Selective adsorption of drug micropollutants from synthetic wastewater using hydrochar derived from carbonisation of unused leaves

ABSTRACT One-step hydrothermal carbonisation method was used to synthesise hydrochar, SRL HC (Saccarum ravannae hydrochar), and SOL HC (Saccarum oficinarum hydrochar), which were employed to remove highly toxic and emerging contaminants drugs, diclofenac, ibuprofen, and naproxen from the synthetic aquatic environment. The presence of hydroxyl and carboxylic acid groups makes these hydrochars an efficient adsorbent. The adsorption experiments were conducted to optimise various parameters such as pH, adsorbent dose, contact time, and temperature. Then maximum adsorption capacity of diclofenac, ibuprofen, and naproxen on SRL HC was found to be 230.04, 201.92, 191.43 mg/g, while the maximum adsorption capacity of SOL HC were 103.40, 77.72, 62.02 mg/g at 303 K. The adsorption of drugs fits well with the pseudo-second-order kinetics model and the Langmuir adsorption isotherm model with a high correlation coefficient (R2 ˃ 0.99). Thermodynamic studies indicated that adsorption was chemisorptive and endothermic. Adsorption in real water samples along with reusability was also performed with these hydrochars.


Introduction
Many extensively used pharmaceutical drugs enter the aquatic environment and have become a class of emerging environmental contaminants.Emerging contaminants may be natural or synthetic, which includes types of pharmaceutical products, personal care products, household products, agricultural products, food additives, natural and synthetic hormones, chemicals, paints, dyes, and heavy metals [1][2][3][4][5][6].These emerging contaminantlike pharmaceutical drugs include a broad range of antibiotics, anti-inflammatory drugs, steroids, painkillers, anti-epileptic drugs, and anti-hypertensive drugs [7][8][9][10].The presence of residue of these drugs has been recently detected in trace amounts in the aquatic environment such as surface water, drinking water, groundwater, rivers, lakes, and wastewater treatment plants [11][12][13].Some of them have been considered anthropogenic markers for water contaminations.It has become an urgent matter of concern because of potential long-term adverse effects on human health and the environment.
Another primary concern is due to their complexity and high bioactivity towards aquatic fauna life [14,15].Therefore, major international organisations such as U. S. Geological Survey, World Health Organization, European Commission, and the U. S. Environmental Protection Agency have growing concerns about water pollution [16].
Among emerging pharmaceutical pollutants, non-steroidal anti-inflammatory drugs (NSAIDs), diclofenac, ibuprofen, and naproxen are the most common drugs identified in the aquatic system due to their extensive consumption.These drugs have high potential medicinal values.Diclofenac can cause a risk of haemodynamic changes in human health and aquatic eco-toxicity.Diclofenac has been detected frequently as a micropollutant because of its high-water solubility, non-biodegradable, and polarity effect [17].Diclofenac has been listed as a priority pollutant by 2013/39/E U, and the upper limit is found at a concentration up to 0.002 ppm [18].Ibuprofen is another NSAID features drug.It is hydrophobic in nature and moderately soluble in water (21 mg/L at 25°C), and has high mobility.Ibuprofen has a long-term detrimental effect on microbes and fish and harms the metabolism of other aquatic organisms and non-targeted organisms [19].Three different vultures in India and Pakistan had reported an unusually high death rate due to the profound use of ibuprofen [20].Naproxen is also used widely in daily life for inflammation, fever reduction, pain, stiffness.Naproxen has low biodegradability, low bioavailability, and has low treatment efficiency due to its resistant nature [14,21,22].Thus, it is essential to raise awareness for removing these pharmaceutical drugs from the aquatic ecosystem.
There are many conventional, and advanced water treatment technologies such as flocculation, coagulation, photodegradation, biodegradation, filtration, chlorination, electrochemical, biological reduction, chemical precipitation, and adsorption are present to treat wastewater [22][23][24].However, adsorption-based techniques are the feasible and preferred method to remove these pharmaceutical pollutants due to simple operational, economic viability, high performance, easiness of installation, suitability over a wide range of concentration, minimising sludge, non-producing by-products, and low energy consumption [25,26].Therefore, the adsorption process is the best promising method for removing trace amounts of pollutants (dyes, metal ions, drugs, and others) from water with proper adsorbent [27,28].However, several imperfections such as low adsorption efficiency, difficult separation, longer contact time, high cost, and many mores limit the practicality of newly synthesised adsorbents for wastewater treatment encompassing active pharmaceutical drugs.Effective adsorption of these drugs in water depends on the physicochemical and molecular structure of drug molecules and also depends on the chemical nature of the resulting adsorbent [14,15,29].Therefore, significant exploration of novel materials was going on, which have high adsorption efficiency, easy separation, fast removal, and good reusability.
Hydrochar (HC) is a carbon-rich material produced by hydrothermal carbonisation of raw biomass, which has aroused great interest because of its high adsorption capacities for polar and non-polar organic pollutants [30].Wet, fresh raw biomass was converted into value-added products such as liquid (water and mixture of bio-oil), solid called hydrochar, and gases (mainly CO 2 ) by hydrothermal carbonisation (HTC) in a pressure vessel at a temperature between 180 and 260°C.The costly pre-drying step is not required in this process.This is the main advantage of this HTC process over other thermochemical processes such as gasification and pyrolysis because the whole HTC process is performed in the aqueous media, which makes the process cost-effective.The main advantage of this HTC process is more negligible pollution risk to the environment, high yield, production of quality products, and ease of operation [31,32].Therefore, in recent studies, these attractive properties have focused on the processing of biomasses, especially to produce carbon-rich hydrochar from raw waste materials of various plant sources and their application towards the adsorption of pollutants from water.Several review papers have already reported the adsorptive properties, such as high adsorption capacities, surface charge, and diverse surface functional groups, for not only inorganic pollutants such as heavy metal ions but also for organic pollutants (polar/ non-polar compounds) in aqueous solutions.
In our continuing effort [33,34] to produce bio-mass-derived adsorbent for waste-water treatment, herein we report the synthesis of hydrochar derived from unused leaves of plant sources (Saccarum ravnnae and Saccarum officinarum).Leaves of Saccarum ravannae and Saccarum officinarum were a new source for the production of hydrochar (Scheme 1).Further, these hydrochars were used for the adsorption of three emerging pollutants drugs like diclofenac, ibuprofen, and naproxen.Therefore, the present study aims to describe the mechanism of the adsorption process and to explain the competitive adsorption behaviour for surface sites present on the adsorbent.Firstly, the analysis was performed with single contaminants, and then the research was conducted in pollutant media synthesised from water collected from different sources to study competitive adsorption.The influencing factors such as pH, adsorbent dose, time of contact study, variation in drug concentration and temperature, and mechanism for adsorption of drugs were systematically investigated.Along with all this, multiple regeneration experiments were also studied to confirm the reusability of these hydrochars.

Synthesis of hydrochar (SRL HC and SOL HC)
The material used to synthesise two hydrochars (SRL HC and SOL HC) was available locally.SRL HC was prepared from dried leaves of Saccarum ravannae and SOL HC was synthesised from leaves remains in the field after extraction of sugarcane (Saccarum officinarum) i.e. agricultural waste, were collected from locals.Saccarum ravannae was a weed grass plant present locally and of no use.The dried leaves of these plants were then washed thoroughly with water to remove dust and other impurities.Leaves were cut into small pieces and kept in the oven for drying.Then dried leaves were grounded into powder, sieved (˂ 200 µm), and stored in an oven for further use.
Hydrochar (SRL HC and SOL HC) was prepared by hydrothermal carbonisation (HTC).5 gm of dried leaves were dispersed in 40 mL distilled water with constant stirring, and then it was transferred to 50 mL stainless steel hydrothermal autoclave reactors (HTC reactor).The HTC reactor was placed in a high-temperature oven and heated to 220°C for 9 hrs.After 9 hrs, the HTC reactor was cooled down to room temperature, and the materials were separated by filtration.The hydrochar from Saccarum ravannae (SRL HC) and Saccarum officinarum (SOL HC) was washed thoroughly with distilled water and ovendried at 105°C for 24 hrs.Then dried SRL HC and SOL HC hydrochars were stored in airtight containers for further characterisation and application in adsorption studies.The prepared hydrochars were characterised in detail as described in supporting information.The SRL HC and SOL HC yields, Hydrochar yield (Y) were calculated by equation (1) [35,36].
The yield (with respect to raw biomass) was 70.4% and 80% for SRL HC and SOL HC, respectively.Stock solutions of all the drugs (Scheme 2) (200 mg/L) were prepared using methanol as a solvent, as these drugs were less soluble in water.

Adsorption experiments
The adsorption efficiency of the hydrochars was compared for all the drugs by adsorption studies.The initial concentrations of all the three drugs in the initial experimental studies were set to 10 mg/L in distilled water by diluting the stock solutions of drugs.Stock solutions of all the drugs were prepared using methanol as a solvent, as these drugs were less soluble in water.A calibration curve for drugs was obtained from the spectra of standard solutions (1-10 mg/L).10 mg of the adsorbents were added in 25 ml of (10 mg/L) drug solutions.The solutions were kept in a shaking incubator with a speed of 180 rpm at room temperature for 12 hrs.The influence of pH on adsorption was experimented with by adding 10 mg of SRL HC and SOL HC into 25 ml 10 mg/L aqueous drugs solutions, and the pH of the solutions range from 2 to 10.For studying the influence of adsorbent dosage, the experiments were performed by varying the adsorbent dose from 5 to 20 mg.Varying dosage of the hydrochars was added into 25 ml of 10 mg/L aqueous drugs solutions at optimum pH≈4.Then the solutions were kept in shaking at the speed of 180 rpm at room temperature for 6 hrs.The effect of contact time on the adsorption was studied for diclofenac, ibuprofen, and naproxen, 11 mg, 14 mg, and 17 mg of SRL HC, and 10 mg, 12 mg, and 13 mg of SOL HC was added into 25 ml of 10 mg/L aqueous drugs solutions at optimum pH≈4.At different time intervals, the residual drug concentration was examined by taking out a sample from original solutions.The influence of initial drug concentrations on adsorption was studied by varying the range of concentration of drugs from 10 mg/L to 200 mg/ L. 11 mg, 14 mg, and 17 mg of SRL HC and 10 mg, 12 mg, and 13 mg of SOL HC were dispersed into 25 ml of solutions by varying concentration of drug at optimum pH≈4.The residual concentrations of diclofenac, ibuprofen, and naproxen drugs in the reaction mixture after adsorption were analysed with a UV Vis spectrophotometer at a particular wavelength of 276 nm, 221 nm, and 230 nm, respectively.The removal efficiency (%) and adsorption capacity of diclofenac, ibuprofen, and naproxen by SRL HC and SOL HC were respectively evaluated by the following equations (2 and 3): Where C o (mg/L) is the initial drug concentration in solutions, and C e (mg/L) is the final drug concentration after adsorption.q e (mg/g) is the adsorption capacity of the drug per gram of adsorbent.V (L) is the volume of drug solutions and m(g) is the mass of adsorbent SRL HC and SOL HC applied.

Regeneration studies
The reusability of hydrochar-derived adsorbent (SRL HC and SOL HC) is an important parameter studied through regeneration studies.95% ethanol solution provided the best result and was used as regenerating solvent for recycling in the present studies.Hydrochars used for adsorption studies were restored in the following way: used material was soaked in 50 ml of 95% ethanol solution and kept for shaking at 200 rpm for 6 hrs.Then adsorbent was filtered, washed with double distilled water, and dried at 100°C for 48 hrs before further use.The same adsorbent hydrochars SRL HC and SOL HC were used in diclofenac, ibuprofen, and naproxen adsorption and recycled five times for adsorption studies.

FESEM and EDX analysis
The FESEM studies help to identify the microstructure and surface morphology of the hydrochars (Figure 1(a,b)).The hydrochars were found to be smooth, nonporous, and spherical after hydrothermal processing [37].It may be due to the deformation and decomposition of cellulose present on plant biomass.The elemental composition of hydrochars was observed by EDS analysis (Figure 1(c,d)).Hydrothermal processing of plant biomass increases the carbon content of the hydrochars [23].The carbon content of SRL HC hydrochar was greater than SOL HC hydrochar.Other elements such as silicon (Si) and oxygen (O) can be seen in the elemental analysis of both the hydrochars [38].Biomass used for the synthesis of hydrochar; Saccarum ravannae (SRL HC) and waste leaves of sugarcane Saccarum officinarum (SOL HC), was rich in silicon and oxygen content therefore they are also detected in elemental analysis.

FT-IR analysis
FT-IR study was conducted to identify the functional groups on the surface of the hydrochars (Figure 2(a,b)).The broad peak at 3340 cm −1 supports the presence of the free -OH groups of lignin and cellulose on the surface of both hydrochar.The bands present at 2914, 2854, 2920, and 2290 cm −1 of SRL HC and SOL HC were attributed to -C-H bonds in aliphatic methylene groups.Additionally, bands present at 1707, and 1708 cm −1 , of hydrochar SRL HC and SOL HC were denoted as stretching vibration of amide, ketones groups, esters, and aldehydes, while the bands present at 1595 cm −1 were ascribed for -C = O asymmetric stretching in the carboxylic group.The peaks present at 1615 cm −1 were defined for -C = C vibrations in the aromatic ring of lignin, while the peaks present at 1449 and 1450 cm −1 were associated with carbonyl in the plane bending group.Both the spectra have a peak at 1035 cm −1 which resembles the C-O-C stretching vibrations of esters, phenols, and ethers [17].The bands between 1000-700 cm −1 represent the aromatic C-H vibrations, which is due to an increase of aromaticity during the hydrothermal process [39].

Raman analysis
Raman study was performed to identify and characterise the defect in the graphite structure of both the hydrochar (Figure 2(c,d)).The strong G band at 1580 cm −1 reflects the in-plane vibrations of sp 2 carbon and corresponds to the graphitic carbon structure in the hydrochar.Meanwhile, the D band present at 1343 cm −1 represents the disordered amorphous carbon structure and defects in hydrochars.The relative ratio of D to G bands (I D /I G ) gives graphitic defects in carbon materials.The I D /I G value of SRL HC was higher than SOL HC, implying the lower degree of aromatisation of SRL HC hydrochar [36,[40][41][42].

Powder XRD analysis
The peak present at 15.6° in the PXRD spectra, corresponds to cellulose in plant biomass and manifests the carbonisation of hydrochar (Figure 2(e,f)).It was also revealed that the inorganic crystalline phases of SiO 2 are present in both the hydrochar.PXRD pattern confirms the amorphous nature of the adsorbent.The lack of inorganic substances in lignocellulosic plant biomasses may be the reason for the amorphous nature of hydrochars.Major peaks present at 22.45 and 22.54° were characteristic of cellulose type 1 and silica.The peaks at 26°, related to the graphite carbon structure, indicate the stacking of a few graphene-like layers into the hydrochars.The weak intensities of peaks indicate small disordered graphitic domains, i.e. the degree of graphitisation of hydrochars is low [23,[41][42][43].

BET analysis and Zeta potential study
The BET surface area of hydrochars SRL HC and SOL HC were calculated to be 27.26 and 26.21 m 2 /g (Fig. S2a, b).This result shows that the hydrothermal process enhances the development of pores in hydrochar.Both BJH and BET methods were used to evaluate the pore size and surface area distribution.The surface area of SOL HC was slightly low compared to SRL HC [35,36,44,45].The zeta potential of hydrochars SRL HC and SOL HC were measured and depicted in Fig. S2c and d.The surface charge was (-)17.75mv and (-) 13.46 mv for SRL HC and SOL HC.Thus, both the hydrochars are negatively charged.Both the hydrochar are stable [36].

Effect of initial pH and adsorbent dosages on the drug removal
The influence of pH on the removal of drugs was investigated over a range of pH 2 to 12 (Figures 3a and 4a).The result reveals that the removal percentage of those samples of drugs is affected by pH.It was observed that more than 90% removal occurs in acidic conditions, and after that decrease in the removal, the percentage was observed for all three drugs.The optimal pH range of 3 to 5 for drug removal may be related to its pKa values.The pKa is 4.2, 4.5, and 4.15 for diclofenac, ibuprofen, and naproxen drug, respectively.The drug molecules exist in the anionic form in the solution where the pH > pk a .Hence, deprotonation of drugs dominates, which also results in a decrease in the removal percentage of drugs due to poor interaction between the negatively charged drugs and the surface of hydrochars [46][47][48].
The effect of adsorbent dosages on the removal of drugs by varying the dose of SRL HC and SOL HC hydrochars from 5 mg to 20 mg (Figures 3b and 4b).The removal percentage of diclofenac, ibuprofen, and naproxen using SRL HC increased from 76.00% to 94.67%, 65.60% to 90.28%, and 56.66% to 85.72%, respectively, by changing the adsorbent dosages.Therefore, the optimum dosage of SRL HC hydrochar is 11 mg, 14 mg, and 17 mg for diclofenac, ibuprofen, and naproxen drug removal.While removal percentage increased from 68.33% to 85.09%, 58.56% to 80.91%, and 50.59% to 75% for diclofenac, Ibuprofen, and naproxen removal by using SOL HC, and the optimum dose was achieved at 10 mg, 12 mg, and 13 mg for removal of drugs.The increased percentage removal of drugs can be explained by the fact that with the gradual increase of adsorbent dosages, there is an increase in active sites on the surfaces of hydrochars and available active sites fully occupied by drugs molecules at low dosages of adsorbent, which results in a lower removal percentage [22].

Effect of contact time and adsorption kinetics
It was found that the adsorption of all the three drugs adsorption on hydrochar SRL HC and SOL HC was very fast (Figures 3c and 4c).The adsorption equilibrium for SRL HC hydrochar was attained in 30 minutes, 60 minutes, and 45 minutes for diclofenac, ibuprofen, and naproxen, respectively.While it has been achieved for SOL HC hydrochar in 60 minutes, 120 minutes, and 60 minutes for diclofenac, ibuprofen, and naproxen, respectively.The rapid adsorption might be due to the presence of many active sites on the surface of the hydrochars.As time continues, a gradual decrease in adsorption occurs due to a reduction in available active sites on the surface of hydrochars.The adsorption mechanism of drugs diclofenac, ibuprofen and naproxen could be explained by kinetics models.These kinetics models were presented by the linear equations of pseudo-first-order and pseudo-second-order kinetics equations ( 4) and (5).
The pseudo-first-order equation is explained by Lagergren and represented by equations (4) [49].
The pseudo-second-order equation is explained by Ho and Mckay and represented by liner mathematical equation ( 5) [50].
The nonlinear Evolich and interparticle diffusion equation were represented by mathematical equation ( 6) and (7).Where q e (mg/g) and q t (mg/g) are the amounts of drugs adsorbed by adsorbent hydrochars at equilibrium and at time t (minutes), k 1 ( min −1 ), and k 2 (g/ mmol.min) are the pseudo-first-order and pseudo-second-order kinetic rate constants, α (mg/g.min) and β (g/min) is the initial adsorption value and the constant of desorption, I is the interparticle diffusion kinetics constant that is related to the boundary layer thickness.
Figure 5(b,e), and represent that the pseudo-second-order kinetics was followed by the adsorbent hydrochars, while Fig. S3a and b, Fig. S3c and d and Fig. S3e and f represent the pseudo-first-order kinetics, Evolich and interparticle diffusion kinetics not followed by the adsorbent hydrochars for the adsorption of drugs.The experimental (q exp ) and calculated adsorption (q cal ) capacities were found to be better following them, and the coefficient of determination (R 2 ) was also greater than 0.99 than pseudofirst-order kinetics, Evolich and interparticle diffusion kinetics (Table 1).The pseudosecond-order kinetics, indicates the chemisorption due to the formation of various chemical interactions between the functional groups present on the surface of hydrochar as well as drug molecules.In general, fast equilibrium was achieved suggest that the prepared hydrochar possessed a high adsorption capacity towards diclofenac, naproxen, and ibuprofen.

Effect of initial drugs concentration and adsorption isotherm
Figures 3d and 4d present the effect of initial drug concentration on the adsorption removal percentage of hydrochars.The maximum adsorption capacity of SRL HC hydrochar was calculated to be 230.04,201.92, 191.43 mg/g for diclofenac, ibuprofen, and naproxen, respectively.The maximum adsorption capacity of SOL HC for adsorption of drugs was 10.3.40,77.72, 62.02 mg/g for diclofenac, ibuprofen, and naproxen, respectively.The removal percentage was decreased by increasing the initial drug concentration.The number of available active adsorption sites for the fixed amount of adsorbent was insufficient to adsorb more drug molecules at higher concentrations; therefore, low removal percentage was observed.
To investigate the adsorption mechanism of drugs molecules on the adsorption of hydrochars, various adsorption isotherms model equations were used.Langmuir and Freundlich's isotherms were applied to describe the adsorption equilibrium.The Langmuir adsorption isotherm assumes, that uniform adsorption of adsorbate occurs on the uniform adsorbent surface, which is monolayer adsorption.Whereas Freundlich adsorption is applied to describe multilayer adsorption appearing on the heterogeneous surface.Langmuir isotherm equation can be represented by equation ( 8) [51].
The nonlinear isotherm Tamkin was represented by mathematical equations (10) [R, and R].
where Ce (mg/L) is the equilibrium concentration of drugs in solutions, q e (mg/g) is the quantity of drugs adsorbed by hydrochars at equilibrium, q max (mg/g) is the maximum adsorption capacity of the hydrochar for drugs adsorption.K L (L/mg) is the Langmuir constant, and K F is the Freundlich adsorption isotherm constant, and 1/n is significant of heterogeneity.A T (1/g) and b T (kJ/mol) are the Temkins constant, R is the universal gas constant and T (K) is the absolute temperature.
The adsorption was fitted well by the Langmuir isotherms (Figure 5(a,d)).Fig. S4a and b, Fig. S4c and d represent the Freundlich isotherm fitting and Temkin isotherm fitting (Table 2).The corresponding experimental data of diclofenac, ibuprofen, and naproxen on hydrochar SRL HC and SOL HC was listed in Table 3.In addition, correlation coefficients (R 2 ) were more than 0.99 for Langmuir isotherm.The dimensionless R L parameter value for the adsorption of drugs using SRL HC and SOL HC was determined by using Langmuir adsorption isotherm model and comes in the range of 0-1, which also shows that adsorption of all the drugs using hydrochar adsorbents is desirable.The high value of A T parameter, and low value of b T parameters calculated by Tamkins isotherm model, which indicates the weak interaction between drugs molecules and adsorbents SRL HC and SOL HC surface.The result indicated that adsorption of drug molecules onto hydrochar involved monolayer adsorption on the outer surface of hydrochars might be due to surface interactions such as π-π interaction and hydrogen bonding [16].Hydrochar SRL HC and SOL HC were composed of various functional groups such as hydroxyl, carboxylic, and carbon oxygen-rich regions, which represent binding site distribution of hydrochars where the drug molecules bind during adsorption.The comparative adsorption capacity for drug adsorption with other adsorbents was listed in Table.S1.

Effect of temperature and adsorption thermodynamics
Thermodynamics studies were performed on three different temperatures to understand the effect of temperature (Figure 5(c,f)) (Table 3) on the adsorption of drugs on hydrochars.A remarkable increase in binding was observed with an increase in temperature.It specifies that the adsorption process was endothermic.Higher temperature facilitates the adsorption of drug molecules on the surface of hydrochars.This phenomenon may be because the movability of drug molecules increases from aqueous solution to solid adsorbent hydrochar, with the increase of temperature, enhancing the interaction between the drug molecules and the adsorbate.Thermodynamic parameters encompass, Gibbs free energy change (ΔG°), entropy change (ΔS°), and enthalpy change (ΔH°), can be calculated from temperaturedependent equilibrium equations ( 8), ( 9), (10) and (11).Where C e (mg/L) is the concentration of the drug in aqueous solution at the equilibrium, q e (mg/g) is the amount of diclofenac, ibuprofen, and naproxen drugs adsorbed by SRL HC and SOL HC at the equilibrium, R (8.314 J/mol K) is the universal gas constant and T (K) an absolute temperature, respectively.The values of ΔG° were negative for all the experimented temperatures, which explained that the adsorption of drugs by hydrochar was spontaneous and feasible.With the increase of temperature, ΔG° values decrease, suggesting that at higher temperatures, higher spontaneity occurs.The positive values of ΔH° specify that the removal of drugs by hydrochar was endothermic i.e. the adsorption process reached equilibrium by consuming energy from within the system.The positive value of ΔS° indicates an excellent commendable affinity between drug molecules and hydrochars, and the randomness increases at the solid/solution interfaces throughout the adsorption process.Therefore, translational entropy of water was higher than drug molecules which in turn was responsible for the increase in randomness at solid/ solution edge [29,53].

Applications in the real water sample
It is necessary to investigate the utility of the hydrochar adsorbents in environmentally applicable conditions.Therefore, water samples were collected from different sources such as distilled water, lake water (IIT Guwahati lake), Brahmaputra river water, and laboratory tap water were collected and filtered with a 0.22 µm filter to remove dust and impurities.These real water samples were spiked with 10 mg/ L diclofenac, ibuprofen, and naproxen drugs.The pH of the collected water samples was measured to be 7.5, 7.5, and 7.3 for tap water, lake water, and river water.The pH was not changed in these water samples.To analyse the removal of drugs in real water samples, approximately 10 mg of SRL HC and SOL HC were dispersed in 25 ml real water samples and kept in rotation at 180 rpm for 12 hrs.The result further proves the remarkable difference in removal of drugs occurs in actual water samples with a removal percentage of more than 60% in all the cases (Figure 6a and b).As SOL HC remove 85%, 76.29%, 70.21% and 80.45% of diclofenac, 75.78%, 68.12%, 67.41%, and 71.13% of ibuprofen, and 70.17%, 61.78%, 62.2% and 66.03% of naproxen from distilled water, lake water, river water, and tap water respectively.While SRL HC remove 90.19%, 79.75%, 78.87% and 85.19% of diclofenac, 85.88%, 75.75%, 73.08%, and 79.50% of ibuprofen, and 77.81%, 68.97%, 69.51% and 73.02% of naproxen from distilled water, lake water, river water, and tap water respectively.Various types of salts present in lake water, river water, and tap water are likely to interfere or inhibit the interaction of drugs molecules.Therefore, removal of drugs was best in distilled water then a slightly decrease in removal occurs in tap water while river water and lake water show less removal of drugs by adsorbent hydrochar SRL HC and SOL HC [23,54].

The plausible mechanism of drug adsorption
Hydrochar can interact with the adsorbate via different adsorptive mechanisms like physical sorption, electrostatic interaction, surface complexation, ionic exchange, ππ interactions, hydrophobic interactions, etc., [55].The adsorption experimental results demonstrated that hydrochars were effective for adsorbing drug molecules primarily by chemisorption processes.Adsorption data confirmations monolayer adsorption of all the aromatic drugs molecules probably via hydrophobic, Van der Waals and π-π interactions.Moreover, hydrochar surface contains abundant oxygencontaining functional groups which would form hydrogen bonds with the functional groups of drug molecules.It is also important to mention here that maximum adsorption occurs at pH around the pk a of the drug molecules i.e. at the charge neutral condition [14,56].FTIR spectra after the adsorption shows the peaks corresponds to the drug molecules onto the hydrochar surface (Fig. S5e, S7e, S9e).FESEM images and PRD patterns noticeably show the difference in surface morphology and crystallinity before and after adsorption of drugs onto hydrochar surface (Fig. S5f, S7f and S9f).Additionally, the adsorption of drugs was also confirmed by elemental mapping (Fig. S6, S8, and S10) [16,20,23,57].

Reusability and recyclability of hydrochars
Excellent reusability of an adsorbent is an essential factor in the evaluation of its potential for further application.In this study, the adsorbed diclofenac, ibuprofen, and naproxen drug molecules on adsorbent hydrochar SRL HC and SOL HC were desorbed by 95% ethanol, and the desorption occurs about 95.94%, 90.44%, 86.96% (SRL HC), and 9.51%, 85.32%, and 80.49% (SOL HC).The recycled adsorbent SRL HC and SOL HC have been added to the drug solution again.Adsorption-desorption cycles were accompanied five times to evaluate the reusability of hydrochars (Figure 7(a,b)).The adsorption removal percentage decreased with the increase in the number of adsorptiondesorption cycles.The decrease in adsorption was observed, this may be attributed due to unadsorbed diclofenac, ibuprofen, and naproxen molecules decreasing the available active sites.There is some loss of adsorbent material that occurs during adsorption and desorption results in a decrease in removal percentage.However, the removal percentage still maintained more than 60% for SRL HC and 56% for SOL HC after the fifth cycle.Hence, SRL HC and SOL HC exhibit excellent reusability for the removal of diclofenac, ibuprofen, and naproxen drugs.

Conclusions
An efficient adsorbent hydrochar SCL HC and SOL HC was synthesised by one-step hydrothermal, a thermochemical reaction, and was efficient for adsorption of diclofenac, ibuprofen, and naproxen from synthetic aqueous media.Hydrochars exhibit a stable molecular structure because of hydroxyl and carbon oxygen-containing groups.Equilibrium occurred within 1 hour in the case of SRL HC, and for SOL HC, equilibrium was achieved in 1 hour for diclofenac, ibuprofen, and naproxen.The maximum adsorption capacity of SRL HC for adsorption of diclofenac, ibuprofen, and naproxen were 230.04, 201.92, 191.43 mg/g, while the maximum adsorption capacity of SOL HC for adsorption of drugs were 103.40, 77.72, 62.02 mg/g obtained.Efficient adsorption of diclofenac, ibuprofen, and naproxen onto hydrochars was attributed to hydrogen bonding, π-π interaction, and electrostatic interactions.Moreover, the kinetics and thermodynamics fit betters with pseudo-second-order kinetics and the Langmuir isotherm model, demonstrating chemisorption and endothermic adsorption process.In addition, the hydrochars SRL HC and SOL HC could be easily regenerated by using ethanol as regenerating solvent.These features noticeably indicate that SRL HC and SOL HC hydrochar may be promising adsorbent materials for the adsorption of drugs from an aqueous environment.In the future, we can explore the use of waste raw biomass to produce biochar/hydrochar with appropriate surface modification to use as a potential adsorbent for water remediation.

Scheme 1 .
Scheme 1. Schematic representation of the work presented.

Scheme 2 .
Scheme 2. Structure of the drugs used in the study.

Figure 3 .
Figure 3. (a) Effect of pH; (b) Effect of adsorbent dosage; (c) Effect of contact time and (d) Effect of drug concentration on the adsorption of diclofenac, ibuprofen, and naproxen drug by the hydrochar SRL HC.

Figure 4 .
Figure 4. (a) Effect of pH; (b) Effect of adsorbent dosage; (c) Effect of contact time and (d) Effect of drug concentration on the adsorption of diclofenac, ibuprofen, and naproxen drug by the hydrochar SOL HC.

Figure 5 .
Figure 5. (a, d) Langmuir adsorption isotherm plot; (b, e) Pseudo-second-order kinetics plot; (c, f) Thermodynamics plot for the adsorption of diclofenac, ibuprofen, and naproxen drug by the hydrochar SRL HC and SOL HC, respectively.

Figure 6 .
Figure 6.Comparative studies on the adsorption of diclofenac, ibuprofen, and naproxen drug by the hydrochar (a) SRL HC and (b) SOL HC in distilled water (DI), lake water, river water, and tap water.

Figure 7 .
Figure 7. Regeneration and recycle studies on the adsorption of diclofenac, ibuprofen, and naproxen drug by the hydrochar (a) SRL HC and (b) SOL HC.

Table 1 .
Kinetic parameters for the adsorption of the drugs on the surface of hydrochar.

Table 2 .
Parameters for the equilibrium isotherm for the adsorption of the drugs on the surface of hydrochar.

Table 3 .
Thermodynamic parameters for the adsorption of the drugs on the surface of hydrochar.