Abstract

Water contamination by nitrogen and phosphorus ions has a direct consequence of eutrophication to the ecosystem. The objective of this study is to investigate the production of hydrochars by acetic acid or sodium hydroxide assisted hydrothermal carbonization (HTC), various activation methods, and the potential of hydrochar as an adsorbent to remove NH4+-N and PO43−-P from wastewater. The results showed that acetic acid or sodium hydroxide assisted HTC and activation with magnesium chloride or air could improve the surface properties of hydrochar. Acetic acid modification generated extensive oxygenated functional groups, while sodium hydroxide modification produced hydrochar with a high N/C ratio and surface aromaticity. Treating hydrochar with magnesium chloride could impregnate nano-magnesium particles on the hydrochar, thereby improving the ability to remove N and P. Air activation of hydrochar resulted in more oxygen containing functional groups. The NH4+-N and PO43−-P adsorption capacities of these hydrochars ranged from 92.6 to 122.4mg/g and 1.6 to 15.8mg/g, respectively. The adsorption capacity of hydrochars in swine wastewater is similar to the results of artificial wastewater. The results suggested that Mg-nanoparticle dispersion and oxygen-containing functional groups played a major role in adsorption than ion exchange and physisorption.

HIGHLIGHTS

  • Hydrochars were produced by acetic acid and sodium hydroxide assisted HTL.

  • Magnesium chloride or air activation could improve the surface properties of hydrochar.

  • Hydrochar activated by using MgCl2 and air substantially increased the adsorption capacity.

  • Mg-nanoparticle dispersion and oxygen containing functional groups played a major role in adsorption.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

With the rapid development of modern society, a large number of human activities, such as excessive fertilization and dumping of industry and sewage, have caused nutrients to pollute water and affect rivers, lakes and ocean coasts (Goel 2006). This type of pollution causes direct environmental and human health problems, as well as impacts on the economy. Excessive nitrogen (N) and phosphorus (P) in wastewater discharge can lead to eutrophication of natural water, which results in the growth of algae in the water, lack of oxygen and the death of animal life (Khan & Mohammad 2014). Therefore, a wastewater treatment process is required to remove N and P from the effluent.

Nitrogen and phosphorus removal technologies are mainly divided into chemical, physical and biological methods. Chemical treatment may produce pollutants (such as sludge) due to the precipitation of phosphorus (Yamashita & Yamamoto-Ikemoto 2014). In terms of biological treatment, it is usually costly and slow because of the need for aeration, microbial growth and other pre-treatments (Bekoe et al. 2018). In the past few decades, physical methods such as reverse osmosis, electrodialysis and adsorption have attracted widespread attention. Due to low cost and high process efficiency, adsorption is widely used as a promising water purification method (Bolisetty et al. 2019). Research on effective and economical adsorbents to remove N and P in wastewater has considerable economic and environmental value (Takaya et al. 2016).

Biochar produced from agricultural wastes provides a viable adsorbent for removing phosphorus and nitrogen from wastewater. In addition, the nutrient-rich biochar after use can be developed into a high-value bio-fertilizer. However, the literature shows that the adsorption results of nitrogen and phosphorus on biochar from different sources and production processes are inconsistent. Kizito et al. (2015) showed that the biochar from wood could remove 73% of NH4+ from an ammonium solution, compared to 60% for the biochar from rice husk. The biochars prepared from wood and rice husk could remove 60 and 53% of NH4+ in the anaerobic digestate (Maurer et al. 2017). Takaya et al. (2016) suggested that the surface area might not be the most important factor that affected the physisorption of ammonia. Jung et al. (2015) showed that the surface area of biochar increased with increasing treatment temperature from 200 to 400 °C, and its P adsorption capacity increased accordingly. Although the specific adsorption mechanism is still unclear, the presence of functional groups and metals may play a key role in the adsorption of N and P (Takaya et al. 2016).

In the past few years, hydrothermal carbonization (HTC) has gained a considerable attention for producing hydrochar (a typical biochar) from carbonaceous materials with a high moisture content (such as municipal sludge, manure, algae, and kitchen waste) (Zhang et al. 2020). HTC processes the feedstock under a self-generated pressure at a mild temperature (e.g., 250 °C) to produce solid hydrochar and aqueous solution containing organic and inorganic compounds. Also, HTC can destroy pathogens, eliminate odors, and remove heavy metals from raw materials. The product of hydrochar is a more hydrophobic solid than that produced by dry torrefaction, which in turn has positive correlation with carboxylic groups on the biochar surface, specific surface area, and pore volume (Xiu et al. 2017; Mao et al. 2019). It has been reported that various factors such as the cation exchange capacity, oxygen containing/acidic functional groups, available anion exchange sites and surface charges, and initial concentration of pollutants affect the adsorption of N and P over hydrochar (Kameyama et al. 2012; Chintala et al. 2014).

In this study, cattail (Typha species) is used as the feedstock, which is a kind of aquatic plant commonly grown on wetlands. It has high insect resistance, high adaptability and high reproduction ability (Zhang et al. 2012). The annual productivity of cattails is considerably high. It was reported that standing crop yields up to 22.4 Mg ha−1 of leaves and 30.9 Mg ha−1 of rhizomes (Dubbe et al. 1988). Improper management of cattails may cause corruption in water, severe secondary pollution, rapid marshes of lakes, and reduced invertebrate production and migratory birds (Kostecke 2002). Studies have shown that harvesting half of the cattails grown in wetlands every year (including rhizomes) will not affect its growth and progress next year (Bansal et al. 2019). Because of these advantages, cattails are considered as a potential renewable feedstock for bioenergy production. Glacial acetic acid (CH3COOH) or sodium hydroxide (NaOH) were used to assist the HTC process of cattails. Both acetic acid and sodium hydroxide have been shown to accelerate the degradation/hydrolysis of the dairy manure and improve the biochar's quality (Chen et al. 2018). The obtained hydrochar was further activated by using MgCl2 or air. To the best of our knowledge, this is the first study on the influence of chemical-assisted HTC of the anaerobic digestate and subsequent MgCl2 or air activation on the N and P adsorption capacity of hydrochar. The objective of this study is to (1) compare the N and P adsorption capacity of hydrochar produced under different HTC temperatures, chemical modification and MgCl2 or air activation; (2) study the N and P adsorption capacity of various hydrochars prepared from anaerobic digested cattail in artificial wastewater and swine wastewater. The research results provide a theoretical and practical basis for the development of hydrochar adsorbents for N and P adsorption from wastewater.

MATERIALS AND METHODS

Materials

Cattail (Typha latifolia), the anaerobic digestion (AD) process, and the composition of cattail digestate have been described elsewhere (Zhang et al. 2020). Cattail was anaerobically digested for 14 days, collected after AD, washed with deionized water to remove loose contents, dried at 105 °C for 24 h in a Thermo Scientific Precision Compact Oven (Waltham, MA, USA). About 2 kg (dry basis) of anaerobically digested cattails were prepared and stored at room temperature for future use.

Artificial wastewater was prepared according to the procedure reported in the literature (Fang et al. 2014). For P-containing wastewater, potassium phosphate monobasic was used for the preparation of the phosphate solution at 125 mg/L. Ammonium hydroxide was used to prepare the N-containing wastewater at a concentration of at 1,000 mg/L. These two solutions were diluted to various concentrations for the measurement of adsorption isotherms.

Swine wastewater was collected from the uncovered lagoon at the swine research facility of the University, which represents a low-strength wastewater. Pretreatment of swine wastewater was carried out by sedimentation and filtration with a Whatman quantitative filter paper with 8 μm pore size to remove large, non-soluble particulate solids.

Hydrothermal carbonization

Approximately 10 g of dried feedstock was mixed with 160 mL of deionized water into a 300 ml stirred Parr reactor (Parr Instrument, Moline, IL, USA). For chemical-assisted HTC, glacial acetic acid (CH3COOH) or sodium hydroxide (NaOH) was added with 160 mL water, giving a concentration of 0.6 M and 2 M, respectively. The choice of the concentrations is based on the results reported in the literature (Reza et al. 2015; Wang et al. 2017).

The temperature of the reactor was increased at a heating rate of about 10 °C/min and held at the desired temperature for 1 h. HTC was performed at three different temperatures of 200, 250 and 300 °C and the stirring speed was set at 100 rpm. The hydrochar produced via water-only, acetic acid-assisted, and sodium hydroxide-assisted HTC at various temperatures was denoted as B200/B250/B300, AB200/AB250/AB300, and NB200/NB250/NB300, respectively.

After HTC treatment, the reactor was cooled down to the room temperature. The hydrochar was filtered out using Whatman grade 1 filter paper and washed with deionized water and ethanol to remove residual bio-oil. The hydrochar was then dried in an oven at 105 °C for 24 h and ground into powder for further analysis. The yield of hydrochar was calculated as:
formula
(1)

Activation of hydrochar

Molten salt activation of hydrochar was prepared with MgCl2, which is one of the common molten salts for activation. For a typical process, the hydrochar (B250) was mixed with MgCl2 at a mass ratio of 1:1, and then the mixture was homogenized with a mortar and pestle. The powdered mixture was transferred to a ceramic crucible and was placed in an electric furnace equipped with a continuous nitrogen flow. After flushing with nitrogen for 30 min, the furnace was ramped at 10 °C/min to the carbonization temperature of 600, 700 or 800 °C and kept at this temperature for 1 h. The furnace was cooled to ambient temperature by switching off the power; meanwhile, the nitrogen flow was maintained until the temperature reached room temperature. The as-obtained product was washed with sufficient amount of water to remove the salts. The activated hydrochar was dried in an oven at 105 °C for 12 h. Hydrochars carbonized at 600, 700 and 800 °C were denoted as MB600, MB700, and MB800, respectively. The yields of hydrochars defined by Equation (1) are reported in the Results and Discussion section.

Air activation of the hydrochar (B250) was conducted at 250 °C. For a typical run, 5 g of hydrochar was heated from the room temperature to activation temperature of 250 °C in an air stream. The sample was held at 250 °C for 2 hr. Then, under the same flow of air, the system was allowed to cool down to the room temperature. The hydrochar was designated as air activated hydrochar.

Characterization

Physical nitrogen adsorption, pore volume, and pore size were measured using a Micromeritics ASAP 2020 surface area and porosity analyzer (Norcross, GA, USA).

Thermogravimetric analyses (TGA) were performed using a TA Instruments SDT Q600 thermogravimetric analyzer (New Castle, DE, USA), in which samples were heated to 250 °C at a heating rate of 20 °C/min in nitrogen.

The morphology of hydrochars was evaluated using a JEOL 7500F scanning electron microscope (SEM, Tokyo, Japan). Energy-dispersive X-ray spectroscopy (EDX) analysis of hydrochar was performed on a JEOL 7500F SEM coupled with an energy dispersive X-ray detector. For each sample (total 13 different hydrochars), the measurement was carried out on 5–10 different particles.

The absorption spectroscopy in the infrared region was measured to determine the frequency of vibrations of the functional groups present in the hydrochar. The spectrum readings were obtained using an Agilent 670 Fourier transform infrared (FTIR) spectrometer with attenuated total reflection (ATR) varied in the range of 4,000–400 cm−1 (Santa Clara, CA, USA).

Ammonium and phosphate adsorption

To evaluate the adsorption capacity of hydrochar to ammonium, 0.1 g of hydrochar was mixed with 100 mL ammonium hydroxide solution in a beaker. The mixture was then shaken on a shaker at 160 rpm and 25 °C for 24 h. After that, the mixture was filtered through a 0.45 μm membrane, and then the ammonium concentration was determined using the TNT 830 ammonia kit with a Hach 6000 UV/VIS spectrophotometer (Loveland, CO, USA).

The same procedure was used to measure the adsorption capacity of hydrochar to phosphate using 125 mg/L phosphate solution. Phosphate concentration of the filtrate was determined using the TNT 845 kit with the Hach spectrophotometer. The adsorption experiments were performed in triplicate, and the average values were reported.

The adsorption capacity Q (mg g−1) of the hydrochar was calculated as:
formula
(2)
where Ce and C0 are the equilibrium and initial NH4+-N or PO43−-P concentrations (mg/L), respectively. V is the volume of the solution and m is the weight of hydrochar (g).

For the adsorption test in swine wastewater, 0.1 g of hydrochar was mixed with 100 mL diluted swine wastewater in a beaker. The mixture was then shaken on a shaker at 160 rpm and 25 °C for 24 h. After that, the mixture was filtered through a 0.45-μm membrane and analyzed for the chemical oxygen demand (COD), NH4+-N, and total phosphorus (TP).

Adsorption isotherms and mathematical models

To measure the adsorption isotherm of ammonium, 0.1 g of the hydrochar was mixed with 50 mL ammonium hydroxide solution with different initial concentrations of 25, 50, 100, 200, 300, 500, 750, and 1,000 mg/L. All samples were shaken on the mechanical shaker at 160 rpm and 25 °C for 24 h. The samples were filtered through a 0.45-μm membrane, and then the ammonium concentration was determined immediately with the Hach spectrophotometer. Based on the difference between the initial and final solution concentrations, ammonium adsorbed on the hydrochar was calculated. To measure the adsorption isotherm of phosphorus, 0.1 g of the hydrochar was mixed with 50 mL phosphate solution with initial concentrations of 20, 50, 80, 100, 150, 200, 250, 300, and 350 mg/L. The remaining procedure was the same as that of ammonium. All isotherm experiments were performed in triplicate, and the average values reported.

The Langmuir and Freundlich models were used for the regression of the adsorption isotherm. The equations of these two models are written as:
formula
(3)
formula
(4)
where KL and Kf are the Langmuir bonding term related to interaction energies (L/mg) and the Freundlich affinity coefficient (mg(1−n)Ln/g), respectively. Smax is the Langmuir maximum capacity (mg/kg), Ce is the equilibrium solution concentration (mg/L) of the sorbate, and n is the Freundlich linearity constant.

RESULTS AND DISCUSSION

Hydrochar production and activation

HTC of anaerobically digested cattails was conducted in deionized water, acetic acid solution and sodium hydroxide solution at 200, 250 and 300 °C for 1 h. The hydrochar yields are summarized in Figure 1. The reaction temperature showed an obvious effect on the hydrochar yield. When the temperature increased from 200 to 300 °C, the hydrochar yields decreased from 40.2% to 31.6%, 37.5% to 28.3%, and 35.7% to 22.7% for the use of water, acetic acid, and sodium hydroxide, respectively.

Figure 1

Solid yields of hydrochar under different conditions.

Figure 1

Solid yields of hydrochar under different conditions.

On average, the increase of the temperature from 200 °C to 300 °C reduced the solid yield by 7–9% of the original mass. The loss of biomass during HTC at a low temperature was mainly attributed to the decomposition of the hemicellulose component. As a result, the hydrochar product treated at 200 °C still showed a color similar to the original feedstock. When the temperature increased, cellulose degradation starts to occur at a temperature above 200 °C due to disintegration through hydrolytic action and further reactions, decreasing the hydrochar yield (Volpe et al. 2018). Because of the p-value of one-way ANOVA analysis (Table S1), the application of acetic acid and sodium hydroxide significantly decreased the hydrochar yield at 250–300 °C (i.e., p-value < 0.05) compared with water-only HTC, which may be caused by the higher hydrolytic effect caused by chemicals added. It is well known that acetic acid could enhance the degradation of hemicellulose and cellulose, while sodium hydroxide could dissolve lignin and partial hemicellulose (Zhang & Wang 2013).

Because raw hydrochars and acetic acid/sodium hydroxide modified hydrochars showed similar properties and adsorption capacity, B250 hydrochar was chosen for further activation. MgCl2 activation of B250 hydrochar was conducted at 600, 700 and 800 °C, and the yield of activated hydrochar was approximately 90% of the original hydrochar. Air activation process was simulated by using TGA. Figure 2 shows the TGA-DSC profiles of B250 hydrochar activated in air. When the temperature increased from the ambient temperature to 250 °C, the weight loss can be divided into two phases (Figure 2(a)). The first phase was between the ambient temperature and 150 °C, which was due to the loss of water present in various forms and represented about 3.5 wt% of the total hydrochar weight. At this stage, the process was endothermic. The second phase happened between 200 and 250 °C, representing a weight loss of approximately 0.5 wt%. The weight loss in the second phase might be due to volatilization of organic matters or oxidation. If the hydrochar was held at 250 °C for 2 h, it would cause an additional 20% mass loss. The process became exothermic (Figure 2(b)).

Figure 2

TGA-DSC graphs of B250 hydrochar, which was heated from 20 °C to 250 °C at 20 °C/min and held at 250 °C for 2 h. (a) Weight/heat flow vs. temperature, (b) weight/heat flow vs. time.

Figure 2

TGA-DSC graphs of B250 hydrochar, which was heated from 20 °C to 250 °C at 20 °C/min and held at 250 °C for 2 h. (a) Weight/heat flow vs. temperature, (b) weight/heat flow vs. time.

Characterization of hydrochar

Surface area

Table 1 gives the surface area of HTC hydrochars and activated hydrochars. The BET surface area of HTC hydrochars increased with the HTC temperature. The HTC hydrochar produced with deionized water at 200, 250 and 300 °C had a relatively small surface area of 4.40, 4.95 and 8.96 m2/g, respectively. These values matched the results reported in the literature (Jaruwat et al. 2018). Hydrochars produced via acetic acid and sodium hydroxide-assisted HTC showed a similar trend. According to the p-value of one-way ANOVA analysis (Table S2), the surface area of hydrochars obtained at 300 °C was significantly higher than that of hydrochars obtained at 200–250 °C. When the temperature increased from 200 to 300 °C, the BET surface areas of the hydrochars increased from 2.52 to 6.86 m2/g and 4.71 to 7.42 m2/g for acetic acid and sodium hydroxide treated samples, respectively. This trend might be caused by the gradual degradation of organic matter in the feedstock and the formation of new vascular bundle and channel structure during the HTC process. But the difference of the surface area of hydrochars obtained via water-only HTL, acid-assisted HTL and base-assisted HTL was not significant.

Table 1

Surface area of different hydrochar samples

Sample IDSpecific surface area (m2/g)at-plot micropore area (m2/g)Pore volume (cm3/g)
B200 4.40 – 0.0091 
B250 4.95 – 0.017 
B300 8.96 1.11 0.0139 
AB200 2.52 – 0.0087 
AB250 2.85 0.69 0.0099 
AB300 6.86 1.85 0.0137 
NB200 4.71 – 0.0101 
NB250 5.62 – 0.01 
NB300 7.42 1.29 0.0161 
MB600 84.85 46.68 0.125 
MB700 131.53 94.80 0.125 
MB800 151.02 108.57 0.161 
Air activated hydrochar 337.75 286.96 0.249 
Sample IDSpecific surface area (m2/g)at-plot micropore area (m2/g)Pore volume (cm3/g)
B200 4.40 – 0.0091 
B250 4.95 – 0.017 
B300 8.96 1.11 0.0139 
AB200 2.52 – 0.0087 
AB250 2.85 0.69 0.0099 
AB300 6.86 1.85 0.0137 
NB200 4.71 – 0.0101 
NB250 5.62 – 0.01 
NB300 7.42 1.29 0.0161 
MB600 84.85 46.68 0.125 
MB700 131.53 94.80 0.125 
MB800 151.02 108.57 0.161 
Air activated hydrochar 337.75 286.96 0.249 

aThe average values of three samples were reported, and the standard deviation was between 0.3 and 3.

Compared with HTC hydrochars, hydrochars activated with MgCl2 showed higher BET surface areas, which were 84.85 ± 1.22, 131.53 ± 2.47, 151.02 ± 2.86 m2/g for 600, 700, and 800 °C treated samples, respectively. Nano-sized MgO dispersion on the hydrochar surface may increase the specific surface area of Mg-loaded hydrochars, which is further illustrated by the SEM images and the magnesium content obtained by the elemental analysis. The hydrochar activated in air at 250 °C showed the highest BET surface area (337.75 m2/g) among all modified hydrochars. Activation in the presence of air leads to the development of porosity, mostly due to the generation of micropores and mesopores (Sajjadi et al. 2019).

Scanning electron microscopy and EDX analysis

SEM images of hydrochars are shown in the supplementary material (Figure S1). SEM analysis of hydrochars allows the visualization of the surface characteristics of the materials. In hydrochars produced via HTC at 200–300 °C, the appearance of a rough surface and a rupture into scales were observed, which may be due to the intrinsic nature of the hydrochar matrix. Chemical modification with acetic acid and sodium hydroxide resulted in considerable cracks and fine particles attached to the surface, forming a system of micro-pore networks. In addition, the sodium hydroxide-modified hydrochar showed sodium hydroxide crystals on the surface, and the length of the crystals was between 1–2 μm.

The morphology of magnesium chloride-treated hydrochars is largely dependent on the treatment temperature. When treating at 600 and 700 °C, a large number of MgCl2 crystal structures emerged and covered the surface. When a treatment temperature of 800 °C was used, molten salt activation happened, and the molten MgCl2 corroded the carbonaceous material to form more cracks and voids, resulting in a higher surface area. These finding are backed up by similar results reported by Fang et al. (2014) in magnesium-modified corn hydrochar. Air-activated hydrochar showed more cracks and carbon particles on the surface compared with B250 hydrochar.

Table 2 gives the elemental compositions of the raw and modified hydrochars determined by EDX. The dominant elements were carbon and oxygen, which were from 59.98% to 82.49% and 11.35% to 32.28%, respectively. Generally, the hydrochar obtained at 200 °C had the lowest carbon content, indicating incomplete volatilization. Hydrochars produced at 250 and 300 °C showed a similar quality. Because a 250 °C HTC could give a higher hydrochar yield, 250 °C was considered as the optimal HTC temperature in this study. Therefore, hydrochar obtained via the 250 °C HTC was further used for activation.

Table 2

Elemental composition results of hydrochars

B200B250B300AB200AB250AB300NB200NB250NB300MB600MB700MB800Air activated
66.37 73.64 73.59 73.68 74.92 75.66 59.98 63.48 63.19 77.46 82.34 82.49 67.61 
31.62 25.32 25.11 24.70 24.76 24.27 31.24 27.67 27.03 18.25 13.36 11.35 32.28 
0.93 0.44 0.31 0.47 0.12 – 0.47 1.06 0.72 0.83 0.66 0.62 – 
0.16 0.07 0.16 0.22 0.12 0.12 0.097 0.12 0.15 – – – 0.04 
Ca 1.41 0.95 0.66 0.82 0.48 – 1.31 3.25 2.08 1.37 1.27 1.32 – 
Na – – – – –  6.66 3.82 6.36 – – – – 
0.19 0.29 – – – – –   – – – 0.07 
Mg 0.15 0.22 0.19 0.11 0.15  0.11 0.21 0.39 3.64 3.65 4.02 – 
Cl – – – – – – – – – 2.95 3.51 0.16 – 
B200B250B300AB200AB250AB300NB200NB250NB300MB600MB700MB800Air activated
66.37 73.64 73.59 73.68 74.92 75.66 59.98 63.48 63.19 77.46 82.34 82.49 67.61 
31.62 25.32 25.11 24.70 24.76 24.27 31.24 27.67 27.03 18.25 13.36 11.35 32.28 
0.93 0.44 0.31 0.47 0.12 – 0.47 1.06 0.72 0.83 0.66 0.62 – 
0.16 0.07 0.16 0.22 0.12 0.12 0.097 0.12 0.15 – – – 0.04 
Ca 1.41 0.95 0.66 0.82 0.48 – 1.31 3.25 2.08 1.37 1.27 1.32 – 
Na – – – – –  6.66 3.82 6.36 – – – – 
0.19 0.29 – – – – –   – – – 0.07 
Mg 0.15 0.22 0.19 0.11 0.15  0.11 0.21 0.39 3.64 3.65 4.02 – 
Cl – – – – – – – – – 2.95 3.51 0.16 – 

The hydrochars also had trace amounts of phosphorus, magnesium, potassium, calcium, and sulfur. Sodium hydroxide-assisted HTC yielded hydrochar with a higher amount of sodium, which is the residue of sodium hydroxide. The presence of magnesium contents on magnesium modified hydrochar is related to the amount of MgCl2 used and the carbonization temperature. The loading of magnesium was between 3.64 and 4.02%. Carbonization at 800 °C considerably reduced the chlorine content of the material, while at 600 °C or 700 °C, magnesium still existed in the form of MgCl2. These results were further corroborated by the dispersion of Mg on the surface and enhanced surface area. The oxygen content of air activated hydrochar increased at the expense of the carbon content, which is attributed to the oxidation of the material.

FTIR analysis

The analysis of functional groups on the surface of hydrochars could help to determine the mechanisms responsible for the adsorption of nutrients in the structure of these materials. The infrared spectra of hydrochars are shown in Figure 3.

Figure 3

Infrared spectra of hydrochars.

Figure 3

Infrared spectra of hydrochars.

The FTIR analysis demonstrated that the character peaks of modified hydrochars differed from each other. The peak at approximately 3,445 cm−1 corresponded to the O–H stretching vibration, which changed noticeably for MgCl2 and acetic acid modified samples. The weakened peak of the O–H vibration indicates the structural change of the feedstock under these chemical treatments. Peaks obtained between 2,820 and 2,990 cm−1 suggested the stretching vibrations of aliphatic C-H. This peak indicated the presence of aliphatic structure in hydrochar samples. The band at 1,737 cm−1 indicated the C = O stretching vibrations in all modified hydrochar, suggesting hemicellulose degradation during HTC above 200 °C. The peak at 1,698 cm−1 corresponding to the carboxyl, ester or carbonyl groups was seen in all hydrochars with little difference, indicating the presence of oxygen-containing functional groups on the hydrochar surface. However, this peak was not clearly noticeable in magnesium-modified hydrochar at 800 °C, which may be due to increased oxygen loss caused by thermal degradation and decarboxylation (Reza et al. 2015). This loss of oxygen in magnesium-modified hydrochar at higher temperature is supported by elemental analysis as well. The adsorption peak at 1,508 and 1,460 cm−1 indicated the presence of aromatic lignin groups in the sample. However, the flattened peak at higher temperature magnesium-modified hydrochar suggested the partial degradation of lignin.

Overall, magnesium modification did not affect the structural formation of most organic functional groups in Mg/hydrochar. The character peaks at 3,200 cm−1 –3,700 cm−1, 1,400 cm−1 –1,690 cm−1, and 550 cm−1 –850 cm−1 represented the hydroxyl (O–H), double bonds (C = O and C = C), and organic chloride (C–Cl), respectively. Air activation reduced certain structures of the hydrochar. But air activated hydrochar was still rich in functional groups of O–H, C = C, carbonyl group, C-O, and so on.

NH4+-N and PO43−-P adsorption

Factors including initial NH4+ concentration, pH, and contact time with solution could determine the adsorption capacity of hydrochar. In this study, the pH of the solution was adjusted to neutral as it has been shown to favor NH4+-N adsorption (Banerjee et al. 2014). As shown in Table 3, hydrochar produced by HTC resulted in around 10% of NH4+-N removal from the artificial wastewater. Hydrochar produced by water-only and acetic acid-assisted HTC resulted in a PO43−-P removal of 2–3%. Sodium hydroxide modification of the hydrochar at 250 and 300 °C increased the PO43−-P adsorption to ∼4.5%. Increased surface area and addition of functional groups may contribute to the increased adsorption capacity.

Table 3

Adsorption capacity of hydrochars

Qe (mg g−1)% NH4+-N adsorbedQe (mg g−1)%PO43−-P adsorbed
Raw hydrochars     
B200 108.7 ± 3.2 10.87 3.1 ± 0.2 2.5 
B250 100.7 ± 0.7 10.07 3.3 ± 0.7 2.8 
B300 97.3 ± 1.6 9.73 2.6 ± 0.36 2.1 
Acetic acid hydrochars     
AB200 112.4 ± 2.6 11.2 3.2 ± 0.9 2.5 
AB250 106.2 ± 2.9 10.6 3.7 ± 1.2 2.9 
Sodium hydroxide hydrochars     
NB200 95. 5 ± 2.5 9.5 1.6 ± 2.6 1.3 
NB250 92.6 ± 3.2 9.2 5.6 ± 3.8 4.5 
NB300 87.4 ± 1.8 8.7 5.7 ± 8.4 4.6 
Magnesium-modified hydrochars     
MB600 122.4 ± 2.2 12.2 11.7 ± 0.7 9.4 
MB700 118.6 ± 2.7 11.8 10.5 ± 1.3 8.4 
MB800 115.8 ± 6.1 11.5 11.9 ± 3.9 9.6 
Air-activated hydrochar 112.4 ± 3.4 11.2 15.8 ± 2.3 12.6 
Qe (mg g−1)% NH4+-N adsorbedQe (mg g−1)%PO43−-P adsorbed
Raw hydrochars     
B200 108.7 ± 3.2 10.87 3.1 ± 0.2 2.5 
B250 100.7 ± 0.7 10.07 3.3 ± 0.7 2.8 
B300 97.3 ± 1.6 9.73 2.6 ± 0.36 2.1 
Acetic acid hydrochars     
AB200 112.4 ± 2.6 11.2 3.2 ± 0.9 2.5 
AB250 106.2 ± 2.9 10.6 3.7 ± 1.2 2.9 
Sodium hydroxide hydrochars     
NB200 95. 5 ± 2.5 9.5 1.6 ± 2.6 1.3 
NB250 92.6 ± 3.2 9.2 5.6 ± 3.8 4.5 
NB300 87.4 ± 1.8 8.7 5.7 ± 8.4 4.6 
Magnesium-modified hydrochars     
MB600 122.4 ± 2.2 12.2 11.7 ± 0.7 9.4 
MB700 118.6 ± 2.7 11.8 10.5 ± 1.3 8.4 
MB800 115.8 ± 6.1 11.5 11.9 ± 3.9 9.6 
Air-activated hydrochar 112.4 ± 3.4 11.2 15.8 ± 2.3 12.6 

Magnesium chloride modification of the hydrochar (B250) increased the adsorption capacity of both NH4+-N and PO43−-P to ∼12% and ∼9.5%, respectively. Compared with the B250 hydrochar, the adsorption capacity of MgCl2-activated hydrochar increased by 20 and 240%, respectively. The NH4+-N adsorption capacity of hydrochar prepared from AD digestate is similar or higher than the results reported in the literature, while the PO43−-P adsorption capacity is lower than the results reported in the literature. Takaya et al. (2016) reported that NH4+-N and PO43−-P adsorption capacities of hydrochar ranged from about 105.8–146.4 mg g−1 and 0 to 30 mg g−1, respectively. Another study showed that the cassava straw and banana straw biochars exhibited the theoretical maximum saturated adsorption capacities of 24.04 mg·g−1 (NH4+-N) and 31.15 mg·g−1 (TP), respectively (Jiang et al. 2019). They suggested that the magnesium loading had strong correlation with NH4+-N and PO43−-P adsorption. In this study, surface area and pore volume showed moderate and weak correlation for NH4+-N and PO43−-P adsorption, respectively. The improved NH4+-N adsorption capacity may be due to the presence of increased negatively-charged functional groups like carboxyl group. Increased PO43−-P adsorption capacity could be attributed to surface complexation with hydroxyl functional group and Mg-phosphate precipitation reaction (Jiang et al. 2019).

Air-activated hydrochar had the most influential impact on P adsorption as it resulted in 15.8 mg/g of P adsorption and 12.6% removal from wastewater. Meanwhile, it also showed a slightly higher NH4+-N adsorption capacity. High surface area of air activated hydrochar (337.75 m2/g) may have allowed high adsorption of both NH4+-N and PO43−-P. Additionally, the increase in negative surface charge due to the presence of acidic functional groups such as the carbonyl group in air-activated hydrochar may have favored the N sorption capacity.

Adsorption capacity in swine wastewater

Swine wastewater was used to test the adsorption capacity of hydrochars, which contained 1,470 mg/L COD, 50 mg/L NH4+-N and 151 mg/L TP. The phosphorus adsorption capacity of hydrochars in swine wastewater is similar to the results of artificial wastewater, which is within a range of 1.5–5.8 mg/g (Table 4). Because the swine wastewater collected from the open lagoon contained a small amount of NH4+-N, almost 100% of NH4+-N was removed by hydrochars. However, with the exception of air-activated hydrochar, most hydrochars had leaching problems. Therefore, before using hydrochar or activated carbon as an adsorbent to treat wastewater, pretreatment is required to reduce leaching.

Table 4

Adsorption capacity of hydrochars in swine watstewater

COD (mg g−1)NH4+-N adsorbed (mg g−1)TP (mg g−1)
Raw hydrochars    
B250 N/A 23.7 ± 0.7 1.8 ± 0.5 
AB250 N/A 24.5 ± 0.5 2.1 ± 0.3 
NB250 N/A 24.6 ± 1.2 2.5 ± 0.7 
Magnesium-modified hydrochars    
MB600 N/A 24.2 ± 0.4 2.3 ± 0.9 
MB700 N/A 24.8 ± 0.2 2.4 ± 1.1 
MB800 N/A 24.7 ± 0.6 2.2 ± 1.9 
Air-activated hydrochar 135.1 ± 5.5 25.0 ± 0.4 5.8 ± 1.2 
COD (mg g−1)NH4+-N adsorbed (mg g−1)TP (mg g−1)
Raw hydrochars    
B250 N/A 23.7 ± 0.7 1.8 ± 0.5 
AB250 N/A 24.5 ± 0.5 2.1 ± 0.3 
NB250 N/A 24.6 ± 1.2 2.5 ± 0.7 
Magnesium-modified hydrochars    
MB600 N/A 24.2 ± 0.4 2.3 ± 0.9 
MB700 N/A 24.8 ± 0.2 2.4 ± 1.1 
MB800 N/A 24.7 ± 0.6 2.2 ± 1.9 
Air-activated hydrochar 135.1 ± 5.5 25.0 ± 0.4 5.8 ± 1.2 

Ammonia and phosphate adsorption isotherms

Based on the initial evaluation, the following five hydrochars were selected for the adsorption isotherm study, including B250, AB250, NB250, MB800, and air-activated hydrochar. The results were fitted into Equations (2) and (3).

Figures 4 and 5 show the adsorption capacities of hydrochars on NH4+-N and PO43−-P, respectively. For NH4+-N adsorption, the increase in initial concentration up to 300 mg/L showed a sharp increase in the adsorption capacity by hydrochars and stabilized at higher initial concentration. The PO43− adsorption showed a similar pattern, in which there was a rapid increase of adsorption capacity with the increase of initial concentration up to 150 mg/L and stabilization at higher initial concentration. Adsorption equilibrium concentration was reached after 24 h at room temperature for both NH4+-N and PO43−-P. Adsorption equilibrium may have resulted in such phenomena. Compared with PO43−-P, overall higher adsorption of NH4+-N might be due to the intrinsic negative charge of hydrochar that is not conducive for P adsorption.

Figure 4

Adsorption isotherm of NH4+-N for 5 different hydrochar (a) B250, (b) AB250, (c) NB250, (d) MB800, and (e) air activated hydrochar.

Figure 4

Adsorption isotherm of NH4+-N for 5 different hydrochar (a) B250, (b) AB250, (c) NB250, (d) MB800, and (e) air activated hydrochar.

Figure 5

Adsorption isotherm of PO43− for five hydrochars. (a) B250, (b) AB250, (c) NB250, (d) MB800, and (e) air-activated hydrochar.

Figure 5

Adsorption isotherm of PO43− for five hydrochars. (a) B250, (b) AB250, (c) NB250, (d) MB800, and (e) air-activated hydrochar.

The isotherm parameters for both models with the linear regression coefficient (R2) are presented in Table 5. The R2 value suggested that the Freundlich model fits the NH4+-N adsorption data better than the Langmuir model for most types of hydrochar. Typically, the Freundlich model indicates heterogenous and multilayer adsorption. Therefore, the results suggest that adsorption could be described by heterogenous and multilayer adsorption. This in turn implies that N adsorption by hydrochar is a sum of all the adsorption sites, where stronger binding sites are occupied first. The maximum adsorption capacity of NH4+-N by B250, AB250, NB250, MB800, and air-activated hydrochar were 147.08, 100, 98.04, 98.04, and 90.90 mg/g, respectively. The results showed that although the adsorption capacity increased at the initial NH4+-N concentration, the saturation of adsorption sites during the isotherm adsorption process led to a decrease in the adsorption capacity at a higher initial concentration. The maximum NH4+-N adsorption capacity of hydrochars has the following order: B250 > AB250 > NB250 = MB800> air-activated hydrochar.

Table 5

Adsorption thermodynamics constants of NH4+-N and PO43−-P of hydrochars

SampleNH4+-N (Langmuir)
NH4+-N (Freundlich)
PO43−-P (Langmuir)
PO43−-P (Freundlich)
Smax (mg/g)KL (L/mg)R2KF (mg/g)nR2Smax (mg/g)KL (L/mg)R2KF (mg/g)nR2
B250 147.08 0.0017 0.94 0.6096 1.29 0.95 3.16 0.07 0.87 1.54 3.29 0.91 
AB250 100 0.0045 0.93 1.2584 1.47 0.94 7.35 0.0074 0.89 0.19 1.79 0.92 
NB250 98.04 0.0051 0.93 1.4083 1.49 0.94 7.077 0.0112 0.97 0.20 1.61 0.98 
MB800 98.04 0.0076 0.92 2.2956 1.63 0.94 11.148 0.016 0.97 0.41 1.67 0.98 
Air-activated hydrochar 90.90 0.91 0.98 3.3201 1.81 0.95 15.87 0.0145 0.97 0.49 1.58 0.98 
SampleNH4+-N (Langmuir)
NH4+-N (Freundlich)
PO43−-P (Langmuir)
PO43−-P (Freundlich)
Smax (mg/g)KL (L/mg)R2KF (mg/g)nR2Smax (mg/g)KL (L/mg)R2KF (mg/g)nR2
B250 147.08 0.0017 0.94 0.6096 1.29 0.95 3.16 0.07 0.87 1.54 3.29 0.91 
AB250 100 0.0045 0.93 1.2584 1.47 0.94 7.35 0.0074 0.89 0.19 1.79 0.92 
NB250 98.04 0.0051 0.93 1.4083 1.49 0.94 7.077 0.0112 0.97 0.20 1.61 0.98 
MB800 98.04 0.0076 0.92 2.2956 1.63 0.94 11.148 0.016 0.97 0.41 1.67 0.98 
Air-activated hydrochar 90.90 0.91 0.98 3.3201 1.81 0.95 15.87 0.0145 0.97 0.49 1.58 0.98 

From Table 5, the R2 value indicates that the results of PO43−-P adsorption also fit the Freundlich model better than the Langmuir model. The Freundlich model suggests that the adsorption of P is heterogeneous and multilayer, which is due to the diversity of adsorption sites and the nature of ions as free or hydrolyzed substances. The literature also showed that the Freundlich model suits phosphate adsorption better because of saturated adsorbents or precipitation reactions (Zeng et al. 2013). The maximum PO43−-P adsorption capacity of hydrochars has the following order: air-activated hydrochar > MB800 > AB250 > NB250 > B250.

The adsorption intensity or surface heterogeneity (1/n) for all hydrochars on both N and P adsorption was less than 1. This value below unity indicates that the adsorption process is primarily governed by the chemisorption process (Oginni et al. 2020). Although not fully understood, it is suggested that the surface area has little effect on adsorption compared to its elemental composition. Xue et al. (2009) reported that chemical composition of adsorbent led to simultaneous ligand exchange and chemical precipitation between adsorbent and PO43−-P. Previous studies have also suggested that surface functionality may not greatly influence phosphate adsorption since biochars tend to be negatively charged (Zeng et al. 2013).

CONCLUSION

This study investigated anaerobically digested cattail hydrochars obtained via acetic acid or sodium hydroxide assisted HTC, various activation methods, and the N and P adsorption capacities of these hydrochars. When the HTC temperature increased from 200 to 300 °C, the hydrochar yields decreased from 40.2% to 31.6%, 37.5% to 28.3%, and 35.7% to 22.7% for the use of water, acetic acid, and sodium hydroxide, respectively. The characterization showed that acetic acid or sodium hydroxide assisted HTC and activation with magnesium chloride or air could improve the surface properties. Acetic acid modification generated extensive oxygenated functional groups, while sodium hydroxide modification produced hydrochar with a high ratio of N/C and surface aromaticity. Treating hydrochar with magnesium chloride could impregnate nano-magnesium particles on the hydrochar, thereby improving the ability to remove N and P. Air activation of hydrochar resulted in a higher oxygen content.

The NH4+-N and PO43−-P adsorption capacities of these hydrochars ranged from 92.6 to 122.4 mg/g and 1.6 to 15.8 mg/g, respectively. The adsorption capacity of hydrochars in swine wastewater is similar to the results of artificial wastewater. Despite the introduction of different surface functional groups, the hydrochars obtained by HTC assisted by acetic acid and sodium hydroxide did not significantly change the adsorption capacity of hydrochar. Hydrochar activated by using MgCl2 and air substantially increased the adsorption capacity. The results suggested that Mg-nanoparticle dispersion and oxygen containing functional groups played a major role in adsorption than ion exchange and physisorption. Both NH4+-N and PO43−-P adsorption fit the Freundlich model well.

ACKNOWLEDGEMENTS

The publication was made possible by the United States Department of Agriculture (USDA NIFA award: NC.X-314-5-18-130-1) and the University of North Carolina's Research Opportunities Initiative (UNC ROI 2017-2020). This work was partially performed at the Joint School of Nanoscience and Nanoengineering, a member of Southeastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542174).

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

REFERENCES

Banerjee
S.
,
Chattopadhyaya
M. C.
,
Srivastava
V.
&
Sharma
Y. C.
2014
Adsorption studies of methylene blue onto activated saw dust: kinetics, equilibrium, and thermodynamic studies
.
Environmental Progress & Sustainable Energy
33
(
3
),
790
799
.
Bansal
S.
,
Lishawa
S. C.
,
Newman
S.
,
Tangen
B. A.
,
Wilcox
D.
,
Albert
D.
,
Anteau
M. J.
,
Chimney
M. J.
,
Cressey
R. L.
,
DeKeyser
E.
,
Elgersma
K. J.
,
Finkelstein
S. A.
,
Freeland
J.
,
Grosshans
R.
,
Klug
P. E.
,
Larkin
D. J.
,
Lawrence
B. A.
,
Linz
G.
,
Marburger
J.
,
Noe
G.
,
Otto
C.
,
Reo
N.
,
Richards
J.
,
Richardson
C.
,
Rodgers
L.
,
Schrank
A. J.
,
Svedarsky
D.
,
Travis
S.
,
Tuchman
N.
&
Windham-Myers
L.
2019
Typha (Cattail) invasion in north American wetlands: biology, regional problems, impacts, ecosystem services, and management
.
Wetlands
39
(
4
),
645
684
.
Bekoe
D.
,
Wang
L.
,
Zhang
B.
,
Todd
M. S.
&
Shahbazi
A.
2018
Aerobic treatment of swine manure to enhance anaerobic digestion and microalgal cultivation
.
Journal of Environmental Science and Health, Part B
53
(
2
),
145
151
.
Bolisetty
S.
,
Peydayesh
M.
&
Mezzenga
R.
2019
Sustainable technologies for water purification from heavy metals: review and analysis
.
Chemical Society Reviews
48
(
2
),
463
487
.
Chintala
R.
,
Schumacher
T. E.
,
McDonald
L. M.
,
Clay
D. E.
,
Malo
D. D.
,
Papiernik
S. K.
,
Clay
S. A.
&
Julson
J. L.
2014
Phosphorus sorption and availability from biochars and soil/biochar mixtures
.
CLEAN – Soil, Air, Water
42
(
5
),
626
634
.
Dubbe
D. R.
,
Garver
E. G.
&
Pratt
D. C.
1988
Production of cattail (Typha spp.) biomass in Minnesota, USA
.
Biomass
17
(
2
),
79
104
.
Fang
C.
,
Zhang
T.
,
Li
P.
,
Jiang
R.-f.
&
Wang
Y.-c.
2014
Application of magnesium modified corn biochar for phosphorus removal and recovery from swine wastewater
.
International Journal of Environmental Research and Public Health
11
(
9
),
9217
9237
.
Goel
P.
2006
Water Pollution: Causes, Effects and Control
.
New Age International Publishers
,
New Delhi, India
.
Jaruwat
D.
,
Udomsap
P.
,
Chollacoop
N.
,
Fuji
M.
&
Eiad-ua
A.
2018
Effects of hydrothermal temperature and time of hydrochar from Cattail leaves
. In:
AIP Conference Proceedings
.
AIP Publishing LLC
, p.
020016
.
Jiang
Y.-H.
,
Li
A.-Y.
,
Deng
H.
,
Ye
C.-H.
,
Wu
Y.-Q.
,
Linmu
Y.-D.
&
Hang
H.-L.
2019
Characteristics of nitrogen and phosphorus adsorption by Mg-loaded biochar from different feedstocks
.
Bioresource Technology
276
,
183
189
.
Jung
K.-W.
,
Hwang
M.-J.
,
Ahn
K.-H.
&
Ok
Y.-S.
2015
Kinetic study on phosphate removal from aqueous solution by biochar derived from peanut shell as renewable adsorptive media
.
International Journal of Environmental Science and Technology
12
(
10
),
3363
3372
.
Kameyama
K.
,
Miyamoto
T.
,
Shiono
T.
&
Shinogi
Y.
2012
Influence of sugarcane bagasse-derived biochar application on nitrate leaching in Calcaric Dark Red Soil
.
Journal of Environmental Quality
41
(
4
),
1131
1137
.
Khan
M. N.
&
Mohammad
F.
2014
Eutrophication: challenges and solutions
. In:
(Ansari, A. & Gill, S. (eds)
)
Eutrophication: Causes, Consequences and Control
.
Springer
,
Dordrecht, the Netherlands
, pp.
1
15
.
Kizito
S.
,
Wu
S.
,
Kirui
W. K.
,
Lei
M.
,
Lu
Q.
,
Bah
H.
&
Dong
R.
2015
Evaluation of slow pyrolyzed wood and rice husks biochar for adsorption of ammonium nitrogen from piggery manure anaerobic digestate slurry
.
Science of The Total Environment
505
,
102
112
.
Kostecke
R. M.
2002
Effects of Cattail Management on Invertebrate Production and Migratory Bird use of Cheyenne Bottoms, KS. PhD thesis
,
Texas Tech University
,
Lubbock, TX
.
Oginni
O.
,
Yakaboylu
G. A.
,
Singh
K.
,
Sabolsky
E. M.
,
Unal-Tosun
G.
,
Jaisi
D.
,
Khanal
S.
&
Shah
A.
2020
Phosphorus adsorption behaviors of MgO modified biochars derived from waste woody biomass resources
.
Journal of Environmental Chemical Engineering
8
(
2
),
103723
.
Sajjadi
B.
,
Chen
W.-Y.
&
Egiebor
N. O.
2019
A comprehensive review on physical activation of biochar for energy and environmental applications
.
Reviews in Chemical Engineering
35
(
6
),
735
776
.
Takaya
C.
,
Fletcher
L.
,
Singh
S.
,
Anyikude
K.
&
Ross
A.
2016
Phosphate and ammonium sorption capacity of biochar and hydrochar from different wastes
.
Chemosphere
145
,
518
527
.
Wang
T.
,
Zhai
Y.
,
Zhu
Y.
,
Peng
C.
,
Xu
B.
,
Wang
T.
,
Li
C.
&
Zeng
G.
2017
Acetic acid and sodium hydroxide-aided hydrothermal carbonization of woody biomass for enhanced pelletization and fuel properties
.
Energy & Fuels
31
(
11
),
12200
8
.
Xue
Y.
,
Hou
H.
&
Zhu
S.
2009
Characteristics and mechanisms of phosphate adsorption onto basic oxygen furnace slag
.
Journal of Hazardous Materials
162
(
2
),
973
980
.
Yamashita
T.
&
Yamamoto-Ikemoto
R.
2014
Nitrogen and phosphorus removal from wastewater treatment plant effluent via bacterial sulfate reduction in an anoxic bioreactor packed with wood and iron
.
International Journal of Environmental Research and Public Health
11
(
9
),
9835
9853
.
Zeng
Z.
,
Zhang
S.-d.
,
Li
T.-q.
,
Zhao
F.-l.
,
He
Z.-l.
,
Zhao
H.-p.
,
Yang
X.-e.
,
Wang
H.-l.
,
Zhao
J.
&
Rafiq
M. T.
2013
Sorption of ammonium and phosphate from aqueous solution by biochar derived from phytoremediation plants
.
Journal of Zhejiang University Science B
14
(
12
),
1152
1161
.
Zhang
B.
&
Wang
Y.
2013
Biomass Processing, Conversion and Biorefinery
.
Nova Science Publishers, Inc
,
New York, NY
.
Zhang
B.
,
Shahbaz
A.
,
Wang
L.
,
Whitmore
A.
&
Riddick
B. A.
2012
Fermentation of glucose and xylose in cattail processed by different pretreatment technologies
.
BioResources
7
(
3
),
2848
2859
.
Zhang
B.
,
Joseph
G.
,
Wang
L.
,
Li
X.
&
Shahbazi
A.
2020
Thermophilic anaerobic digestion of cattail and hydrothermal carbonization of the digestate for co-production of biomethane and hydrochar
.
Journal of Environmental Science and Health, Part A
55
(
3
),
230
238
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).

Supplementary data