Adsorption of Ammonium, Nitrate, and Phosphate on Hydrochars and Biochars

Abstract

Biochar (BC) and hydrochar (HC) have attracted considerable attention owing to their versatile characteristics and proven effectiveness in diverse technical fields. Solid BC is generated as a result of the dry carbonisation process of pyrolysis, in contrast to the slurry HC, which is produced during the hydrothermal carbonisation process. In this study, we evaluated the adsorption potential of two hydrochar samples (HCs) and three biochar samples (BCs) produced from sugar cane bagasse. The adsorption capacity of these samples was tested for ammonium, nitrate, and phosphate ions under various conditions. The BCs and HCs were subjected to characterisation using a CHNS/O analyser, the zeta potential, and Fourier transform infrared (FTIR). Elevating the pyrolysis temperature of the biochar resulted in changes in the fixed carbon and ash contents, while the volatile matter and H/C and O/C atomic ratios decreased. As the residence time increased, the H/C ratio and volatile matter content of the hydrochars (HCs) decreased. However, the fixed carbon content, ash content, and O/C and C/N ratios exhibited an increase. Thermodynamics, adsorption isotherms, and pH were also taken into consideration. The FTIR spectra analysis indicated that the carboxyl and ester functional groups present in both the BCs and HCs displayed reduced peak intensities subsequent to the adsorption of the three ions. While the adsorption was exothermic, we noticed that the adsorption capacity increased with temperature. The results indicate that sorption was homogenous across all binding sites, as evidenced by the optimal fit to the Langmuir isotherm. The research findings indicate that the adsorption capacity of various BC and HC adsorbents is significantly influenced by the surface area of the adsorbents in the case of nitrate and phosphate, but in the case of ammonia, adsorption is dictated by the functional polar groups present on the adsorbent surface.

1. Introduction

The nomenclature employed to refer to the residual material derived from the slow pyrolysis of biowaste, which is primarily composed of carbon, is biochar (BC). Conversely, the residual material obtained from the hydrothermal carbonisation (HTC) process, also called wet pyrolysis, is referred to as hydrochar (HC). Both of the aforementioned products have garnered significant attention owing to their potential applications in a variety of engineering and industrial fields [1]. The production of BC involves a pyrolysis process that necessitates an extra energy input to dry the feedstock. Conversely, the production of HC enables the utilisation of wet feedstock that does not require drying during the process. The biomass undergoes a high-temperature treatment in the presence of water, which serves as a catalyst [2].

BC and HC exhibit promising characteristics and structures that make them suitable for utilisation as soil enhancers. These features include the capacity to enhance crop yield, facilitate the filtration of percolating soil water, and assimilate atmospheric carbon in soil [3]. The variances in the characteristics and functions of BC that have been observed are subject to the influence of various process parameters [4,5,6,7,8]. These parameters include the feedstock, processing temperature, heating rate, and residence time. The synthesis of HC can be achieved through the utilisation of water as the reaction medium at a lower temperature, resulting in a more cost-effective process compared to pyrolysis. The reaction conditions commonly observed in the literature include temperatures ranging from 180 to 260 °C, autogenetic pressures ranging from 2 to 6 MPa, and reaction times ranging from 30 to 240 min [9]. The surface of HC exhibits a higher concentration of functional groups containing oxygen and molecules that have undergone aromatisation. The incorporation of functional groups into the molecule enhances its hydrophilicity, thereby rendering it suitable for augmenting the water retention capacity of soil [10].

Distinguishable variations exist in the physical and chemical characteristics of BCs and HCs. The study by Takaya and colleagues [11] revealed that BC and HC exhibit adsorption capacities ranging from around 0 to 30 mg·g⁻¹ and 105 to 146 mg·g⁻¹ for phosphate and ammonium, respectively. This observed phenomenon can be attributed primarily to the unique physicochemical characteristics of the subject in question. The investigation of the properties and interaction mechanisms of BC and HC adsorption is crucial for the utilisation of soil fertilisers in order to mitigate potential environmental consequences. Eutrophication is a phenomenon that occurs in estuaries and coastal waters as a result of an overabundance of nitrogen and phosphate. This process leads to an escalation in the growth of plant and toxic algae [12]. The application of synthetic fertilisers has been found to have a positive correlation with the release of ammonium, nitrate, and phosphorus into water bodies from agricultural land [13,14]. Research [15,16] findings indicate that BC and HC have the ability to impede the uptake of nitrogen and phosphorus by the soil. The utilisation of sugarcane bagasse BC has been observed to enhance the retention time of nitrate in the vicinity of plant roots, thereby facilitating its uptake by the plants [17]. The optimal conditions were identified for ammonium adsorption on bamboo charcoal, which resulted in the removal of up to 82.4% of ammonium from an aqueous solution [18]. As per existing research, it has been observed that BC has the ability to solubilise phosphate. It has also been noted that modifying the concentration of BC present in soil can lead to improved agronomic performance [19]. As per the findings of a previous study [20], it can be inferred that HC exhibits potential as an adsorbent for the purpose of removing and recovering phosphorus from wastewater. This is supported by the fact that HC has shown to be remarkably efficient in adsorbing phosphate ions under different pH conditions, even in the presence of competing anions.

The investigation of the sorption characteristics of BC and HC is currently in its nascent phase. The majority of research investigations have concentrated on the ion adsorption capabilities of BC, whereas the published findings regarding HC have been notably limited. Understanding the characteristics of both BCs and HCs and their impact on the adsorption mechanisms of ammonium, nitrate, and phosphate is crucial for the improved management of nitrogen and phosphorus in soil and agronomic practices. The aim of this investigation is to examine the impact of bio-based feedstock-derived BCs and HCs, produced under various dry and hydrothermal carbonisation conditions, on the adsorption of ammonium, nitrate, and phosphate.

2. Materials and Methods

2.1. BC and HC Preparation

BC and HC were obtained through the pyrolysis and thermal hydrolysis of sugar cane bagasse, respectively. The biomass underwent an initial shredding process resulting in fragments measuring between 2 and 3 cm in length. The study involved the implementation of slow pyrolysis at 300 °C, 500 °C, and 700 °C for a duration of 60 min within a fixed-bed reactor. The BC produced at these temperatures was labelled as BC300, BC500, and BC700, respectively. The experimental setup comprised a temperature controller cabinet, a quartz tube reactor with an inner diameter of 2.5 cm, and an electric furnace heater. The BCs that were acquired underwent a grinding process and were subsequently rinsed with deionised water. Afterward, they were dried in a furnace at a temperature of 105 °C for a duration of 24 h. The BCs were then stored in airtight containers at room temperature until they were ready for further use.

HC was produced by means of a 2G Parr agitated pressure reactor featuring a removable glass liner. The experimental procedure involved the addition of 200 g of sugar cane bagasse and 1 L of distilled water into the glass liner, followed by thorough mixing for a duration of 30 min. The vessel was hermetically sealed and subjected to a continuous flow of nitrogen gas for a duration of 10 min. The experiment involved subjecting the vessel and its contents to a temperature of 250 °C while simultaneously agitating them for 30 and 180 min. This process yielded two distinct samples, namely, HC30 and HC180. Subsequently, the vessel along with its contents underwent rapid cooling in a cold-water bath maintained at room temperature. The HCs were isolated through the process of vacuum filtration, followed by multiple washes with distilled water. The resulting HCs were then subjected to drying in an oven set at 105 °C for a period of 24 h. Finally, the HCs were stored in plastic receptacles for further use.

Activated carbon (AC) from Sigma-Aldrich was subjected to characterisation and subsequently employed in sorption experiments to facilitate a comparison between the BCs and HCs.

2.2. Characterisation of BCs and HCs

The BC, HC, and AC samples were subjected to analysis utilising a CHNS/O analyser, specifically the Elementar Variant LE Cube. The moisture content of the char samples was determined in accordance with the ICS 75.160.10, DD CEN/TS 14774-3:2009 standard [21]. The ash and volatile matter contents were determined using ICS 75.160.10, DD CEN/TS 14775:2009 [22] and ICS 75.160.10, DD CEN/TS 15148:2009 [23], respectively. The pH values of the BCs and HCs were measured by means of a pH metre (PHM 84, Radiometer, Denmark) equipped with glass REF 451 and calomel pHG 201-8 electrodes. The ratio of BC or HC to distilled water was maintained at 1:20 (m/v) during the pH determination process. The study employed a Fourier transform infrared (FTIR) spectrometer, specifically the Cary 630 FTIR spectrometer from Agilent Technologies Inc. (Santa Clara, CA, USA). Spectra were obtained within the 400–4000 cm⁻¹ range, using a resolution of 4 cm⁻¹ and 64 scans per sample. The biochar surface area and pore volume were determined by using the Brunauer–Emmett–Teller (BET) method, using a Gemini 2375 V5.01 surface area analyser (Mcromeritics Instrument Co., Norcross, GA, USA).

The zeta potentials of the BCs, HCs, and AC were determined. The experimental procedure involved mixing 0.5 g of each sample with 100 mL of deionised water at a pH of 7. The impact of NH₄⁺, NO₃⁻, and PO₄³⁻ adsorption on the zeta potential was assessed by introducing 0.5 g of each char sample to a solution containing 100 mg·dm⁻³ of NH₄⁺, NO₃⁻, and PO₄³⁻. Prior to this, the pH of the suspensions was adjusted to a range between 4.0 and 9.0 using 0.1 mol·dm⁻³ of HCl or NaOH. The solutions under investigation were subjected to mechanical agitation at a rate of 250 revolutions per minute for a duration of 30 min. The suspensions underwent ultrasonic dispersion for a duration of 1 h at a temperature of 30 °C, utilising a bath-type sonicator with a frequency of 40 kHz and a voltage of 300 W. Subsequently, the solution underwent filtration using filter paper of the Whatman 42 variety. The zeta potential of each supernatant solution was determined by means of a Malvern Zetasizer Nano instrument (Malvern Instruments, Malvern, UK). The replicability of all analyses was ensured.

2.3. Adsorption of NH₄⁺, NO₃⁻, and PO₄³⁻

Initial analyses were performed to determine the most effective adsorber dosage. Adsorption experiments were conducted over a range of adsorbent masses, specifically between 0.1 and 1 g with increments of 0.1 g. The concentration of the adsorbate used was 100 mg·dm⁻³. A notable variance in the adsorber material dosage was observed at 0.5 g, which was utilised in subsequent experimental trials.

2.3.1. Analysis of NH₄⁺, NO₃⁻, and PO₄³⁻

An analysis of NH₄⁺ was performed utilising a Varian Spectrophotometer (UV-Vis-4000) in accordance with the phenate method [24]. The determination of NO₃⁻ and PO₄³⁻ was carried out using ion chromatography equipment (Dionex DX-120 Ion and ICS1100, Manasquan, NJ, USA). The analysis points were obtained by calculating the mean of three distinct parallel sample solutions. Statistical significance was determined at a level of p < 0.05.

2.3.2. pH Effect

In order to determine the effect of pH, 0.5 g of an adsorbent material was mixed with 100 cm³ of a solution containing 100 mg·dm⁻³ of either NH₄⁺, NO₃⁻, or PO₄³⁻. Before adding the biochar, the pH of the solutions was adjusted to lie within the range of 3 to 9. Using a PHM 84 pH metre equipped with glass REF 451 and calomel pHG 201-8 electrodes, this was accomplished. For every pH measurement, a unique experimental setup was established. The suspensions were agitated for 24 h on an agitator operating at 250 revolutions per minute and a temperature of 25 °C. Each sample was filtered using a paper filter with a pore size of 0.5 micrometres. The pH of the subsequent filtrate was then measured. The concentrations of NH₄⁺, NO₃⁻, and PO₄³⁻ were measured using the previously described method.

Equation (1) was utilised to determine the equilibrium adsorption capacity:

$$q_e=\frac{V\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{m}$$

(1)

where q_e is the adsorption capacity at equilibrium (mg·g⁻¹); V is the volume of the solution (dm³); C₀ and C_e are the initial and equilibrium concentrations of NH₄⁺, NO₃⁻, and PO₄³⁻ (mg·dm⁻³); and m is the weight of the adsorbent (g).

2.3.3. Adsorption Isotherms

In this experiment, we added the BCs, HCs, and AC (0.5 g) to 250 cm³ Erlenmeyer flasks containing NH₄⁺, NO₃⁻, and PO₄³⁻ solutions. The solutions had varying concentrations of 25, 50, 100, 200, and 500 mg dm⁻³, and the pH was kept constant at 7. The suspensions underwent agitation on a shaker operating at 200 rotations per minute and a temperature of 25 °C for a duration of 24 h. The filtration process involved the use of a paper filter with a pore size of 0.5 micrometres to filter the suspensions. The filtrates were subjected to the measurement of NH₄⁺, NO₃⁻, and PO₄³⁻ concentrations.

The adsorption data of the NH₄⁺, NO₃⁻, and PO₄³⁻ on the BCs, HCs, and AC were analysed using the Langmuir and Freundlich isotherm models. The Langmuir model is described in Equation (2) [25]:

$$\frac{C_e}{q_e}=\frac{1}{K_L{q}_m}+\frac{C_e}{q_m}$$

(2)

where C_e is the equilibrium concentration (mg·dm⁻³), and constant q_m (mg·g⁻¹) and K_L are the characteristics of the Langmuir equation (dm³·mg⁻¹) and can be determined from the linearised form (plots of C_e/q_e vs. C_e). The Freundlich model is expressed according to Equation (3) [26]:

$$ln{q}_e=ln{K}_F+\frac{1}{n}ln{C}_e$$

(3)

where K_F is the Freundlich adsorption capacity (mg·g⁻¹), and 1/n is the Freundlich constant. The above equation can be linearised to calculate the parameters K_F and n (plots of log q_e vs. log C_e).

Studies of the thermodynamics and the effect of temperature are presented in Section 3.6. The experiments were conducted as follows: the BCs and HCs were studied at 20, 35, and 50 °C using initial NH₄⁺, NO₃⁻, and PO₄³⁻ concentrations of 100 mg·dm⁻³ at pH 7. Each sample was filtered through a 0.5 μm paper filter, and the concentrations of NH₄⁺, NO₃⁻, and PO₄³⁻ were measured.

3. Results and Discussion

3.1. BC and HC Characteristics

Ultimate and proximate analyses of the BC and HC samples produced under various conditions are presented in

Table 1. Ultimate and proximate analyses of BC and HC samples produced under different pyrolysis and hydrothermal carbonisation conditions.

	Ultimate Analysis wt.%					Atomic Ratios			BET Surface Area m2/g	Pore Volume cm3/g	Proximate Analysis wt.%
Material	N	C	H	S	O	C/N	H/C	O/C			Ash	Volatile Matter	Fixed Carbon
HC30	0.49	67.2	6.15	0.0	19.9	138	0.092	0.297	1.4	0.004	4.83	48.8	46.4
HC180	0.36	65.2	5.77	0.0	21.2	179	0.088	0.325	1.8	0.004	6.30	45.9	47.8
BC300	0.39	56.4	5.67	0.0	28.3	145	0.100	0.502	2.4	0.003	6.30	58.7	35.0
BC500	0.57	69.1	2.96	0.0	13.7	121	0.043	0.199	80.4	0.032	10.1	15.8	74.2
BC700	0.54	76.3	1.37	0.0	0.97	142	0.018	0.013	84.3	0.030	17.8	5.72	76.5
AC	0.48	86.1	0.30	0.0	6.51	179	0.003	0.076	400	1.365	3.60	4.44	92.0

. The results indicate that an increase in residence time led to a corresponding increase in the fixed carbon and ash contents, as well as in the O/C and C/N ratios, for the HCs. Conversely, the volatile matter and H/C ratio decreased with an increase in residence time. The results suggest that the residence times of 30 and 180 min did not significantly affect the production of hydrocarbons, as the values obtained were closely aligned. The observed difference in the C/N ratio can be attributed to the increase in N losses that occurs with longer residence times. The findings obtained are consistent with the outcomes and patterns documented in prior research [27,28,29]. The BET method was applied to determine the pore structure. The BET surface area and pore volume of the BCs produced at a low temperature and the HCs are similar, and the values of both parameters increased significantly as the pyrolysis temperature increased, as confirmed in other studies [30,31]. The surface area of the BC stayed at a stable level after passing 500 °C, as presented in Table 1.

The results indicate that an increase in carbonisation temperature led to an increase in the concentrations of fixed carbon and ash, while the volatile matter and atomic ratios (H/C and O/C) decreased for the BCs. The obtained results are consistent with those previously reported in the scientific literature [16,32] for various carbonisation temperatures. According to the findings of previous research, the pyrolysis process primarily results in the transformation of hemicelluloses and cellulose into gaseous products, whereas lignin is predominantly transformed into char at elevated temperatures. The results of this study suggest that increasing the pyrolysis temperature can increase the degree of carbonisation of BC. According to previous research [16], this finding suggests that the BCs produced by this process may have potential benefits for carbon sequestration. Compared to the BCs and HCs, the AC was found to have the highest concentration of fixed carbon and the lowest concentrations of ash and volatile matter [33].

An FTIR spectral analysis plays a crucial role in the identification of distinct functional groups responsible for ion adsorption [18]. As shown in Figure 1, the FTIR spectra of HC30, HC180, BC300, BC500, BC700, and AC were collected and analysed in the presence and absence of NH₄⁺, NO₃⁻, and PO₄³⁻ adsorption.

The vibrational spectra of the HCs and BCs were compared, and it was observed that the number of vibrational peaks was significantly greater in the HC spectra than in the BC spectra. The vibrational peaks at 2850 cm⁻¹, 1698 cm⁻¹, and 1206 cm⁻¹ were attributed to C-H stretching, aromatic C=O, and C-O stretching vibrations, respectively, as depicted in Figure 1. The techniques of hydrolysis and pyrolysis exhibited distinct differences in terms of chemical structure. The observed peaks in the spectra can be attributed to the presence of carboxyl and ester functional groups, as previously reported [34]. The study in [35] reported comparable transmittance values in the range of 4000 to 650 cm⁻¹ for two samples of HCs. This suggests that there were no significant chemical structural modifications resulting from an increase in the residence period.

A correlation was observed between the pyrolysis temperature and the presence of functional groups in the BCs, with a decrease in the number of functional groups as the temperature increased. The obtained spectra exhibited distinct peaks at specific wavenumbers, namely, 2907 cm⁻¹ for methyl CH stretching vibrations, 1703 cm⁻¹ for aromatic carbonyl/carboxyl C=O, 1595 cm⁻¹ for aromatic C=C and C=O, 1028 cm⁻¹ for aliphatic COC and OH, and 814 cm⁻¹ for aromatic CH [36]. The observed shifts in the appearance of the bands are consistent with the findings of previous studies [37,38] and were attributed to the increase in pyrolysis temperature. According to Zhou and colleagues [18], the peaks observed in the range of 1000 to 600 cm⁻¹ are associated with the undulating vibration of aromatic C-H. With an increase in pyrolysis temperature, a reduction in the polar functional groups (OH and C-O) was observed, as indicated by the smaller peaks in their respective spectra [16]. Upon the adsorption of NH₄⁺, NO₃⁻, and PO₄³⁻, both the biochars (BCs) and hydrochars (HCs) exhibited comparable peaks. Nevertheless, the peaks observed in the BC spectra were relatively feeble, whereas those in the HC spectra were relatively robust. Consistent with previous studies [16,39,40], our FTIR spectra analysis revealed the absence of a significant band subsequent to phosphate adsorption. Upon analysing the wavelengths of particular peaks depicted in Figure 1, slight deviations were observed, consistent with findings reported previously [41,42]. The present study demonstrates the complexation phenomenon between ammonium ions and ionised -OH groups, as well as the bonded -OH bands of carboxylic acids. This was evidenced by a discernible shift in the wavenumber of multiple peaks within the range of 3500 to 3200 cm⁻¹. The confirmation of the ion-exchange phenomenon between the protons of aliphatic C-H and the symmetric stretching vibration of CH₂ with ammonium ions has been established through shifts observed at 2920 and 2860 cm⁻¹, as previously reported [42].

3.2. Zeta Potential

The zeta potential values of the HCs, BCs, and AC were measured after the adsorption of NH₄⁺, NO₃⁻, and PO₄³⁻, as presented in Figure 2. The samples were analysed for their zeta potential at pH 7, which was found to range from −18.7 to −32.6 mV. This suggests that the particles present in the samples possessed negatively charged surfaces. The results of this study indicate that the AC exhibited the highest average zeta potential, while the HC displayed the lowest. An increase in the pH of the suspension resulted in an elevated discrepancy in the zeta potential between the systems. The findings of this study indicate that HCs, BCs, and AC have the ability to selectively adsorb NH₄⁺, NO₃⁻, and PO₄³⁻. Moreover, it was observed that the influence of specific adsorption became more pronounced at elevated suspension pH levels. Similar findings [16,43] were reported in studies on the utilisation of crop straw feedstock-based biochar for the adsorption of copper and phosphate, respectively.

Furthermore, the zeta potential of the HCs, BCs, and AC became more negative with increased pH values, suggesting that the amount of negative charge increased with the increase in pH and that the adsorption capacity should be lower. The electrostatic adsorption of ions on solid charged surfaces does not tend to affect the surface charge or surface potential of colloidal particles, as the adsorbed ions exist in the diffuse layer of the electric double layers on the particles. Nevertheless, the specific adsorption of ions changes the surface charge and the surface potential of colloidal particles, since these ions go into the Stern layer of the electric double layers and form chemical bonds with the solid particle surfaces. In addition, during the process of cation specific adsorption, some positive charges are transferred to the surface of biochars. This process could lead to a less negative surface charge or even a net positive charge [44]. Accordingly, the zeta potential of the BCs became less negative or changed from negative to positive.

3.3. Adsorption of Ammonium

The impact of carbonisation temperature and residence duration on the adsorption capacity and surface area of the chars was highly significant. The equilibrium isotherms and the adsorption capacity variation as a function of the char used and the initial NH₄⁺ concentration are illustrated in Figure 3. The results of this study indicate that HC180 exhibited a higher adsorption capacity compared to HC30. However, BC700 demonstrated a lower adsorption capacity in comparison to BC500 and BC300. The results indicate that BC300 exhibited the most significant NH₄⁺ adsorption capacity, while HC30 demonstrated the least. Upon increasing the initial concentration of NH₄⁺ from 25 to 500 mg·dm⁻³, the observed mean values for the HCs exhibited a range of 0.63 to 2.67 mg·g⁻¹, while the mean values for the BCs ranged from 2.59 to 9.05 mg·g⁻¹. In terms of NH₄⁺ adsorption capacity, the AC was surpassed only by BC300.

The adsorption behaviour of the BCs was observed to be influenced by the initial concentration of NH₄⁺ in the solution, as well as the temperature of carbonisation. The results of the observations indicate an inverse relationship between NH₄⁺ adsorption capacity and carbonisation temperature. Specifically, as the temperature of carbonisation increased, the capacity for NH₄⁺ adsorption decreased. Comparable results were obtained by Wang et al. [14] on biochars derived from oak sawdust through pyrolysis at temperatures between 300 °C and 600 °C. The results of this study indicate that the adsorption of NH₄⁺ is augmented by BCs that are generated at lower temperatures. As per prior research, it can be observed in Figure 1 that the BC produced at a higher pyrolysis temperature lacked the presence of aromatic C=O and C=C, -CH₂-, CO, and CC functional groups. As a result, it was observed that the functional polar groups present in the material underwent a reaction with NH₄⁺ during the adsorption process. This led to a significant increase in the adsorption capacity of the BC that was produced at lower temperatures, even its surface area was greatly lower than that produced at higher temperatures.

The observed results indicate that there was no significant variation in the NH₄⁺ adsorption capacity of the HCs that were assessed. The results suggest that varying the residence time between 30 and 180 min does not significantly impact the NH₄⁺ adsorption capacity. In their study, Takaya et al. [11] conducted hydrothermal carbonisation at 250 °C to synthesise HC and compared it with BCs obtained from different biomass sources at low and high pyrolysis temperatures (400–450 °C and 600–650 °C, respectively). The study found that there were variations in the physicochemical properties of the char and the refining conditions. However, the NH₄⁺ sorption capacities ranged between 105.8 and 146.4 mg·g⁻¹. As per the findings of the study, it can be inferred that the NH₄⁺ adsorption capacity is not primarily influenced by the surface area [11,45].

The present study employed the Langmuir and Freundlich isotherm models to investigate the adsorption behaviour of NH₄⁺, NO₃⁻, and PO₄³⁻ on various samples of BCs, HCs, and AC. The results of the analysis are presented in

Table 2. Langmuir and Freundlich isotherm parameters for the adsorption of ammonium, nitrate, and phosphate on BCs, HCs, and AC.

System		Material
		BC300	BC500	BC700	HC30	HC180	AC
		Ammonium
Freundlich	KF	1.177	1.016	0.755	0.192	0.186	1.117
	n	2.737	2.700	2.502	2.187	2.134	2.778
	R2	0.886	0.846	0.896	0.866	0.944	0.886
Langmuir	q0	9.833	8.183	8.137	2.970	3.434	9.050
	KL	0.028	0.032	0.020	0.016	0.012	0.029
	R2	0.968	0.946	0.980	0.975	0.993	0.967
		Nitrate
Freundlich	KF	0.036	0.073	0.091	0.033	0.093	0.093
	n	1.565	1.583	1.632	1.971	1.523	1.539
	R2	0.688	0.674	0.651	0.826	0.748	0.708
Langmuir	q0	1.637	3.051	3.261	0.698	1.399	4.529
	KL	0.008	0.009	0.010	0.013	0.007	0.008
	R2	0.879	0.880	0.871	0.953	0.902	0.893
		Phosphate
Freundlich	KF	0.119	0.272	0.301	0.033	0.098	0.477
	n	1.992	2.147	2.193	1.971	2.107	2.092
	R2	0.829	0.809	0.860	0.826	0.805	0.832
Langmuir	q0	2.413	4.184	4.699	0.698	1.615	8.292
	KL	0.013	0.017	0.014	0.013	0.016	0.014
	R2	0.955	0.948	0.956	0.953	0.947	0.943

. According to the existing literature, the Langmuir isotherm model posits that sorption occurs uniformly across all binding sites. Conversely, the Freundlich isotherm model suggests that the surface is heterogeneous, with an uneven distribution of adsorption capacity over the surface [46]. The Langmuir model demonstrates superior descriptive capabilities for the adsorption process of NH₄⁺, NO₃⁻, and PO₄³⁻ on the BCs, HCs, and AC, as evidenced by the higher R² values. Through the utilisation of both BCs and HCs, it was found in the study in [11] that the Langmuir isotherm model exhibited a higher degree of accuracy in describing NH₄⁺ adsorption in comparison to the Freundlich isotherm model.

3.4. Adsorption of Nitrate

The maximum and the minimum NO₃⁻ adsorption values were reached by the AC and HC30, respectively (Figure 4). The average values ranged from 0.11 to 0.39 mg·g⁻¹, and 0.59 to 3.46 mg·g⁻¹, respectively, when increasing the initial NO₃⁻ concentration from 25 to 500 mg·dm⁻³.

This study revealed that the adsorption capacity of NO₃⁻ was comparatively higher in BC700 as compared to in BC500 and BC300. This indicates that an increase in pyrolysis temperature leads to an improvement in the NO₃⁻ adsorption capacity of BCs. In a previous study [14], a range of BCs were analysed. These BCs were produced through the pyrolysis of oak sawdust at different temperatures, ranging from 300 °C to 600 °C.

The results of this study indicate that higher pyrolysis temperatures led to an improvement in the adsorption of NO₃⁻. The results indicate that HC180 exhibited a notably greater adsorption capacity compared to HC30 for the target compounds. This suggests that an increase in residence time from 30 to 180 min led to an increase in the quantity of NO₃⁻ that could be adsorbed by the material.

3.5. Adsorption of Phosphate

Similarly to the NO₃⁻ adsorption capacity using the studied chars, the maximum and the minimum PO₄³⁻ adsorption values were reached by the AC and HC30, respectively (Figure 5). The average values ranged from 0.12 to 1.23 mg·g⁻¹, and 0.57 to 7.15 mg·g⁻¹, respectively, when increasing the initial PO₄³⁻ concentration from 25 to 500 mg·dm⁻³.

The adsorption capacity results for PO₄³⁻ in the BCs were found to be comparable to those observed for NH₄⁺ and NO₃⁻. It was observed that the adsorption capacity was relatively higher for BC700 as compared to BC500 and BC300. This finding further supports the earlier conclusion that a reduction in BC pyrolysis temperature results in an increase in the PO₄³⁻ adsorption capacity.

The findings of Trazzi et al. [16] indicate that the adsorption and desorption behaviour of PO₄³⁻ on BC derived from sugar cane bagasse and Miscanthus x giganteus grass is contingent upon the initial concentration of PO₄³⁻, pyrolysis temperature, and residence duration. Likewise, Peng et al. [47] found that the adsorption capacity of PO₄³⁻ was higher when utilising BC obtained through the pyrolysis of sawdust from Pinus trees at a temperature of 750 °C as compared to that produced at 550 °C.

Regarding the HCs, it was observed that HC180 demonstrated a significantly higher adsorption capacity in comparison to HC30. The findings indicate that the augmentation of the residency period from 30 min to 180 min resulted in a rise in the quantity of adsorbed PO₄³⁻. Dai et al. [20] were successful in obtaining higher values compared to our investigation by utilising an engineered HC that was synthesised through the HTC of lanthanum-pre-treated rice straw. In a pH range spanning from 2.5 to 10.5, the adsorption capacities of P on the HC in question were observed to exceed 50 mg·g⁻¹. Furthermore, the impact of competing anions (such as Cl⁻, NO₃⁻, and SO₄²⁻) on P adsorption was found to be negligible [20].

In order to compare the properties of BC and HC, the study in [11] analysed the sorption capacities of PO₄³⁻ in BCs and HCs that were prepared at different temperatures using different types of waste materials. The PO₄³⁻ sorption capacities of these materials ranged from 0 to 30 mg·g⁻¹. The study conducted by the researchers revealed that the alterations in the pyrolysis temperature of the BC and the composition of the feedstock did not have a substantial effect on the capacity of char adsorption. Additionally, the study found that the HC generally exhibited a lower adsorption capacity than the BCs. The augmentation of the initial ion concentration has been observed to intensify the adsorption process, plausibly owing to the amplified concentration gradients, thereby leading to a more comprehensive occupation of the responsive adsorption locales [45]. Previous research has indicated that the adsorption of PO₄³⁻ increased with an increase in the initial concentration of the ion. However, the efficiency of adsorption decreased, likely due to a reduction in the number of active adsorption sites available at higher initial PO₄³⁻ concentrations, as demonstrated by other studies [48].

The Langmuir model demonstrated superior R² values for PO₄³⁻ adsorption, suggesting that this model could more precisely depict the adsorption on the BCs, HCs, and AC, as shown in Table 2. Comparable outcomes were identified in the existing literature for alternative BC [49]. The findings of Takaya et al. [11] suggest that the linearised Freundlich isotherm model provided a slightly better description of the adsorption mechanism. This conclusion was based on the higher R² values and stronger correlation observed between the experimental and calculated q_e values.

3.6. Adsorption Thermodynamics

In the design of sorption systems, two categories of thermodynamic properties are essential: directly measurable properties, like temperature and the equilibrium constant, and properties that cannot be directly measured, such as entropy (S) and free energy (G). To determine the thermodynamic properties that cannot be directly measured, the well-known thermodynamic relation Gibbs free energy (G) is employed. Both Gibbs free energy and entropy play a crucial role in determining the characteristics of sorption chemical reactions or the overall nature of the sorption process. Evaluating these thermodynamic parameters is instrumental in gaining insights into the nature of the adsorption process [50]. The differential heat of adsorption was calculated using the Clausius–Clapeyron expression in the form

$$∆H=-2.303R\left(\frac{T_1{T}_2}{T_1{-T}_2}\right)log{C}_2-log{C}_1$$

(4)

where C₂ and C₁ are the equilibrium concentrations at temperatures T₂ and T₁, respectively. For each adsorption isotherm, we obtained C₂ and C₁ at a fixed q_e value using the Langmuir isotherm expression. Then, the obtained C₂ and C₁ values were substituted in Equation (4) to obtain the differential heat of adsorption. Equation (5) can be used to obtain the adsorption isosteric heat, which is a positive quantity and is given by the following relationship:

q_st = −ΔH

(5)

By employing other thermodynamic parameters, including the Gibbs free energy, ΔG, and the entropy change, ΔS was obtained from the following thermodynamic relation:

TΔS = ΔH − ΔG

(6)

The free energy change can be calculated using the expression

ΔG = RT lnC_e/C_s

(7)

where C_s is the solubility of the solute, and it depends on the adsorption temperature. The free energy change obtained using Equation (7) is independent of the temperature within the range of temperatures studied.

Figure 6a–i show the variation in the free energy change, change in the heat content, and entropy change for the transfer of the solutes (single components) ammonium, nitrate, and phosphate from the bulk solution to the adsorbed state on the adsorbent surface of BC700, HC180, and the AC at 50 °C for all the studied systems. Irrespective of the solute and adsorbent type, we observe a general trend for the variation in the thermodynamic parameters as a function of the amount adsorbed. The Gibbs free energy increases with coverage. This can be expected as the equilibrium concentration increases with the adsorption at a fixed T. The differential heat of adsorption decreases with coverage or adsorption, q_e. At lower q_e, the adsorption is dominated by solute–solid interactions; however, as the adsorption or the coverage increases, the adsorption is dictated by both solute–solute and solute–solid interactions, and this explains the decrease in the ΔH value with the increase in q_e. The solid–solute interactions at lower surface adsorption can be due to the adsorption onto the active sites with the highest binding affinity; these typically arise from pores of a size that allows them to commensurate with solute ions or the functional groups. For the case of entropy, it clearly decreases with the increase in the surface coverage. This is an expected observation as the system moves from a disordered state to a more ordered adsorbed state. From the differential heat of adsorption, we calculate the adsorption isosteric heat at zero coverage by estimating that q_st = −ΔH at q_e = 0. The calculated q_st values at zero coverage for BC700, HC180, and the AC are plotted as a function of the solute. In Figure 7, it can be seen that HC180 has strong adsorption towards nitrate ions. This does not mean that this adsorbent possesses the highest adsorption capacity, but it has the potential to target the solute molecules at lower concentrations. The adsorption isosteric heat ranges from 5.6 to 27.5 kJ/mol, 15 to 53.6 kJ/mol, and 3.5 to 23.8 kJ/mol for the biochar, hydrochar, and activated carbon, respectively. According to q_st at zero coverage, BC700 shows a strong affinity towards ammonium, followed by nitrate and phosphate. For the case of nitrate and phosphate removal, HC180 shows a stronger affinity towards this solute, followed by BC700 and then AC. Another noteworthy observation is that the q_st of BA700 for ammonium and HC180 for nitrate is around 28 and 54 kJ/mol, respectively. These values indicate that adsorption proceeds via chemisorption during the early stage of adsorption.

A large number of previous studies have indicated that the adsorption of ammonium [51,52,53], nitrate [53,54,55,56], and phosphate [57,58,59] is typically exothermic, agreeing with our results (note that the ΔG values obtained in this work are independent of the temperature).

To gain additional insights into Figure 6, we plotted the adsorption capacity of NH₄⁺, NO₃⁻, and PO₄³⁻, which exhibit an upward trend with an increase in the process temperature in Figure 7. Based on the data, it can be inferred that the adsorption of all three ions onto BC700, HC180, and the AC was facilitated by higher adsorption temperatures. This clearly shows no correlation with q_st at zero coverage. This can be expected, as the adsorption at higher concentrations is independent of the binding affinity of the adsorption at zero coverage. Nevertheless, Figure 7 delivers an important message that higher temperatures promote more favourable adsorption, which agrees with some of the earlier works [60,61].

3.7. Comparison with Other Adsorbents

Table 3. Ammonium, nitrate, and phosphate adsorption capacities of different BCs and HCs formed under different conditions.

Carbonaceous Materials	Conditions for the Adsorbent	Adsorbed Ion	Solution Concentration mg dm−3	Adsorption Capacity mg g−1	Ref.
Oak-based HC	Hydrothermal carbonisation 250 °C	NH4+	43	6.0	[62]
Oak-based BC	Slow pyrolysis at 450 °C			9.2
	Slow pyrolysis at 650 °C			8.9
Peanut shell, corncob, or cotton stalk BC	Pyrolysis at 300, 450, or 600 °C for 2 h	NH4+	50	16–18	[63]
Wheat straw BC	Pyrolysis at 500 °C for 1.5 h	NH4+	50	0.33	[37]
Corn straw BC				0.92
Peanut shell BC				0.54
Sugarcane bagasse BC	Pyrolysis at 500 °C for 1 h	NH4+	50	4.46	This study
Sugarcane bagasse HC30	Hydrothermal carbonisation at 250 °C for 30 min			1.37
Corn cob HC	230 or 260 °C for 30 min	NH4+	20–3000	≈34–107	[64]
KOH-activated HC	Corn cob HC mixed with 200 mL of 3 M KOH			≈48–140
Oak-based hydrochar	Hydrothermal carbonisation 250 °C	NH4+	1000	109.7	[11]
Oak-based BC	Slow pyrolysis at 450 °C			129.4
	Slow pyrolysis at 650 °C			123.5
Macadamia nutshell BC	Pyrolysis at 1000 °C for 1 h	NO3−	10		[65]
Sugarcane bagasse BC	Pyrolysis at 800 °C for 2 h	NO3−	10	0.06	[66]
Bamboo chip BC				0.15
Japanese cedar BC				0.23
Rice husk BC				0.07
Poultry manure BC				0.02
Pine sawdust BC	Pyrolysis at 500 °C for 4 h	NO3−	30	0.45	[67]
Corn straw BC				0.15
Peanut hull BC				0.38
Rice straw BC				0.15
Wheat straw BC	Pyrolysis at 500 °C for 1.5 h	NO3−	50	0.021	[37]
Corn straw BC				0.032
Peanut shell BC				0.024
Sugarcane bagasse	Feedstock	NO3−	50	2.1	[68]
Sugarcane bagasse BC	Pyrolysis at 200 to 600 °C for 4 h			11. 6
Modified sugarcane bagasse BC	Sugarcane bagasse BC added in epichlorohydrin and N-dimethylformamide			28.2
Sugarcane bagasse BC	Pyrolysis at 500 °C for 1 h	NO3−	50	1.34	This study
Sugarcane bagasse HC30	Hydrothermal carbonisation at 250 °C for 30 min			0.35
Pine sawdust BC	Pyrolysis at 550 °C for 15 min	PO43−	30	10	[47]
	Pyrolysis at 750 °C for 15 min			14
Activated carbon residue BC	Downdraft gasifier 1000 °C	PO43−	50	21	[69]
Oak sawdust BC	Pyrolysis at 500 °C for 30 min	PO43−	61	11	[19]
Sugarcane BC	Pyrolysis at 300 °C for 20 min	PO43−	100	1.63	[16]
	Pyrolysis at 700 °C for 20 min			10.0
Miscanthus BC	Pyrolysis at 300 °C for 20 min			1.83
	Pyrolysis at 700 °C for 20 min			11.4
Corn BC	Pyrolysis at 300 °C for 3 h	PO43−	100	≈35	[17]
	Pyrolysis at 600 °C for 3 h			≈40
Sugarcane bagasse BC	Pyrolysis at 500 °C for 1 h	PO43−	100	2.76	This study
Sugarcane bagasse HC30	Hydrothermal carbonisation at 250 °C for 30 min			0.45
Sewage sludge HC	Hydrothermally carbonised for 5 h at 210 °C 21 and 24 bar	PO43−	150	2.3	[70]
KOH-activated HC	Produced sewage sludge HC was washed with 1 M KOH			14.2
Oak-based hydrochar	Hydrothermal carbonisation at 250 °C	PO43−	400	26.6	[11]
Oak-based BC	Slow pyrolysis at 450 °C			5.5
	Slow pyrolysis at 650 °C			3.6
Poultry manure BC	slowly pyrolysed at 350 °C after Al doping	PO43−	3000	701	[71]
Sugarcane BC	slowly pyrolysed at 350 °C after Al doping			759

presents the adsorption of NH₄⁺, NO₃⁻, and PO₄³⁻ by various BCs and HCs. Our research findings indicate that the quantity of adsorbed ions was influenced by the parameters of the carbonisation process and the initial ion concentrations present in the solution. When evaluating the sorption capacities of different carbonaceous materials, it is imperative to consider these parameters.

Based on the data in Table 3, it appears that the BCs and HCs employed in this study share similar sorption capacities with other adsorbents used in earlier investigations. Higher NH₄⁺ adsorption was observed in HCs derived from oak [11] and HCs activated with KOH [62] compared to HCs derived from other biomass sources. Our HC30s from sugarcane bagasse adsorbed far less ammonium than those in Table 3. When compared to wheat straw BC [34], BC500 was at least ten times more efficient in removing nitrates from the environment.

The sorption capacities of the BCs and HCs examined here are comparable to those of previously investigated adsorbents, as shown in Table 3. Compared to HCs generated from other biomass sources, HCs activated with KOH and HCs derived from oak exhibited much greater NH₄⁺ adsorption. Table 3 displays the ammonium adsorption capacities of several HCs. Sugarcane bagasse HC30 exhibited the lowest value. BC500 was shown to be at least ten times more effective than wheat straw BC at adsorbing ammonium.

Table 3 shows that the BCs and HCs used here compare favourably to other adsorbents in terms of their sorption capacities. KOH-activated HC and oak HC were shown to have much greater NH₄⁺ adsorption than HCs generated from other biomass sources. Table 3 shows that, compared to the other HCs, sugarcane bagasse HC30 was much less effective at adsorbing ammonium. BC500 was at least 10 times more effective at adsorbing ammonium than BC produced from wheat straw.

For the best NO₃⁻ adsorption results, use BC [55] produced from refined sugarcane bagasse. In acidic solutions, adsorption was shown to be more efficient than in basic ones. Table 3 suggests that, among the BCs, BC500 had the highest nitrate adsorption rate.

The highest PO₄³⁻ adsorption capacity of the modified sugarcane BC was achieved through slow pyrolysis at 350 °C with Al enrichment, similar to what was carried out in a prior work [66]. The BCs’ improved P adsorption capacity can be attributed to the material being doped with Al cations, which facilitate bonding between ions and carboxylic groups. Surface irregularity and specific surface area are proven to have a major impact on BCs’ adsorption performance [67]. According to Table 3, alternative HCs, such as sewage sludge HC, KOH-activated sewage sludge HC, and oak-based HC, are more efficacious than the sugarcane bagasse HC30 used in the present study [66]. As a result, the adsorption capacity differs substantially depending on the type of biomass, the degree to which it was pre-treated or activated, and the conditions under which it was manufactured.

4. Conclusions

The utilisation of BCs and HCs has experienced a significant surge owing to their demonstrated efficacy and versatility in various interdisciplinary domains. In the present study, sugarcane bagasse underwent pyrolysis and hydrothermal carbonisation at various temperatures to yield BC and HC. As the pyrolysis temperature was elevated for the BCs, there was a noticeable increase in the fixed carbon, surface area, pore volume, and ash content, whereas the volatile matter and the H/C and O/C atomic ratios experienced a decrease. The observed trend indicated an increase in the fixed carbon content, ash content, and O/C and C/N ratios, as well as volatile matter and H/C ratios, with an increase in residence time for the hydrocarbons. Upon the adsorption of NH₄⁺, NO₃⁻, and PO₄³⁻, a reduction in the intensities of carboxyl and ester functional groups was observed in the FTIR spectra of both the BCs and HCs. Furthermore, the research findings demonstrate that the Langmuir isotherm model exhibited a marginally better conformity than the Freundlich isotherm model, implying that the adsorption process was homogeneous across all adsorption sites. At elevated temperatures, the exothermic reaction pertaining to the adsorption of the three ions was expedited. The adsorption capacity of the BC and HC adsorbents was observed to be significantly influenced by the type of biomass employed, the extent of pre-treatment or activation of the chars, and the preparation parameters. However, in the case of ammonia adsorption on the BCs, the functional polar groups present in the material underwent a reaction with NH₄⁺ during the adsorption process and had a much stronger effect than the surface area.