with water. Some studies have reported that because the raw material used in the HTC process contains a mixture of complex molecules and compounds (life biomass wastes), the morphology of the final product varies. For example, Niinipuu et al. [46][44] used horse manure, fiber sludge, bio-sludge, and sewage sludge as raw material in the HTC process at 180 °C, 220 °C, and 260 °C; from the SEM images, it was revealed that all the HC produce had different morphologies depending on the raw material used. For horse manure HC, the structure bears a resemblance to that of lignocellulosic biomass, and the sewage sludge HC exhibited amorphous particles intermixed with fibers, whereas only large amorphous particles were observed for bio-sludge-derived HC. In the case of fiber sludge, only fiber structures with a similar appearance were observed. The morphology of HCs produced using corn stalks as solid raw material in the HTC process at 200 °C and different reaction times was investigated by Lei et al. [47][45]. Corn stalks contain a large amount of cellulose and hemicellulose (47.98% and 21.78%); thus, the final HCs produce spherical particles over an 8 h reaction time. Usually, the pretreatment of biowastes is required. For example, dried avocado seeds were mixed with NaOH 0.1 M, under reflux, over a 2 h residence time and at a temperature of 70 °C to eliminate dyes and tannins before separation and washing [48][46]. Citric acid was added together with Ziziphus mauritiana L. fruit pulp and distilled water for carbonization under acidic conditions [49][47].

2.2. Effect of the Temperature

Numerous studies and reports have been conducted regarding the impact of reaction temperature for biowaste processed by HTC. It has been found that temperature is the key element influencing the product characteristics of HTC [17,50,51][16][48][49]. The reaction temperature has a significant impact on HC characteristics, presenting an almost-linear relationship with carbon content; thus, with a temperature increase, the carbon yield decreases [52,53][50][51]. The temperature increased the amount of glucose in the water-soluble component which, at higher temperatures, further broke down into 5-HMF [54][52]. The hydrolysis rate of biomass fragments also depends on the temperature [55,56][53][54]. Hemicellulose degrades from 220 to 315 °C, followed by cellulose which decomposes in a temperature range from 315 to 400 °C, and lignin at over 220 °C [57][55]. The temperature also affects the rate of polymerization [58][56]. The effects of different temperatures (200 °C, 225 °C, and 250 °C) and processing times (4 h, 6 h, and 8 h) on HC processed from oil palm shell (OPS) were studied by Budiman et al. [59][57]. Based on the best properties obtained by the determination of the amount of iodine and surface area of HC, the authors concluded that a temperature of 225 °C and a residence time of 8 h are the optimal parameter values for conducting the HTC process.

2.3. Effect of the Pressure

During the HTC process, pressure is self-generated and rises automatically. Higher reaction pressures result in a decrease in hydration and decarboxylation processes, which are often the main mechanisms in this process [17][16]. Nevertheless, it has been found that this effect has a negligible impact on HTC and natural charring [60][58].

2.4. Effect of Water as a Catalyst

The literature on biomass reactions in water has been widely reviewed and reported. Water can be a solvent and catalyst during the biowaste HTC process, and it has been observed to accelerate the carbonization process [61][59]. Water is more ionized at higher temperatures, which allows for hydrolysis, ionic condensation, and splitting; therefore, it acts as a reaction medium and a catalyst for the formation of organic molecules [62][60]. Additionally, water’s dielectric constant strongly decreases in HTC conditions, whereas its self-diffusion coefficient increases. As a result, under these circumstances, water behaves as a polar solvent with some organic characteristics. The properties and distribution of the product are influenced by how much water is present in the biomass feedstock [63][61]. It has been observed that the addition of HCl to the HTC water leads to the increase in adsorption efficiency of HC, lowering the temperature and reaction time [64][62]. In comparison with HC produced without acid, HC obtained in the HCl-assisted HTC process presented more oxygenated functional groups, higher aromaticity and hydrophobicity, larger pores, and superior thermal stability. Furthermore, in the case of sugar bagasse transformed into HC for crystal violet dye and tetracycline removal, the maximum adsorption capacities of 207.16 mg/g and 68.25 mg/g, respectively, were recorded by the HCl-assisted HTC process.

2.5. Effect of the Reaction Time

The qualities of the product were discovered to be impacted by the reaction time [47][45]. Low reaction times and temperatures (<200 °C) result in a structure with a higher proportion of furan groups, whereas lengthy residence durations (above 24 h) and high temperatures (>200 °C) result in an arene-rich structure, which is either the product of condensed polynuclear aromatic hydrocarbons structures or three-membered furan units [45][43]. Longer reaction times were observed to significantly increase the yield of HC [39][37], which contrasts with the lower yield obtained at longer reaction times [17][16], [52][50]. Heilmann et al. [53][51] advised to use shorter reaction times after observing that the reaction lengths do not really affect HTC.

3. Utilization of HCs for the Decontamination of Wastewaters

In order to prevent and control the pollution of surface water and groundwater resources, the governments of many developed and developing countries have established various firm rules and regulations to maintain the quality of available water resources. In practice, preventing and controlling water pollution is different from controlling water quality. Water quality standards may differ depending on the intended use of the water; pollutant limits may be prescribed depending on the primary use, such as drinking, cooking, washing, bathing, and agricultural work [81][63]. In some cases, these limits and standards are not properly enforced unless they are closely related to human consumption. Governments regulate the water quality by implementing general rules, which will help to avoid water quality decline and maintain natural states. Due to their unique qualities, some wastes, such as biosolid (sewage sludge), biowaste (for example, compost from municipal waste), or fly ash from burning coal, are suitable for use on land as a nutrient source and organic matter, or as a soil amendment. Human activities can lead to the conversion of soil into a kind of reservoir for the accumulation of all possible chemicals, such as xenobiotics (halogenated organic substances (AOX), di(2-ethylhexyl)phthalate (DEHP), polychlorinated biphenyls (PCB), organo-tins (MBT, DBT, and TBT), organic dyes (methylene blue (MB) [82[64][65][66][67],83,84,85], malachite green (MG) [86[68][69][70],87,88], crystal violet (CV) [75,89][71][72], Congo red (CR) [87[69][73],90], and rhodamine B (RB) [91,92][74][75]), heavy metals [93,94,95,96,97][76][77][78][79][80] and metalloids and polycyclic aromatic hydrocarbons (PAHs), and various pharmaceuticals [33][31], which can leakage from soil to water sources. Due to their high toxicity and lack of biodegradability, these types of pollutants have drawn considerable attention when it comes to polluting water. HC materials made from sustainable resources have a variety of known uses, including an effective and eco-friendly method of removing contaminants from water. They have been shown in numerous studies to be capable of adsorbing both organic [98,99,100][81][82][83] and inorganic molecules [101,102,103][84][85][86].

3.1. HCs for Organic Contaminants Retention from Wastewaters

3.1.1. Dyes

Dyes interfere with bacterial growth and prevent aquatic plants from producing oxygen through photosynthesis; therefore, they are an environmental concern [104][87]. Even at low concentrations, these phenomena can be seen. The primary source of the dyes is industrial effluent. The elimination of organics and the removal of metal ions are two separate processes. Carbon-based adsorbents have been explained by three main mechanisms: (i) hydrogen bonds, (ii) π–π interactions, and (iii) the creation of donor–acceptor complexes [105,106][88][89]. Another possible adsorption mechanism may occur between electrostatic and dispersive interactions [107][90]. The amphoteric characteristics of carbonaceous materials have an immediate impact on the adsorption mechanism [108][91]. These characteristics largely depend on the content of heteroatoms, which determines the electron density, hydrophobicity, and surface charge [109][92]. Reactive dyes are thought to be released into downstream effluents in amounts of 200,000 tons per year [92][75], leading to some cases in which dye concentrations in aqueous effluents can reach values up to 800 mg/L. Treating these dye-containing effluents poses considerable problems in the wastewater industry. From an ecological perspective, it is crucial to remove dyes from water sources [86][68]. The HTC process can be used to transform various biowastes into HC for elimination of dyes from wastewater. Valuable biowastes, such as coconut shells [110][93], coffee husks [111][94], hazelnut shells [112][95], chickpea stems [113][96], shrimp shells [114][97], cigarette butts [29][28], and forestry [115[98][99][100],116,117], have been exploited to produce HCs for dye removal from aqueous waters. Several recent studies that have used the HTC process on biowastes to produce adsorbents for organic pollutants from wastewaters, process conditions, and adsorption performance in terms of adsorption capacity (Qe) and removal efficiency are presented in Table 1.
Table 1.
 HTC conditions and adsorption performance of HC for dyes removal for wastewater treatment.
Biowaste HTC Conditions Adsorption, Pollutant Retention Efficiency Ref.
T (°C) Residence time (h) Activation Dye Qe (mg/g)/Removal (%)
Pine needles (PNs) 225 5 - MG 52.9/92 [116][99]
H2O2 97.1/96
Pine wood 300 4 NaCl MB 86.7/n.a. [93][76]
Bamboo 180 24 - MB 91.74/n.a. [118][101]
Pretreated cotton stalk 220 6 KOH MB 198.0 ± 9.8/n.a. [119][102]
Coffee husk 180 6 KOH MB 357.38/n.a. [111][94]
Sugarcane bagasse 240 10 - MB 116.65/n.a. [82][64]
NaOH 334.74/n.a.
Hazelnut shell 250 7.5 KOH MB 524/n.a. [112][95]
Chickpea stem 200 5 KOH MB 96.15/77.86 [113][96]
Coconut shell 200 2 NaOH MB 200.01/98 [110][93]
Shrimp shell 180 12 Acetic acid Methyl orange (MO) 755.08/n.a. [114][97]
Phycocyanin-extracted algal bloom residues (PE-ABR) 200 10 - MG 89.05/92.4 [111][94]
Avocado seeds 230 3 - Indigo carmine 49 [48][46]
Sewage sludge 180 3 - CV n.a./99 [75][71]
Cigarette butts (CBs) 190 48 and 72 NaOH MB 561.73 and 548.72/n.a. [29][28]
PVC + bamboo 200 24 NaOH MB 234.46/n.a. [120][103]
As observed from Table 1, most studies refer to the removal of MB from wastewater with the help of HCs derived from agro-residual wastes [82,111[64][94][95][96],112,113], woody biomass [94[77][102],119], and composites based on PVC and bamboo [120][103]. In a study by Roldan et al. [121][104], N- and S-doped mesoporous carbon prepared through a single-vessel HTC process were used for water treatment to adsorb MB and RB. They found that, generally, MB had a better adsorption capacity than RB, which may be mostly attributable to size-related factors, because the second contaminant has larger dimensions. Both molecules are cationic; therefore, there is little difference in how they interact with the HCs. The materials perform better before carbonization, explained by the lower degree of graphitization and the higher content of oxygen functional groups, which offers a more hydrophilic surface, improving the electrostatic interactions with MB and RB. By generating various pore sizes in the HCs, the dopant in this instance significantly enhances the adsorption capabilities. For porous S-doped carbon activated with ZnCl2, the adsorption capacities were around 123 mg/g for MB and 106 mg/g for RB. Alatalo et al. [122][105] studied the removal of MB from aqueous media using meso/microporous soft carbonaceous materials prepared by the HTC tempering method in basic media. The HCs were prepared from fructose and activated with a mixture of LiCl/ZnCl2 in a single-vessel HTC at 180 °C. The first HC was prepared with pure fructose (FruLi) with polar oxygenated surface functionalities, and the second HC (FruLi + TCA) was made of fructose and 2-thiophenecarboxyaldehyde (TCA) with thiophene sulfur, the mixture being doped in the final HC structure. These HCs were subjected to MB adsorption tests in the pH range of 3–8, but only a minimal effect was seen. Additionally, Alatalo et al. examined the impact of temperatures between 20 and 60 °C. These studies revealed that the adsorption effectiveness somewhat increased between 20 and 40 °C, possibly as a result of a decrease in solution viscosity, which increased the diffusion rate of adsorbent molecules into the internal pores [98][81]. The maximum adsorption capacities for FruLi and FruLi + TCA were 96 mg/g and 64 mg/g, respectively, at equilibrium conditions (temperature 20 °C, pH 6, contact time 24 h). A very high adsorption capacity for MB (735 mg/g) was reported by Correa et al. [123][106], using activated carbon made from various coals. A carbon–silicate composite produced by the HTC process was utilized by Xiong et al. [124][107] to remove MB, achieving a maximum adsorption capacity of 418 mg/g, which was significantly higher than what was found for HCs that were not activated or modified. HCB48 and HCB72 hydrochars synthesized from 5 g CBs in 37 mL of deionized water at 48 and 72 h, respectively, without inert, were used as adsorbents for MB, as chemically activated (HCB48-ATV, HCB72-ATV) and non-activated forms [29][28]. The yield of HBCs was 26.81% (wt./wt.) for HCB48 and 23.95% (wt./wt.) for HCB72. The maximum MB removed occurred at pH 11.0 for activated HCB, at 20 min equilibration time, if the specific area of HCB activated was low (2.30 and 3.74 m2/g tested by BET). The adsorption mechanisms were explained by the electrostatic interactions produced at high pH, due to the negative surface charge of adsorbents, which attracts the positively charged MB. Among Langmuir, Freundlich and Dubinin–Radushkevich models investigated, it was observed that the Langmuir isotherm better described the adsorption process.
 

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