Coal Discards and Sewage Sludge Derived-Hydrochar for HIV Antiretroviral Pollutant Removal from Wastewater and Spent Adsorption Residue Evaluation for Sustainable Carbon Management

Abstract

The effects of various parameter interactions on the textural structure of hydrochars produced via hydrothermal (HTC) and co-hydrothermal (Co-HTC) treatments of coal discards and sewage sludge (wastes), as well as the subsequent use of the hydrochars (HCs) synthesized for HIV drug (nevirapine and lamivudine) removal from wastewater, were investigated in this study. The HTC and Co-HTC process improved the carbon content of the raw material by 13.47%, 7.08%, and 30.65% for hydrochar coal tailing (HCT), hydrochar coal slurry (HCS), and hydrochar from coal–sewage blend (HCB), respectively. The Co-HTC-derived HCB had a high S_BET of 20.35 m²/g and pore volume of 0.38 cm³/g, leading to significant adsorptive reductions of nevirapine (NEV) and lamivudine (LAM) (97.19% and 93.32%, respectively). HCT and HCS displayed high NEV and LAM adsorption capacities (50 mg g⁻¹, 42 mg g⁻¹ and 52 mg g⁻¹, 41 mg g⁻¹), respectively, despite being less effective than HCB (53.8 mg g⁻¹, 42.8 mg g⁻¹). In addition, the use of spent adsorption residues for energy storage applications was investigated further. The findings showed that spent adsorption residues are an effective carbonaceous material precursor to produce electrical double-layer capacitors (EDLCs).

1. Introduction

South African coal beneficiation plants produce more than sixty million tons of coal waste annually [1]. The majority of these coal wastes are disposed of in tailings piles and slurry dams [1]. As a result, concerns about the solubilization and/or volatilization of toxic substances in coal waste are seen as a major threat to the nation’s environmental waste management [2,3]. South Africa’s population has grown rapidly during the past two decades, from 48.8 million in 2005 to 51.58 million in 2010 and 57.7 million in 2018. More than half the population is currently living and working in densely populated urban areas [4]. This rapid population growth, combined with growing urbanization, has put additional strain on existing wastewater treatment plant (WWTP) infrastructure, resulting in increased sewage sludge production at these facilities [5]. Several lines of evidence suggest that landfilling sewage sludge is not a sustainable solution for sewage sludge management [6]. Despite the potential concerns regarding soil and subsoil pollution, roughly 80% of wastewater treatment plants continue to dispose of their sewage sludge in specified locations [7]. The need for innovative techniques to minimize sewage sludge waste quantities has become an environmental and societal imperative.

On the other hand, data from the World Health Organization (WHO) g Lobal Health Observatory report that 70% of the estimated 36.7 million persons g Lobally living with HIV/AIDS at the end of 2016 were from Africa [8]. South Africa’s antiretroviral therapy (ART) program is the world’s largest, with an estimated 3.9 million people receiving antiretrovirals (ARVs), accounting for roughly 24% of the g Lobal ART program as an estimated 19.8 million people use ART in the world. ARVs are used to treat and prevent a variety of viral infections, including HIV, influenza, hepatitis, and herpes [9]. In South Africa, they are mainly used for the treatment of HIV/AIDS. South Africa uses the most antiretroviral drugs per capita of any country [10,11]. Although several studies have shown that ARVs are sterilized during wastewater treatment, little work in the literature has been done on the presence of dissolved ARVs, their metabolic products, and removal strategies in wastewater [9]. P Recent research in this area by Kebede (2020), Ndilimeke Akawa (2021), and Zitha (2022) demonstrated the existence of antiviral and other pharmacological compounds in bodies of water [12,13,14]. According to these studies, nevirapine and lamivudine were found in South African ambient water at a maximum concentration of 1.640 ng·L⁻¹, posing a substantial environmental risk due to their potential influence on aquatic life and medication resistance in humans [14]. As a result, there is a need for enhanced pharmaceutical component removal methods from water bodies. So far, research into the removal of pharmaceutical components from water bodies has been limited, owing to the high expense of prospective researched strategies such as adsorption, degradation, and phytoremediation [12,14]. Furthermore, recent research conducted by Späth (2021) employing biochar as adsorbents revealed a moderate ARV removal efficacy in wastewater, which may be attributed to the textural properties of the adsorbent materials as well as the adsorption process parameters employed [15]. Despite the fact that a significant amount of biochar (100 g/L) was required for the efficient removal of nevirapine and lamivudine from wastewater at ambient temperatures (rather than the 45 °C reported in most studies), Späth’s research demonstrated that the results were comparable to those achieved using high-cost synthetic adsorbents and that there was still potential for improvement. South Africa has a high HIV/AIDS prevalence, and it is anticipated that an increasing load of antiretroviral drugs may circumvent the WWTP systems and infiltrate the country’s water networks [16,17,18]. The premise of this work is that the physicochemical and textural characteristics of HTC-derived biochar may be enhanced for usage as low-cost potential adsorbents for pharmaceutical component removal from water bodies.

In this study, we focus on two antiretroviral drugs, namely (i) Nevirapine (NVP), which is an oral drug used to treat and prevent retroviral infections, particularly those caused by human immunodeficiency virus type 1 (HIV-1). HIV-1 is a virus that primarily infects CD4-T cells, which are immune system cells. HIV-1 ART, on the other hand, neither cures nor destroys the virus; rather, it suppresses or delays its reproduction [19]. It is typically used in conjunction with other ARV medicines. Nevirapine is a non-nucleoside reverse transcriptase inhibitor (NNRTI) antiretroviral medication (ARVD) commonly used to prevent HIV transmission to the fetus. The nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) impede the HIV viral RNA’s reverse transcription into its DNA, which is essential before the virus can integrate into the host cell. This is accomplished by interfering with the viral enzyme reverse transcriptase, which oversees the performance of this activity. After intake, it is excreted in urine at a rate of 2.7% [20,21]. (ii) Lamivudine (3TC) is a first-generation nucleoside reverse transcriptase inhibitor that was licensed in 1995 for the treatment of HIV-1 infection and hepatitis B virus (HBV) infection. EPIVIR (2013); EPIVIR–HBV (2016). Since 2002, the WHO has recommended HIV treatment regimens, and 3TC or emtricitabine (FTC) are almost always the preferred components of fixed-dose combos (WHO, 2016). Only one study used HPLC/MS to determine its content in water, which was less than 1 ng·L⁻¹ [22,23,24].

The release of micropollutants into the environment via wastewater treatment plant (WWTP) effluents is one of the most important concerns with micropollutants in sanitation. For a long time, these discharges were recognized as a major source of micropollutant introduction into the environment, and their centrality to environmental contamination has been established [22,23]. Pharmaceutical residues and hormones, pesticides, phthalates, artificial sweeteners, and personal care products are all detected in these discharges at trace amounts (ng/L to µg/L) [23,24]. One of the current challenges is the removal of micropollutants from WWTP discharges. This is crucial for creating cutting-edge innovations, guiding management strategies, and optimizing treatment operations and routes to reduce micropollutant concentrations in discharges and hence in the environment. It is well-known that traditional WWTPs significantly reduce hydrophobic, volatile, or biodegradable chemicals while having little or no effect on many micropollutants such as pharmaceutical compounds [22,23,24,25]. As a result, there is a need for additional pollution reduction solutions, and the implementation of additional tertiary treatments aimed at the eradication of micropollutants is a viable option; among these the use of activated carbon adsorption, in particular, appears to be a highly adaptable, efficient, and fairly priced method.

Though activated carbon adsorption has been widely used in drinking water treatment, it has only recently attracted more attention in wastewater for the elimination of micropollutant concentrations in WWTP discharges. There is relatively little research on the elimination of micropollutants, and in particular, of ARVs micropollutants from WWTP discharges [26,27,28,29,30].

This is the motivation behind this research, which aims to synthesize low-cost AC from sewage sludge and coal discards for the removal of ARV micropollutants from WWTPs, thereby contributing to the closure of the waste loop in WWTPs by optimizing the carbon yield of HCs synthesized from CD and SS, examining their efficiency for the adsorption of dissolved pharmaceutical micropollutants in water (nevirapine and lamivudine), and then investigating the use of the spent adsorption residues obtained for energy storage applications.

2. Experimental Method

2.1. Materials

The standard non-nucleoside nevirapine (NEV) and nucleoside lamivudine (LAM) reverse transcriptase inhibitor tablets supplied by Dischem pharmacy of South Africa with 99.9% purity were utilized for adsorption tests. The tablets were crushed and dissolved in ultra-pure water obtained from a Direct-Q^®3 with a pump (Millipore, Merck system). The dissolution was carried out in a laboratory g Lass beaker using a predetermined volume of ultra-pure water (pH 6.8) at 20 °C for stirring speed of 50 rpm and time of 30 min.

Coal tailing (CT) and coal slurry (CS) were collected from a coal beneficiation plant in Mpumalanga, South Africa, for use in this study (Figure 1A,B). Before use, the collected samples were dried in an ambient atmosphere, loaded, and transported in airtight bags to the laboratory, then dried once more at room temperature. The dried samples were mixed for homogenization and split into two portions from which one was screened into various particle sizes and the other was pulverized for physicochemical characterization (proximal, ultimate, total sulfur, SEM, and FTIR). The sewage sludge (SS) used in this study (Figure 1C) was collected from a wastewater treatment plant (Ekurhuleni Water Care Company (ERWAT)) and split into two portions. The first portion was dried in a laboratory dryer at 105 °C for 24 h and crushed for physiochemical characterization, while the second portion was used for HTC tests as received.

2.2. Methods

Batch experiments were conducted to evaluate the adsorption performance of the synthesized HCs for the removal of NEV and LAM from synthetic aqueous solutions made using ultra-pure water at an initial concentration of 20 μmol L⁻¹. The adsorption test was performed in volumetric polypropylene flasks of 200 mL. The flasks were filled with a mixture of HCs and prepared aqueous solutions (containing the dissolved medicines) and shaken on a Platform shaker at 80 rpm at 25 °C. (Labcon-Instrulab). Two adsorption parameters were considered: the concentration of adsorbent^® and the residence time (t). For the experiments, 1 g of each HC was combined with a similar amount of water solution. To assess the adsorption of NEV and LAM on the selected HCs concentration, 10 g L⁻¹, 20 g L⁻¹, 50 g L⁻¹, 100 g L⁻¹, and 300 g L⁻¹ were used for a range of residence duration between 10 and 360 min. After allowing the mixtures to equilibrate for 24 h to guarantee saturation, they were centrifuged for 15 min at 1500 rpm. The supernatant from each flask was then filtered through a 0.22 µm Whatman filter paper. The filtrates were then tested against corresponding produced aqueous solutions using a UV-1800 spectrophotometer and the gas chromatograph linked to the mass spectrometer Shimadzu (GCMS-2010) The experiments were repeated three times to guarantee the accuracy of the results. The dissolved drug concentrations (NEV and LAM) in the initial aqueous solutions were used as a reference for calculating adsorption percentages using Equation (1):

Adsorption (%) = (Ci − Cf) /Ci × 100

(1)

where Ci (µmol L⁻¹) is the concentration of the transcriptase inhibitor drugs in the corresponding initial solution and Cf (µmol L⁻¹) is the remaining drug concentration in the liquid phase at the end of the adsorption experiment with each HCs.

The hydrothermal carbonization (HTC) and co-hydrothermal (Co-HTC) experimental test runs were designed using Design-Expert software (DoE) (Version 13, State–Ease, Inc., Minneapolis, MN, USA). For experimental designs, the central composite design (CCD) and the custom design (CD) were utilized to know the number of runs that will be required to optimize the HTC and Co-HTC runs, respectively. HTC experiments were conducted using variations in temperature, pressure, and time ranging from 150 to 270 °C, 10 to 27 bar, and 10 to 180 min, respectively. However, the Co-HTC experiments were performed using variations in residence time ranging from 10 to 360 min for the same range of temperature and pressure utilized in HTC experiments, i.e., 150 to 270 °C, 10 to 27 bar, respectively. The Co-HTC experiments used a coal feedstock combination (CT + CS) of 50% coal tailing and 50% coal slurry. For the sake of simplicity, a 1:1 coal tailing to coal slurry blend was adopted. The coals–sewage sludge blend ratio (CT + CS: SS) used for Co-HTC experiments varied from 0:25 to 25:0 (wt:wt). High fixed carbon (FC) content hydrochars by means of the DoEwere selected using the response surface methodology (RSM) and the optimal combined methodology (OCM) techniques for HTC and Co-HTC, respectively. RSM and OCM were adopted to understand the interactions of the HTC and Co-HTC factors on the fixed carbon contents of the HC produced from the individual CT, CS, and from the mixture of coals with sewage sludge, respectively. In addition, RSM and OCM were used to develop appropriate models to enable the prediction of the optimum operating conditions required to produce high-carbon content hydrochars. The analysis of variance (ANOVA) was used to evaluate the statistical significance of the designed models.

The spent HC residues from nevirapine and lamivudine adsorption experiments were chemically activated using a KOH reagent to produce activated carbons (ACs). Several studies on the activation process of coal-based materials have shown that the impregnation ratio of KOH/carbonaceous material (CM) has an effect on the textural features of the generated ACs [27,31,32]. Furthermore, a recent study done by Abdulsalam Jibril et al. (2019) found that increasing the activation temperature from 400 °C to 800 °C and the KOH/CM impregnation ratio from 1/1 to 4/1 increased the specific surface area (SBET) of the generated ACs. SBET values of 1925 m²/g for run-of-mine coal, 1826.41 m²/g for coal waste, and 1484.96 m²/g for coal slurry ACs were obtained [33]. The optimum conditions established by Abdulsalam Jibril et al. [34] were adopted in this investigation to synthesize ACs from nevirapine and lamivudine adsorption tests. To remove residual alkali, the ACs were rinsed with 0.5 N HCl and then distilled water [34]. The washed samples were then dried in an oven at 60 °C overnight before being kept in airtight bags for physicochemical analysis. ACs were labeled as follows: AC–HCT–NEV, AC–HCT–LAM, AC–HCS–NEV, AC–HCS–LAM, AC–HCB–NEV, and AC–HCB–LAM. Because of the abundant charge storage sites on the highly porous surface of ACs, they may be used as electrode materials in supercapacitors [35]. As a consequence, ACs derived from nevirapine and lamivudine adsorption tests were evaluated as electrode materials in ionic solutions (Na (+) SO₄ (2−)).

2.3. Physicochemical and Textural Analyses

The concentrations and dissolved organic compounds of transcriptase inhibitor drugs (NEV and LAM) in the initial solution and produced filtrates from adsorption experiments were determined using the UV-1800 spectrophotometer and the gas chromatograph linked to the mass spectrometer Shimadzu (GC-MS-2010). The drug release concentration (CR%) was evaluated at a wavelength of 270 nm. The GC-MS instrument was set at an initial temperature of 140 °C. The initial temperature was held for five minutes then increased to 50 °C at a heating rate of 4 °C/min and maintained for 12.5 min. The samples were injected at 220 °C using the spitless mode. The analysis was performed using the following procedure: sampling time: 1.00 min; flow control mode: pressure: 100.00 kPa, total flow: 50.00 mL/min; column flow: 1.13 mL/min; linear velocity: 3.00 mL/min. The identification of the organic compounds in the PW was conducted using the NIST 14 database and compared with published mass spectra.

Spectra of nevirapine and lamivudine standards were built in the range from 200 to 350 nm using quartz cuvettes with 1 cm of path length and water as blank. Solutions of NEV and LAM standards at 15 g mL⁻¹ prepared in water were used in this analysis. Ten tablets of each drug were weighed and powdered. The equivalent of 150 mg of nevirapine and lamivudine was weighed and transferred into a 200 mL volumetric flask with ultra-pure water. This solution was filtered, and 1.0 mL was transferred into a 100 mL volumetric flask using water to obtain a solution at 15 g mL⁻¹. A calibration curve with five points, in the range from 5 to 25 g mL⁻¹, was produced at the wavelength of 270 nm (ICH Q2 (R1) 2005).

A variety of analytical techniques were used to compare the physicochemical properties of the initial absorbent (HCs) and the residues obtained after adsorption experiments, namely the proximate analysis of 1 g of −212 µm of the sample was carried out using the Thermo Gravimetric analyzer equipment (Leco TGA 701) in accordance with ASTM D5142. The FTIR analysis was conducted on a spectrometer Perkin Elmer Spectrometer coupled with a diamond attenuated total reflectance accessory in the wavenumber range between 4000 and 450 cm⁻¹. The interpretation of the spectra was performed based on reference tables provided by Smith Brian (2018). The scanning electron microscopy (SEM) was done using a Carl Zeiss Sigma Field scanning electron microscope connected to the Oxford X-act EDS detector. The analysis was carried out in this procedure according to the operational mode of the equipment. The SEM/EDS analysis settings were on 10 kV and a working distance (WD) of 7.2–8.2 mm using a backscattered electrons (BSE) signal. The SEM/EDS analysis provided the surface structure and the homogeneity of the samples [36]. The porous properties were obtained from Brunauer Emmet Teller analysis (BET) using a nitrogen adsorption test at 77 K. For BET analysis, the autosorb iQ gas sorption instrument (Quantachrome Instruments, Boynton Beach, FL, USA) was used while the Quantachrome^® ASiQwinTM software (Boynton Beach, FL, USA), which interfaces the autosorb iQ to a computer, was used for data acquisition and data reduction.

3. Results and Discussion

3.1. HTC and Co-HTC Statistical Analysis

Table 1. Hydrothermal carbonization and co-hydrothermal carbonization are optimum operating solutions.

Name	HCT	HCS	HCB	POAE
Parameters
Temperature (°C)	150	150	208.70	0–0.33
Pressure (bar)	27	27	22.55	0–0.22-
Time (Min)	92.13	10	331.02	0.35–7.4
CT + CS: SS	-	-	20.02:4.98	1.96
Results (%) on a dried basis
Ash content	38.13	21.12	17.43	0.07–1.09
Fixed Carbon	40.20	57.19	58.88	0.5–1.21
Volatile Matter	21.67	21.69	23.69	1.08–0.37
Mass yield	96.18	96.05	84.67	1.35–2.00

HCT: Hydrochar coal tailing. HCS: Hydrochar coal slurry. HCB: hydrochar from the Co-HTC of coals and sewage sludge. SS: Sewage sludge. POE: Percentage of absolute error.

shows the optimum parameters evaluated using the design of experiment (DoE version 13) on the HTC (coal tailing and coal slurry) and Co-HTC (coal sewage sludge blend) experimental results obtained from the proximate analysis. From the ANOVA, the regression models (Equations (2)–(4)) developed using response surface methodology (RSM) for HTC and optimal combined methodology (OCM) for Co-HTC were then used to assess the impact of the process factors on the fixed carbon (FC) of the produced hydrochar (HCs). Due to the high F-value and low p-value, the results show that all models are of statistical significance [33]. Temperature, pressure and residence time, and the coal–sewage sludge blend ratio all influenced the FC that the HCT, HCS, and HCB produced. The regression models obtained from the RSM of the HTC and OCM of Co-HTC experimental data revealed that temperature and the coal–sewage sludge blend ratio are the most significant model terms.

$$\begin{array}{cc}\hfill HCT Fixed Carbon& \\ & =37.66-1.20A+0.1090B+0.0040C-0.0837AB\hfill \\ & -0.0388AC+0.0337BC+1.04A^2+0.113B^2-0.2714C^2\hfill \end{array}$$

(2)

$$HCS Fixed Carbon=53.01-3.29A+0.5520B-0.3410C$$

(3)

$$\begin{array}{cc}\hfill HCB Fixed Carbon& \\ & =30.46A+45.36B+55.13AB+9.11AC+6.58AD\hfill \\ & +13.69AE-0.6847BC-0.1279BD-4.28BE-15.83ABC\hfill \\ & -10.52ABD+14.57AB\hfill \end{array}$$

(4)

where A is the temperature, B is the pressure, C is the time, D is the mass of sewage sludge (SS), and E is the mass of the mixture of coal tailing and coal slurry (CT + CS).

The effects of operating conditions on the FC of produced hydrochars (HCs) are depicted in Figure 2 and Figure 3. The curvature seen in Figure 2A and Figure 3 describes the significant impact of the interaction between the HTC and Co-HTC parameters. A linear slope observed in Figure 2B indicates insignificant interactions (high p-values > 0.2) between the HTC of CS process parameters. As a result, quadratic and linear models accurately fitted the HTC of CT, Co-HTC of CB, and HTC of CS, respectively. Temperatures above 150 °C reduce the FC of the produced HCT and HCS. However, increasing the temperature to 208 °C and the (CT + CS: SS) ratio to 5:1 increases the FC of the produced HCB. The results shown in Figure 3 show that an interaction effect of reaction temperature and coal–sewage sludge mass ratio had a significant effect on the FC of produced HCB.

3.2. Optimum Operating Conditions

The optimization of input factors and response surface constraints concluded the analysis and evaluation of the resulting data. The goal was to assess the input variables in order to maximize FC for the generated HCs. The input parameters for the HTC process ranged from 150 °C, 10 bar, and 10 min as lower values and 270 °C, 27 bar, and 180 min as upper limits. On the other hand, the Co-HTC produced optimization solutions that were established using input factors of 150 °C, 10 bar, 10 min, and 0:25 g (CT + CS: SS) as lower limits and 270 °C, 27 bar, 360 min, and 25:0 g (CT + CS: SS) as the upper limits. The conditions that produced the highest desirability factor were selected as the optimum process parameters. The optimum HTC conditions obtained were 150 °C, 27 bar, and 92.13 min, while the ideal Co-HTC conditions were 208 °C, 22.5 bar, 331 min, and 20.02:4.98 (CT + CS: SS mass ratio). Results in Table 1 show that the optimized predicted results were in agreement with the experimental measurement.

absolute error (POAE) (fraction) = (Experimental-Predicted)/Experimental

(5)

However, results from this study are more conservative than those obtained in previous research that used a different feedstock to optimize HTC and Co-HTC operating conditions [37]. However, the implemnetation of the optimal conditions found in this study could have a significant positive effect on the economics of the HTC and Co-HTC process, and so this work lays the groundwork for the development and economic feasibility of the operating conditions used for other similar thermal processes, such as pyrolysis and gasification [37,38].

3.3. Adsorption Results

Table 2. Experimental results of NEV and LAM adsorption on the produced hydrochars.

Adsorbent (HC) Concentration and Residence Time			HCT							HCS							HCB
	NEV (μg L−1)		STD	pH	LAM (μg L−1)	STD	pH	NEV (μg L−1)		STD	pH	LAM (μg L−1)	STD	pH	NEV (μg L−1)		STD	pH	LAM (μg L−1)	STD	pH
Raw solution	5.53			5.58	4.58		5.34	5.53			5.58	4.58		5.34	5.53			5.58	4.58		5.34
HCs concentration (gL−1) for a residence time of 90 min
10	3.87		1 × 10³	5.83	3.37	1.8 × 10³	5.44	4.42		2.3 × 10³	5.67	3.87	1.7 × 10³	5.48	3.81		2.7 × 10³	5.67	4.13	2.1 × 10³	5.46
25	2.41		1 × 10³	6.02	2.35	1.8 × 10³	5.76	3.98		2.3 × 10³	5.94	3.61	1.7 × 10³	5.61	1.94		2.7 × 10³	5.89	3.16	2.1 × 10³	5.59
50	0.81		1 × 10³	6.21	1.63	1.8 × 10³	5.98	1.18		2.3 × 10³	6.17	1.94	1.7 × 10³	6.01	0.65		2.7 × 10³	6.05	0.96	2.1 × 10³	5.77
100	0.53		1 × 10³	6.38	0.37	1.8 × 10³	6.08	0.33		2.3 × 10³	6.42	0.47	1.7 × 10³	6.17	0.15		2.7 × 10³	6.1	0.30	2.1 × 10³	5.84
300	0.68		8.1 × 10³	6.52	0.47	1.8 × 10³	6.14	0.44		2.3 × 10³	6.48	0.55	1.7 × 10³	6.25	0.26		2.7 × 10³	6.22	0.40	2.1 × 10³	5.91
Time (minutes) for an HC concentration of 100 g L−1
10	1.57		8.1 × 10³	5.92	2.75	1.8 × 10³	5.59	2.60		2.3 × 10³	6.08	2.37	1.7 × 10³	5.79	1.27		2.7 × 10³	5.67	1.66	2.1 × 10³	5.48
30	0.77		1 × 10³	6.12	0.96	1.8 × 10³	5.87	1.35		2.3 × 10³	6.24	0.78	1.7 × 10³	6.05	0.62		2.7 × 10³	5.88	0.75	2.1 × 10³	5.61
90	0.53		1 × 10³	6.38	0.37	1.8 × 10³	6.08	0.33		2.3 × 10³	6.42	0.47	1.7 × 10³	6.17	0.15		2.7 × 10³	6.11	0.30	2.1 × 10³	5.84
180	0.53		1 × 10³	6.38	0.37	1.8 × 10³	6.08	0.33		2.3 × 10³	6.42	0.47	1.7 × 10³	6.17	0.15		2.7 × 10³	6.11	0.30	2.1 × 10³	5.84
360	0.53		1 × 10³	6.38	0.37	1.8 × 10³	6.08	0.33		2.3 × 10³	6.42	0.47	1.7 × 10³	6.17	0.16		2.7 × 10³	6.11	0.30	2.1 × 10³	5.84

presents the results of the NEV and LAM adsorption experiments conducted with the synthesized HCs. The uptake results for the different HCs (HCT, HCS, and HCB) are shown in Figure 4A,B, respectively. The highest uptake of NEV and LAM was achieved at 90 min and at an adsorbent concentration of 100 g/L. For all the tested HCs, increasing the process time (t) and lowering the adsorbent concentration improves NEV and LAM uptake. This is in line with Yang’s (2007) finding that as process time and adsorbate concentration increase, the proportion of saturation in the adsorption process increases [39]. This happens because of the adsorbate remaining in contact with the adsorbent surface for a long time [40]. The adsorbate eventually saturated the porous media as there was a plateau after 90 min (Figure 4B). Additionally, as the adsorbent concentration increased, the uptakes of NEV and LAM on all HCs used decreased. The higher the adsorbent concentration, the less readily dissolved drugs moved from the liquid (AS) to the solid pores (HC), and the stronger the driving force of liquid adsorption into the solid (HC) [41].

HCT: Hydrochar coal tailing. HCS: Hydrochar coal slurry. HCB: hydrochar from the blend of coal tailing, coal slurry, and sewage sludge. NEV: Nevirapine. LAM: Lamivudine. HC/AS: hydrochar–aqueous solution (charged with inhibitor drugs dissolved) solid/liquid ratio. STD: Standard deviation. The results demonstrate that the HCB has a higher drug uptake performance (5.37 μg L⁻¹ of NEV and 4.2737 μg L⁻¹ of LAM) than the HCT and HCS. This may be due to HCB’s higher porous textural structure (high S_BET and pore volume). In all cases, NEV uptake was higher than LAM uptake under similar operating conditions. This could be attributable to NEV’s higher capillary uptake degree in aqueous solution compared to LAM. Overall, the highest NEV and LAM uptakes were 90.32% and 91.78% for HCT, 93.97% and 89.67% for HCS, and 97.19% and 93.32% for HCB.

The results obtained showed that the HCB owned the highest NEV and LAM uptake efficiency (Figure 4). Hence, the GC-MS analysis was performed on the final filtrates from the HCB adsorbent to identify the remaining organic elements dissolved.

Table 3. Organic compounds in filtrate from NEV adsorption by HCB.

Initial Solution from the Mixture (HCB + Nevirapine)				Final Solution after Adsorption (HCB + Nevirapine)
Peak	Retention Time (minutes)	Organic compound Name	Conc (%)	Retention Time (minutes)	Organic compound Name	Conc (%)
1	3.287	Cyclotrisiloxane, hexamethyl- (CAS)	4.19	11.536	Hexadecanoic acid (CAS)	26.19
2	3.412	Cyclotrisiloxane, hexamethyl- (CAS)	21.28	12.515	9,12-0ctadecadienoic acid (Z, Z)–(CAS)	35.02
3	3.557	Propanoic acid, 2-hydroxy- (CAS)	4.15	12.615	Octadecanoic acid	23.82
4	3.655	DIHYDROXYACETICACID	0.93	12.899	Hexadecanoic acid, (3-bromoprop-2-ynyl)	2.7
5	3.72	Propanal, 3-ethoxy- (CAS)	1.01	13.018	Hexadecanoic acid, 1-[[[(2-aminoethoxy)hydroxyphosphinyl]oxy]methyl]-2-[(1-oxotetradecyl)oxy]	1.52
6	4.382	Propanoic acid, 2-hydroxy- (CAS)	19.64	13.495	Cis-11-Eicosenoic acid	0.97
7	4.72	trisiloxane, 1,1,1,5,5,5-hexamethyl-3	0.84	13.596	Eicosanoic acid	1.89
8	4.765	Cyclotetrasiloxane, octamethyl- (CAS)	7.61	13.72	SII.1CONE OIL	0.48
9	4.948	1,3-Dioxane (CAS)	2.19	13.809	9,12-0ctadecadienoyl chloride, (Z,Z)-	0.67
10	6.025	0-TRIS-TRIMEfHYLSILYI.rEPIN	0.97	13.94	Cis-9-Hexadecenal	0.98
11	6.098	Cyclopentasiloxane, decamethyl- (CAS)	4.5	14.344	11-Cyclopropyl-4-methyl-5, ll-dihydro-6.	3.76
12	6.87	3,3,5-Triethoxy-1,1,1,7,7,7-hexamethyl-5	0.33	15.003	Silane, dimethyl(dimethyl(3-methylpentyl)	0.38
13	7.461	Cyclohexasiloxane, dodecamethyl	2.18	17.538	Docosane (CAS)	0.33
14	8.695	3-Ethoxy-1,1,1,7,7,7-hexamethyl-3,5,5	0.51	18.814	Tetratetracontane (CAS)	0.68
15	8.86	1-Butylimidazole	3.39	20.38	Tetratetracontane (CAS)	0.61
16	8.915	Emylcamate	1.7
17	8.985	N-(4-ISOPROPYLBENZVL) -N,1-DIME	1.51
18	9.794	trimethylsilyl [2,3-di(trimethylsiloxy)pher	0.84
19	10.035	Chloro(dimethyl)phosphine	0.29
20	10.125	Erythro-dl-0-ethylthreonine	0.28
21	10.734	Trimethylsilyl) -{bis[methyl(trimethylsilyh	0.36
22	11.572	3-Butoxy-1,1,1,7,7,7-hexamethyl-3,5,5	0.59
23	11.65	Histidine, 4-chloro-2-trifluoromethyl	0.46
24	12.155	1-Methyl-l-n-tetradecyloxy-1-silacyclope	0.27
25	12.23	Alpha-(N-methoxy-N-(2;1.;l.-trifluoroethanol	0.62
26	12.338	1,1,1,3,5,7,7,7-0ctamethyl-3,5-bis (trime	0.68
27	12.83	Acetamide, N-(2-cyano 4,5-dimethoxy pht	0.56
28	12.945	Silane, l,l’-[oxybis(methylene sulfonyl-2,1	0.68
29	12.985	Carbonic acid, isohexyl 2;1.;1.-trichloroeth	0.57
30	13.025	EICOSAMETHYLCYCLODECASILQ)	0.77
31	13.185	Succinylacetone diethoxime, trimethylsily	0.29
32	13.265	Pentanoic acid, butyl ester (CAS)	0.29
33	13.373	Morpholine, 4-(2,4-dinitrophenylbenzoyl)	0.63
34	13.585	d-Altronic acid	0.32
35	13.67	Butyl heptanoate	0.56
36	13.719	Heptasiloxane, hexadecamethyl	1.07
37	14.03	N-Acetyl- alpha-aminooxy-propionic acic	0.45
38	14.392	Benzene, (1-methoxyethenyl)	0.65
39	14.501	Heptasiloxane, hexadecamethyl-(CAS)	0.62
40	15.314	5-Chloro-IH-l;1.,4-triazole-3-carboxamid	0.42
41	15.435	Benzeneacetic acid, alpha.,3,4-tris[(trike	1.55
42	15.855	(1-methylethyl)-thiirane	0.36
43	15.92	Ethyl 2-nitropropionatc	0.62
44	17.548	Nonane, 5-(1-methylpropyl} -	0.93
45	18.824	Hepladecane, 2,6,10,15-tctramcthyl- (CA	2.94
46	20.26	D-Glucose, 6-acctamido-6-deoxy-, diethy	0.61
47	20.398	Sulfurous acid, octadccyl 2-propyl ester	2.2
48	20.47	1-Phenazinecarboxylic acid, 6-[l-[(l-oxo)]]	0.28
49	22.255	1,2,4-Triazolo [3,4-a] phthalazine-7-hydrn	0.26
50	22.318	Octadccane, 5-methyl	1.05
			100

Conc: concentration.

and

Table 4. Organic compounds in filtrate from LAM adsorption by HCB.

Initial Solution from the Mixture (HCB + Lamivudine)				Final Solution after Adsorption (HCB + Lamivudine)
Peak	Retention Time (minutes)	Organic compound Name	Conc (%)	Retention Time (minutes)	Organic compound Name	Conc (%)
1	3.34	Cyclotrisiloxane, hexamethyl- (CAS)	1.54	3.35	Cyclotrisilox ane, hexamethyl- (CAS)	0.29
2	3.375	3,4-Dimethylbenzoicacid, TBDMS deri•	0.7	3.518	Cyclotrisiloxane, hexamethyl- (CAS)	0.56
3	3.441	Cyclotetrasiloxane, octamethyl- (CAS)	8.83	4.81	Cyclotetrasiloxane, octamethyl- (CAS)	0.24
4	4.56	Cyclotetrasiloxane, octamethyl- (CAS)	0.54	5.467	1,2,3-Propanetriol (CAS)	98.52
5	4.63	BENZENE, (1,1-DIMETHYLETI YL)	2.55	6.28	Disiloxane, l, l,3,3-tetramethyl-l,3-bis [3]	0.39
6	4.704	Tridecane (CAS)	2.11
7	4.792	Cyclotetrasiloxane, octamethyl- (CAS)	7.76
8	5	Butyric acid, 2-phenyl-, hept-2-yl ester	1.06
9	5.364	Heptane, 5-ethyl-2-methyl-(CAS)	1.3
10	5.44	Ethyl neopentyl carbonate	0.47
11	5.612	l-MEfHYL-2-PHENYLCYCLOPROPJ	3.03
12	5.718	Undecane (CAS)	1.81
13	6.099	Cyclopentasiloxane, decamethyl- (CAS)	0.66
14	6.153	Cyclopentasiloxane, decamethyl- (CAS)	0.84
15	6.757	Dodecane (CAS)	2.9
16	6.875	Carbonic acid, 2-ethoxyethyl neopentyl	0.63
17	7.165	Oxalic acid, hexyl neopentyl ester	0.76
18	7.19	2-Propyl-5-oxohexanoic acid	0.86
19	7.234	1-Heptadecanamine (CAS)	2.81
20	7.465	CYCLOHEXASILOXANE, OODECM	1.1
21	7.531	Tridecane (CAS)	4.51
22	8.011	Tetradecane (CAS)	1.59
23	8.269	Hexadecane (CAS)	2.32
24	8.33	5-Allyl-6-methyl-2-phenyl-5,6-dihydro-4	0.54
25	8.695	(E)-2-[(tert-butyl) dimethylsily! J-2- [I’-me	1.93
26	8.746	Octadecane, l,1’- [1,3-propanediylbis (ox)	0.95
27	8.941	Pentadecane (CAS)	3.08
28	9.144	1-Bromo-3,7-dimethyloctane	0.57
29	9.235	Decanoic acid, nonyl ester	0.59
30	9.264	PENTASIWXANE, 1, l,3,3,5,5,7,7,9,9-	1.6
31	9.42	Dodecane, 1,2-dibromo- (CAS)	0.51
32	9.583	Pentadecane (CAS)	2.12
33	10.198	Heptadecane (CAS)	2.31
34	10.435	Tridecane, 6-propyl-	1.92
35	10.575	3beta,17beta-Diacetoxy-17-isopregn-5-e	0.45
36	10.772	Octadecane (CAS)	1.29
37	10.875	9-Oxa-10-thiatricyclo [3.3.I. I (2,7) Jdecar	0.63
38	11.019	Undecane, 6-ethyl-	0.73
39	11.105	(−)-lsopulegol	0.51
40	11.161	DIMETHOXYGLYCEROLDOCOSYL	0.55
41	11.324	Tricosane (CAS)	1.06
42	11.843	NONADECANE	1.57
43	12.346	Tetradecane (CAS)	1.07
44	12.825	Heptacosane (CAS)	0.69
45	12.91	Pentasiloxane, 1,1,3,3,5,5,7,7,9,9-decam	0.5
46	13.01	1-Hcptcnc, 2,6,6-trimethyl-	0.7
47	14.433	11-Cyclopropyl-4-methyl-5, l l--dihydro-t	21.89
48	15.435	Benzene, 1-(1, l-dimethylcthyl) -4-(2-ethyl	0.44
49	18.775	Pentasiloxanc, dodecamethyl- (CAS)	0.55
50	19.056	3-Thiazolidinccarboxamidine, 2-imino-	0.57
			100

Conc: concentration.

demonstrate that the final adsorbates include fewer dissolved organic components than the antiretroviral-seeded solutions. Organic acids, sugars, and methyl compounds were among the organic molecules identified in traces in the final adsorbates. Further chemical analysis of the dissolved organic molecules in the final adsorbates revealed a preponderance of acid, aromatic, alcohol, and esters compounds, as well as modest levels of aliphatic hydrocarbons. The organic chemicals are produced when NEV and LAM break down (dissolve) in water, and it is anticipated that their presence in discharge water will pose environmental risks to possible receiving waterbodies [15].

The most abundant organic acids were hexadecanoic acid, propanoic acid, 2-hydroxy, and 9,12–0ctadecadienoic acid, followed by l-l -Cyclopropyl-4-methyl-5,11-dihydro-6H, 6-0xa-bicyclo [3.1.0] hexan-3-one, and hexadecanoic acid. As depicted in Figure 4, the GC-MS outcomes for LAM adsorption followed the same pattern as NEV adsorption (higher adsorption performance of HCB). In the adsorbate of LAM, hexenoic acid, methyl and heptadecyl ester, heptanol, 1,2,3-propanetriol, and alkanes were the most common organic molecules. The GC-MS analysis reveals a slightly more extensive dispersion of organic compounds for both NEV and LAM, and the presence of organic components in the final adsorbates likely indicates the necessity for an additional removal stage before contaminated water is released into bodies of water.

3.4. Proximate, Ultimate, and BET Analysis

Table 5. Physicochemical properties of waste precursors produced hydrochar and spent residues from NEV and LAM adsorption.

Analyses	CT	HCT	HCT–NEV	HCT–LAM	CS	HCS	HCS–NEV	HCS–LAM	SS	CB	HCB	HCB–NEV	HCB–LAM
Proximate Analysis (wt. %, adb)
Inherent Moisture	4.17	1.07	1.81	1.04	3.94	1.73	1.58	1.26	8.12	2.71	1.72	1.76	1.45
Ash content	38.64	37.96	33.12	33.96	23.22	22.78	19.78	16.78	36.07	37.75	19.56	13.56	19.56
Volatile Matter	21.44	21.04	22.86	24.49	21.28	18.38	19.2	23.72	47.07	21.92	20.31	22.16	25.26
Fixed Carbon	35.75	39.94	42.21	40.51	50.98	57.11	59.44	58.24	8.75	37.62	58.41	62.52	60.73
Ultimate Analysis (wt. %, adb)
Total Carbon	42.82	49.80	55.23	53.98	61.85	66.90	70.90	69.45	29.7	45.64	67.04	73.85	68.44
Hydrogen	3.01	2.87	2.42	2.39	3.56	2.98	2.28	2.41	4.88	3.05	2.78	1.75	1.57
Nitrogen	1.14	1.72	2.74	2.66	1.39	1.99	2.63	2.84	4.15	1.82	2.43	3.05	2.95
Oxygen	8.79	5.83	3.9	4.21	4.78	2.75	1.91	3.99	15.22	8.2	6.08	5.29	4.15
Total Sulfur (wt. %, adb)	1.43	0.75	0.78	1.76	1.26	0.87	0.92	1.27	1.86	0.83	0.39	0.74	1.88
BET analysis
SBET (m2/g)	6.066	11.82	0	0	6.37	14.35	0	0	-	6.17	20.35	0	0
Total pore volume (cm3/g)	0.034	0.071	0	0	0.048	0.094	0	0	-	0.09	0.38	0	0

CT: Coal Tailing. CS: Coal Slurry. SS: Sewage sludge. CB: Blend (CT + CS: SS). HCT: hydrochar CT. HCS: hydrochar CS. HCB: hydrochar CB, HC–NEV: residue from nevirapine’s adsorption. HC–NEV: residue from nevirapine’s adsorption. HC–LAM: residue from lamivudine’s adsorption. adb: air dried basis. $\mathrm{Oxygen}\%=100-\left(\mathrm{Inherent} \mathrm{moiture}+\mathrm{Ash}+\mathrm{Total} \mathrm{Carbon}+\mathrm{Hydrogen}+\mathrm{Nitrogen}+\mathrm{Total} \mathrm{sulfur}\right)$. BET: Brunauer Emmet Teller. SBET: Specific surface area.

provides the proximate and ultimate analysis results of CT, CS, SS, selected hydrochar (HCs) from hydrothermal carbonization (HTC) and co-hydrothermal carbonization (Co-HTC), and adsorbent residues from the adsorption process under optimal conditions. The HTC and Co-HTC processes proved the ability to enhance the fixed carbon content of raw materials by releasing mineral matter, oxygen, and sulfur from their molecular structures [42,43]. The HTC and Co-HTC processes increased the FC concentration of the raw samples in order to produce hydrochar. The optimum HTC and Co-HTC conditions maintained the liquid state of water and promoted the hydrolysis of aliphatic components, dissolution, and release of mineral materials, resulting in the formation of porous solids [44]. In addition, the optimum HTC and Co-HTC conditions retained a considerable amount of the atomic carbon content of the feedstock in the solid HC produced, validating the HTC and Co-HTC process’s potential for sequestration and repolymerization [45]. The rise in total carbon content in the generated HC may be related to a decrease in hydrogen, oxygen, and total sulfur contents (Table 5), while the decrease in sulfur content in the produced hydrochar likely contributes to the enlargement of pores [46]. Following the adsorption process, the nitrogen adsorption isotherms of synthesized fresh HCs and spent HCs residues exhibited a decrease in the amount of nitrogen absorbed (Figure 5). This confirmed that pores formed during HTC and Co-HTC synthesis of HCs were occluded [47]. Figure 6, Figure 7 and Figure 8 demonstrate that the lateral interaction between dissolved drug molecules and HCs likely contributed to a change in the surface texture of the absorbent. It is also likely that drug molecules repopulate the mesopores created when the initial sulfur and oxygen content is oxidized. As seen in Table 5, this decreases the SBET and total pore volume of HCs in the ensuing spent residues.

Table 5 illustrates the increase in the volatile matter (VM) and total carbon in the spent residues comparative to the source materials and the synthesized HCs. This provides evidence of NEV and LAM dissolved adsorption on HCs. The NEV and LAM are organic compounds with cyclic carbon structures incorporating nitrogen and sulfur, especially in the case of LAM [48]. As a result of their adsorption on HCs, the nitrogen and total sulfur content of the solid residues increase.

The FTIR spectra of NEV and LAM provided by John Wiley & Sons (2022) in Figure 9A,B were used as a reference to illustrate the changes of the chemical structural features of the hydrochar after adsorption of the drug molecules (Figure 10, Figure 11 and Figure 12) [49]. Spectra were interpreted using reference tables supplied by Smith Brian (2018) [50]. The enhanced peaks at wavelengths between 1700 cm⁻¹ and 1350 cm⁻¹ observed in Figure 10, Figure 11 and Figure 12 indicated NEV and LAM adsorption. The adsorption peaks identified at the wavelengths 3250 cm⁻¹, 3000 cm⁻¹, and 2250 cm⁻¹ were attributed to the absorbed NH₂, aliphatic C-H, and S-H stretch characteristics of the decomposition of NEV and LAM in water. The broad adsorption peaks of S-H stretch vibration validated the better performance of HCB for the adsorption of LAM (Figure 12), followed by HCT (Figure 10) and then HCS (Figure 11). According to Table 5, the high porosity of HCB enhanced its absorption capacity. The peaks found at 1650 cm⁻¹ wavelength suggested the existence of double-bound carbonyl groups (C=O) associated with aromatic stretch, confirming the presence of cyclic hydrocarbons with carbonyl attached from NEV and LAM [51]. At 1600 cm⁻¹ bands, which correspond to aromatic rings in the NH₂ scissor structure, HCT–LAM has the highest peak intensity, followed by HCB and HCS (Figure 10). This demonstrated the ability of HCT to adsorb -NH₂ groups because of its polarity caused by the hydrolysis equilibrium induced by the substitution of higher -OH bending vibrations groups of C₁₁H₈O (HCT) resulting in a lower probability of hydrogen bond formation [49]. The adsorption of -NH rings from NEV was characterized by the peaks seen at 1495 cm⁻¹ and 1465 cm⁻¹ in wavelength. In the HCB-NEV spectrum (Figure 12), the strength of the peaks was found to be greater than in the HCS (Figure 10) and HCT (Figure 11) spectra, respectively. This was owed to the increased aromatic ring presence in HCB as compared to HCT and HCS, which decreased the basicity of -NH and led to the delocalization of the lone pair of electrons on nitrogen. The displaced pair of electrons could then participate in hydrogen bonding (nitrogen alkyl groups) for the formation of imines or organic compounds containing nitrogen double bonds [52]. At a wavelength of 1420 cm⁻¹, the OH and aldehyde bend vibrations were detected in the spectra of all residues, demonstrating the adsorption of NEV. This may be facilitated by the basic light produced by the dissolution of NEV, which promoted the substitution of CH₃ groups by OH- and aldehydes. Peaks at 1385 cm⁻¹ were attributable to the symmetric deformation associated with the -CH₃ and –C-CH₃ groups formed by the interaction of hydrophobic hydrochar with the NEV [53]. The -NH group was found at 1350 cm⁻¹ in the spectra of NEV spent residues, demonstrating the connection between the aromatic ring and the dissolved amine groups of the non-nucleoside drug medication. Numerous peaks between 900 and 800 cm⁻¹ correspond to aromatic species in aromatic rings, trans- and cis-CH₂ in long saturated aromatic -CH-CH chains, C-O stretching vibration of ether groups, and O-H bending vibrations in phenolic, phenoxy, and hydroxybenzene structures. The FTIR results demonstrate the expansion of existing peaks and the emergence of new peaks in spent residues revealing a complex interaction between the HCs and the adsorbates.

The morphological characteristics of spent adsorbent residues were compared to those of the freshly synthesized HCs.

The results show that the surface of the HCs (Figure 6a, Figure 7a and Figure 8a) has a more uniform porous structure than the surface of the spent residues (Figure 6c,e, Figure 7c,e and Figure 8c,e), which lacked a discernible porous structure. The formed pores on the surfaces of HCs generated by HTC and Co-HTC process conditions were covered by absorbates that are roughly aggregated on their surfaces (NEV, LAM) (Figure 6c,e, Figure 7c,e and Figure 8c,e). During the adsorption process, the surface structural rearrangement of HCs, as seen on the surface of spent residues, may be an indication of the ability of HCs to trap drug molecules in its mesopores [26]. Sousa et al. (2022) demonstrated the ability of carbon compounds (activated carbon) formed from microwave pyrolysis of brewery wastes to adsorb dissolved pharmaceuticals (sulfamethoxazole, trimethoprim, and ciprofloxacin) [27]. The study showed that the adsorption performance of the carbon material is dependent on the material’s textural (porous) features and the adsorption equilibrium time. Effective contact between the adsorbent and adsorbates had an effect on the absorbent’s well-developed porous structures (0.67 cm³ g⁻¹). This caused a dispersed distribution of adsorbates within the pores and altered the morphological characteristics of the absorbents [27]. This study, therefore, investigated the porous qualities of the generated HCs (total pore volume of 0.071 cm³ g⁻¹, 0.048 cm³ g⁻¹, and 0.38 cm³ g⁻¹) as a potential characteristic for their use in the adsorption of dissolved pharmaceutical contaminants. The SEM pictures of the spent residues (Figure 6, Figure 7 and Figure 8a,c,e) reveal the efficiency of pharmaceutical components’ capture in the pores of the absorbents as minute particles that obscure the spots on the surface of HCs. In addition, the elemental composition of the adsorption residues (Figure 6, Figure 7 and Figure 8b,d,f) revealed a rise in carbon, sulfur, and the appearance of nitrogen peaks in comparison to the element spectra of HCs. This indicated evidence of components absorbed from the dissolved NEV and LAM onto the pores of the HCs, correlating with the proximate and ultimate analyses.

Kebede (2020), Ndilimeke Akawa (2021), and Zitha (2022) identified antiviral and other pharmaceutical compounds in bodies of water throughout their studies [12,13,14]. According to these findings, the maximum concentrations of nevirapine and lamivudine in South African ambient water were discovered at 1640 ng·L⁻¹, posing a substantial environmental danger due to their potential harm to aquatic life and medication resistance in humans [14]. As a result, pharmaceutical-compound-removal strategies for bodies of water are required. Extensive investigations of the removal of these medications from water bodies have not yet been completed, most likely due to the significant cost of prospective remediation technologies [12,14].

Table 6. Absorbents reported for the removal of ARVDs and other pharmaceutical products in different water matrices.

Adsorbent	Adsorbate (Pharmaceutical)	Adsorbent Concentration (g L−1)	Contact Time (min)	Uptake mg g−1	Reference
Commercial carbon	Nalidixic acid	0.08	325	1595	Patiño (2016) [54]
Carbon nanotube	Nalidixic acid	0.08	150	583
Graphite	Nalidixic acid	0.08	10	62
Akaganeite-carbon	Doxycycline	0.027	108	13.4	Zhang (2016) [55]
Coffee residue/Fe3O4	Tetracycline	0.1	85.9	181	Oladipo (2016) [56]
Bone char	Naproxen	0.01	78	0.11	Reynel (2015) [57]
Walnut shell carbon	Cephalexin	0.1	117.1	211	Nazari (2016) [58]
Peach stones carbon	Ibuprofen	0.01	426	55	Álvarez. (2016) [59]
	Tetracycline	0.01	1446	132.6
Activated biochar	Ranitidine	0.2	63	12	Mondal (2016) [60]
Akaganeite-carbon	Doxycycline	0.027	-	15	Zhang (2016) [55]
Date stones carbon	Levofloxacin	0.15	-	2.1	Darweesh (2017) [61]
Pomegranate carbon	Amoxicillin	0.05	-	5107	Yaghmaeian (2014) [62]
AC from oak acorn	Paracetamol			45.45	Nourmoradi (2018) [63]
AC from Babassu coconut				71.39	Ferreira (2015) [64]
AC from Dende coconut				70.62
NH4Cl-induced AC				233	Mashayekh-Salehi and Moussavi (2016) [65]
Acid-treated beverage sludge-activated carbon				145.4	Streit (2021) [66]
AC from vegetable	Ibuprofen			115.1	Frohlich (2018) [67]
AC from agricultural by-products				12.6	Baccar (2012) [68]
AC from oak acorn				96.15	Nourmoradi (2018) [63]
AC from vegetable	Ketoprofen			79.1	Frohlich (2018) [67]
Acid-treated beverage sludge-activated carbon				57.66	Streit (2021) [66]
Nanofibers fabricated from Mondia whitei	ARVs	0.04	0–120	75–320	Kebede (2020) [12]
Graphene-based material	Nevirapine	0.2	1500	48.31	Adeola (2021) [69]
Graphene-based material	efavirenz	0.2	500	4.41
AC from Spent brewery grains	sulfamethoxazole	0.025	60	217	Sousa (2022) [27]
	trimethoprim	0.025	60	229.35
	ciprofloxacin	0.025	60	205.7
AC from primary paper mill sludge	sulfamethoxazole	0.8	-	185.14	Silva (2019) [26]
Functionalized biochar	sulfamethoxazole	0.01		80.79	Ahmed (2017) [31]
Graphene oxide	lamivudine	0.1	120	10.08	Lan (2022) [32]
Molecularly imprinted polymers (MIPs)	Abacavir		60	167	Terzopoulou (2016) [70]
Graphene wool	Efavirenz			4.41
	Nevirapine			48.31
Alginate, polyvinylpyrrolidone and activated carbon				44.4	Ndilimeke. (2021) [13]
	Zidovudine			42.2
Nanofibers (Mondia White roots)	Nevirapine			200.5	Kebede (2020) [12]
	Didanosine			75.9
	Ritonavir			86.9
	Efavirenz			152.1
	Stavudine			160
Biochar made from- olive	Abacavir			Optimum value not reported, exhibited poor removal efficiencies	Späth (2021) [71]
Residues, tomato residues	Atazanavir
Rice husks, and the African	Darunavir
Palm tree Raphia farinifera	Lamivudine
	Nevirapine
	Raltegravir
HCT	Nevirapine	100	90	50	This study
	Lamivudine			42
HCS	Nevirapine			52
	Lamivudine			41.1
HCB	Nevirapine			53.8
	Lamivudine			42.8

HCT: Hydrochar coal tailing. HCS: Hydrochar coal slurry. HCB: hydrochar from the blend of coal tailing, coal slurry, and sewage sludge. AC: Activated carbon.

highlights selected recent adsorption studies for pharmaceutical product removal. Despite the fact that a large amount (100 g L⁻¹) was required in this study for the efficient removal of nevirapine and lamivudine from water at ambient temperatures (rather than the 45 °C reported in most research), this investigation shows that HCs perform similarly to other synthetic adsorbents. Furthermore, because coal waste is readily available in South Africa and the HTC process is environmentally friendly, hydrochars formed from HCs could serve as excellent, cost-effective adsorbents for numerous pharmaceutical compounds discovered in water bodies.

3.5. Production and Evaluation of AC from Spent Hydrochar Residues from Nevirapine and Lamivudine Adsorption Tests

The Fourier transform infrared (FTIR) spectra of spent hydrochar residues from nevirapine and lamivudine adsorption tests and produced ACs are displayed in Figure 13 and Figure 14. The observation of these spectra revealed the impact of the activation process which led to a significant decrease, and in some cases disappearance, of adsorption peaks. This demonstrates the effectiveness of functional group removal resulting in the transformation of chemically active sites [72].

The FTIR spectra of the produced ACs presented in Figure 13 and Figure 14 depicted few peaks which indicates fewer functional groups (Smith Brian, 2018). The peak at 3430 cm⁻¹ is observed only on the spectra of AC–HCB–LAM (Figure 14) and could be assigned to the remaining amines stretch after the activation process [73]. The peak observed at the adsorption band of 2900 cm⁻¹ and 750 cm⁻¹ on the spectra of AC–HCT–LAM and AC–HCS–LAM (Figure 13) indicates the presence of C–H aliphatic stretching vibrations. arising from C-CH₃ groups of the hydrochar [74]. The peaks at 1650 cm⁻¹ observed on the spectra of AC–HCB–LAM confirmed the presence of amine group NH₂ (Figure 14) [73]. The identified peaks at 1050 cm⁻¹ indicate the presence of the C–O bond and of –C–OH from carboxylic acids, alcohols, and phenols [75] whereas the peaks at 1125 cm⁻¹ possibly result from C=C vibrations of aromatic rings in the AC–HCB–NEV and AC–HCB–LAM (Figure 14) [76].

The morphologies of the produced ACs from spent hydrochar residues from nevirapine and lamivudine adsorption tests are presented in Figure 15. The results show a surface transformation from irregular and rough surfaces (Figure 7 and Figure 8c,e) to a more porous surface of ACs (Figure 15). In addition, the distribution of pores on the surface images of ACs seemed heterogeneous (Figure 15). This is possibly due to the chemical activation reaction between KOH and the carbon structure of the residues, and most of the organic volatiles evolved, leaving behind the ruptured surface of ACs with many pores [77]. The diffusion of KOH molecules into the pores of the carbonaceous processor during the activation process tended to increase the KOH–carbon interaction and created more pores ultimately increasing the S_BET and pore volume of the produced ACs [77].

The porosity helps in the diffusion of electrolyte ions at the charging–discharging process of supercapacitor cells and thus increases the capacitance performance of the synthesized device [73].

The BET results obtained from the analysis of the N₂ adsorption/desorption isotherm at 77 K for the produced ACs are represented in

Table 7. BET analysis results of the synthesized ACs using spent hydrochar residues from nevirapine and lamivudine adsorption tests.

Material	Surface Area (m2/g)	Total Pore Volume (cm3/g)	Micropore Volume (cm3/g)	Average Pore Diameter (nm)
AC–HCT–NEV	1047.5	1.14	0.986	2.23
AC–HCT–LAM	748.2	0.47	0.353	2.55
AC–HCS–NEV	618.4	0.373	0.289	2.41
AC–HCS–LAM	582.8	0.442	0.255	3.04
AC–HCB–NEV	436.5	0.42	0.16	3.85
AC–HCB–LAM	603.6	0.473	0.262	3.13

BET: Brunauer Emmet Teller. AC–HC–NEV: Activated carbon from nevirapine’s adsorption spent residue. AC–HC–LAM: Activated carbon from lamivudine’s adsorption spent residue.

. The highest S_BET of 2047.5 m²/g and the lowest SBET of 436.5 m²/g, but higher than the lowest S_BET of common activated carbons (290 m²/g), were obtained from the AC–HCT–NEV and AC–HCB–NEV, respectively [78]. This could be due to the relatively high carbon (C-C) bonds identified in the HCT–NEV spectra presented on Figure 10 compared to other spent residues. Under activation conditions, carbon bonds break and create complexes with KOH on the carbon silica matrices [79]. The average pore diameter and pore volume were in the range of 2.23 nm to 3.13 nm and 0.373 cm³/g and 1.14 cm³/g, respectively, which make the produced ACs predominated by mesopores (2–50 nm). Though Darweesh (2017) has demonstrated that mesoporosity is favorable for adsorption-based applications, the synthesized ACs could be beneficial for electrochemical applications [61]. Table 7 summarizes the BET results for the produced ACs using spent hydrochar residues from nevirapine and lamivudine adsorption tests. The results indicate that the S_BET increase is influenced by the generation of pores on the carbon silica matrices [80]. Possibly, the KOH–carbon matrix interaction favored the dilatation of the carbon structure which expends the cross-linked matrix of the CM and generates pores after the acid-washing step [81].

3.6. Electrochemical Characterization

Figure 16a depicts the cyclic voltammograms (CV) of the produced ACs at a scan rate of 100 mV/s, demonstrating that all sweep curves are symmetric on the positive and negative sweeps and the currents at 0 V for electrodes made of different ACs change. The electrodes built from AC–HCT–NEV (A1) and AC–HCT–LAM (A2) outperform the other four AC electrodes tested (0.016 mA, 1 V, and 0.015 mA, 1 V), as indicated by the high current and nearly ideal standard rectangle [82]. This is consistent with the high S_BET, total volume, and micropore volume of the two ACs shown in Table 7. This demonstrates that the textural structure of the AC electrode has a significant impact on the electric conductivity qualities [83]. The roughly rectangular cyclic voltammetry shape implies that the AC electrodes have rapid dynamics and good charge propagation. The charge storage mechanism of the ACs was described from the evaluation of the scan rates effect. The observation of the results (Figure 16a) revealed that the AC electrode exhibited potential independent double-layer capacitance between appreciable voltage ranges of polarization [84]. Figure 16a shows an approximately ideal double-layer trend, and the rectangular sharp specific to electrochemical capacitance was seen at the tested scan rate.

Equation (6) below was used to determine the specific capacitance of the AC electrode from the CV presented in Figure 16a.

$$C=\left(\frac{I_a-I_c}{2v.m}\right)=\left(\frac{\Delta I}{2v.m}\right)$$

(6)

where C is the specific capacitance (F/g), DI is the average of anodic and cathodic current, v is the potential scan rate (V/s), and m is the mass of the electrode.

The specific capacitance of the produced AC electrode was in the range of 20 to 100 Fg⁻¹ at scan rates of 10 to 100 mVs⁻¹. The results obtained are in agreement with prior work which shows that the specific capacitance of electrodes increase is a complex fact not depending only on the specific surface area SBET [85].

Additionally, the results presented in Figure 16b show that the specific capacitance depends on the porous structure of the electrode (Table 7). The presence of macropores in AC–HCT–NEV may favor electrolyte ion displacement into the porous material. As a result, the electrolyte ions can only settle in some of the electrode’s pores to form the double layer [86]. At a current density of 10 A/g for ions transportation, the micropores’ accessibility decreases from AC–HCT–NEV (A1) to other ACs and even becomes inaccessible for AC–HCB–NEV (C1) and thus does not assist in the total double-layer capacitance of the porous electrode, resulting in a null specific capacitance [87]. The resonance of narrowed pores (micropores) could increase the ions’ mobility at high temperatures [87]. The results of this investigation reveal that when the S_BET of the electrode increases, so does the specific capacitance. Furthermore, additional properties of the porous carbonaceous material, such as pore size distribution and surface chemistry, influence its electrochemical capacity (AC–HCB–NEV and AC–HCB–LAM). Furthermore, the ACs produced from NEV and LAM adsorption residues using HC absorbents exhibited good reversibility and stability, as observed from the ideal double-layer behavior shown in Figure 16a [88], implying that the produced ACs could be used as excellent electrodes for supercapacitor devices with current densities ranging from 1A/g to 5 A/g in energy storage applications.

Figure 17 shows the electrochemical impedance spectroscopy (EIS) results of AC electrode, which might be used to test different circuit models for data fitting. Electro-active regions, specific capacitance, and charge transfer resistance of electrodes obtained from CV and EIS studies were evaluated [89]. The spectra depict the contact resistance and material resistance (electrode materials, electrolyte, and separator) values that prevent ions from entering the pores of electrode materials, as well as ion mobility in the solution and separator. The real axis is set at 3.5 Ω, suggesting that the AC–HCT–LAM (A2) electrode has a lower contact resistance and higher conductivity than the other five electrodes [88]. At lower frequencies, the slope of the AC curves increases from AC–HCT–LAM (A2), which is roughly vertical, to AC–HCB–NEV (C1), which acquired a considerable slope, indicating that the ions in the electrolyte are difficult to enter the pores of AC–HCB–NEV, as previously described. As a result, the manufactured AC electrodes have a high potential electrode of an electrochemical double-layer capacitor due to their wide electro-active area [89]. The findings indicate that the structural characteristics of the carbon–silica linkages impact the electrochemical properties of the ACs. The release of volatile matter during the activation process creates pores in the structure of the spent adsorption residues, whereas silica, a component of ash content from the HCs precursors (Table 1), remains in the matrix of the produced ACs, reducing its porosity and, as a result, its electrochemical conductivity [90]. As a result, applying strong compression pressure to the synthesized ACs may shatter the individual particles and condense the materials. This might pull individual particles closer together with other nearby particles, increasing the conductivity of the ACs [88,90]. The higher density of the AC produced at high pressure causes potential electron mobility across the material, increasing its electrical conductivity. Nonetheless, the reduced density of AC at low compression pressure leaves large voids between the particles of the carbon matrix, reducing electron mobility dramatically. As a result, the electrical conductivity of the AC used decreases.

4. Conclusions

This study examined the effects of various parameter interactions on the textural structure of hydrochars produced via hydrothermal (HTC) and co-hydrothermal (Co-HTC) treatments of CD and sewage sludge (wastes), as well as the subsequent utilization of the hydrochars (HCs) synthesized for HIV drug (nevirapine and lamivudine) removal from wastewater. The HCB synthesized from Co-HTC had a high SBET of 20.35 m²/g and pore volume of 0.38 cm³/g, resulting in significant adsorptive removals of NEV and LAM (97.19% and 93.32%), respectively. Despite being less effective than HCB, HCT and HCS displayed good NEV and LAM adsorption capacities. The use of coal waste and sewage sludge for the synthesis of hydrochar for the adsorption of dissolved pharmaceutical-resistant substances might be a cost-effective and environmentally friendly option in wastewater treatment. Furthermore, the use of spent adsorption residues for energy storage applications was examined further. The results reveal that spent adsorption residues are an excellent carbonaceous material precursor for the manufacture of EDLCs. This research has shown that using low-cost HTC technologies to generate value-added carbonaceous material (hydrochar) from coal waste and sewage sludge waste could be a long-term recycling strategy that reduces disposal costs while also addressing any environmental contamination caused by the release or solubilization of toxic compounds in these wastes as a result of landfill disposal. However, additional kinetics study and thermodynamic analyses are necessary to offer a deeper knowledge of HTC and Co-HTC reaction processes, as is the development and testing of a continuous ARVs components adsorption method, as well as its implications for large-scale industrial use. A life cycle evaluation of the hydrothermal treatment and adsorption procedures may also be necessary to determine the long-term sustainability of the options described in this study.