Linear and nonlinear investigations for the adsorption of paracetamol and metformin from water on acid-treated clay

Natural clays are considered a safe, low-cost, and sound sorbent for some pharmaceutical and body care products from water. Metformin (MF) and paracetamol (PA) are of the most consumable drugs worldwide. A portion of natural clay was treated with distilled water, and another part was treated with hydrochloric acid. The water-treated clay (WTC) and the acid-treated clay (ATC) were characterized by scanning electron microscopy-energy dispersive spectroscopy, X-ray diffraction, Fourier transforms infrared spectroscopy, and nitrogen adsorption isotherm. Batch experiments were employed to investigate the influence of contact time and solution parameters on the adsorption of PA and MF on WTC and ATC. 30 min attained the equilibrium for all sorbent-sorbate systems. Both sorbents fitted the pseudo-second-order kinetic model with a preference to the nonlinear fitting, and the mechanism of adsorption partially fitted the liquid-film diffusion model. The PA and MF adsorption on WTC and ATC fitted the Freundlich model in preference to nonlinear fitting. The adsorption of pollutants on both sorbents was spontaneous, exothermic, and physisorption in nature. Even at low concentrations, both WTC and ATC showed efficiency above 80% in removing PA and MF from tab water, groundwater, and Red seawater. These findings nominated natural clay as an alternative to the costly nanomaterials as sorbents for removing pharmaceutical contaminants from water.

The presence of organic pollutants in water systems is a devastating problem facing the global environment these days. The growing demand and intensive use of Pharmaceuticals and personal care products (PPCPs) lead to the accumulation of their residues in water and wastewater systems 1,2 . These PPCPs may enter the water systems from humans and animals urine and feces, expired drugs discarded, washing of machinery, and discarding improperly manufactured pharmaceuticals [3][4][5] . The availability of PPCPs in water systems may cause epidemic bacteria and other microbes to gain immunity against these drugs on long exposure periods, leading to the evolution of drug resisting microbes, and finally, treatment failure 6,7 . Different illumination practices were encountered to remove the PPCPs residues from wastewater, including oxidation, coagulation-precipitation, reverse osmosis, electro-dialysis, and ion exchange [8][9][10][11] . The high cost of these processes and the need for continuous monitoring are considered among the main disadvantages of these technologies 12 . Adsorption is a simple procedure that can be carried out with low-cost sorbents like carbonaceous materials or natural clay minerals [13][14][15][16][17][18][19][20] . Although carbon materials have a large surface area, clay materials can be free because they are found naturally. Among all known adsorbents used in water treatment, clay is the cheapest and safest to the degree that it can be used as excipients or active substances in many pharmaceutical products [21][22][23][24][25][26][27][28][29] . Metformin (MF) and paracetamol (PA) are among the most consumable drugs worldwide [30][31][32] . PA was one of the eighteen PPCPs found in a Shallow lake-China, while both MF and PA were of the thirteen PPCPs found in the red-sea water 33,34 . In addition, PA consumption is expected to increase enormously as it had been recommended to treat the symptoms of the COVID-19 pandemic [35][36][37][38][39] . Natural clay is a safe and low-cost material, making it a suitable alternative to nano-sorbents if it has an adequate adsorption capacity. a natural clay NC was collected from the Eldoushain area, Nile river state, Sudan (16°45′43.1″N 33°35′25.0″E) [40][41][42][43] . To improve the adsorption capacity, water treated clay (WTC) and an acid-treated clay ATC were employed for the removal of MF and PA from real water samples. Since some recent Adsorption studies. Adsorption of MF and PA in an aqueous solution by WTC and ATC was studied. The adsorption was possibly due to Van der Waals interactions and/or electrostatic attraction between the pharmaceutical pollutant and the sorbents [54][55][56] . The impact of WTC and ATC mass on the trend of adsorption for MF and PA was investigated. As the sorption sites increase by increasing the mass of sorbent, the adsorption percentage increased; meanwhile, the adsorption capacity decreased inversely. The efficiency of WTC and ATC in the removal of PA and MF are comparable to those found in the literature with the advantage of low cost for the sorbent in this study [57][58][59][60][61][62][63] . www.nature.com/scientificreports/ The contact time study revealed that 20 min sufficed the adsorption of MF and PA on ATC to reach the equilibrium; on the other hand, it takes 30 min for the adsorption of PA and MF on WTC (S. 1). The impact of ionic strength on the adsorption of PA and MF on WTC and ATC was investigated. The decrease in adsorption efficiency as the ionic strength increased may be attributed to the influence of salinity on the adsorption process (S. 2a). Also, the significant reduction in the adsorption efficiency of the MF may be attributed to the five partially negative nitrogen atoms that may repulse with any negative sites on the sorbent surface and/or negative sorbates 64,65 . The pH effect on the adsorption of PA and MF on WTC and ATC was investigated. The best adsorption percentage for MF on both WTC and ATC was achieved at a pH value of 4.0; on the other hand, the best adsorption percentage for PA on both WTC and ATC was achieved at a pH value of 6.0 (S. 2b). Moreover, the influence of initial fed concentrations of PA and MF on WTC and ATC adsorption efficiency was studied. The adsorption percentage for both sorbents was inversely proportional to the initial fed concentration for both drugs. This trend illustrates that both sorbents can be very effective for treating MF and PA at low concentrations, as expected to be in the polluted water resources.
Adsorption kinetic order. The some studies in the literature highlighted the misleading results of the linear pseudo-second-order kinetic equations 66 . Considering that, non-linear pseudo-first-order (NLPFO) (Eq. 1), non-linear pseudo-second-order (NLPSO) (Eq. 3), linear pseudo-first-order (LPFO) (Eq. 2), and linear pseudosecond-order (LPSO) (Eq. 4) were all employed for the kinetic study investigations 67-69 . where k 1 (min -1 ) is the PFO rate constant, k 2 (g mg -1 min -1 ) is the PSO rate constant, q t and q e (mg g -1 ) are the adsorbed mass of drug per unit mass of sorbent at time t and equilibrium, respectively.
The adsorption rate constant were calculated from the slope of the corresponding regression 70 . The adsorption kinetic orders were determined by the best linear fit judged usually by the best correlation coefficient (R 2 ) value 66 . Also, the residual sum of squares (RSS), the value of Chi-squared (χ 2 ), were considered as they reflects the goodness of regressions ( Fig. 3; Table 1). The adsorption of MF and PA on WTC and ATC fitted the PSO kinetic models since it revealed the lowest χ 2 , the highest R 2 , a semi-typical q e, and the lowest RSS value. Worthmentioning that the q e predicted from the NLPSO was almost typical to the experimental values for all sorbent-sorbate systems in this study.
Adsorption rate-controlling mechanism. The adsorption rate-control-mechanism was investigated for the adsorption of PA and MF on WTC and ATC by the employment of the intra-particle diffusion model (IPDM) (Eq. 5) and the liquid film-diffusion model (LFDM) (Eq. 6) 67,71 .
where k ip (mg g −1 min -0.5 ) is the rate constant of the IPDM. C i : concentration in mg g -1 (parameter relates to the boundary layer thickness).
where k LF (min -1 ) is the equilibrium fractional attainment.
Although the K IP values were much more than the K LF values indicating the preference of IPDM, it worth mentioning that the high C i values indicated a high boundary-layer resistance to the IPDM 72 . The regression results of LFDM and IPDM fittings (Fig. 4) revealed that LFDM mainly controlled the adsorption of PA and MF on WTC and ATC since it had the least RSS, the highest R 2 , and the least χ 2 as demonstrated in Table 1. Besides, the multi-linearity curve produced by IPDM evident that the adsorptions of PA and MF on WTC and ATC were complicated ones that may be controlled by two or more mechanisms 73,74 . This complexity may be attributed to the diverse nature of the clay sorbent; the presence of diverse types of clay minerals may impart to the difference in diffusion models.
Adsorption isotherms. The Langmuir and Freundlich isotherms were employed to investigate the adsorption of PA and MF on WTC and ATC. The linear and nonlinear forms of the Langmuir model (LLM and NLLM) were utilized to investigate the monolayer adsorption with no sorbate penetration in the surface-plane explained  (7) and (8), respectively. The adsorption nature was further investigated by the Langmuir isotherm separation factor (S L ) (Eq. 9).
where C e is the drug concentration at equilibrium, q m and K L (L mg −1 ) represent the maximum adsorption capacity, and the Langmuir constant corresponds to the monolayer adsorption, respectively. As a multilayered adsorption model, the linear and the nonlinear forms of the Freundlich model (LFM and NLFM) were employed in this study (Eqs. 10, 11, respectively).
is the Freundlich constant, while n (arbitrary) is the Freundlich heterogeneity factor related to the adsorption capacity. Worth mentioning that an n value ranged between 0 and 10 indicates the favourability adsorption process 75 . Figure 5 illustrated the fitted curves of isotherms while the obtained isotherms parameters were monitored in Table 1. The adsorption of PA and MF on WTC and ATC does not fit the LLM or NLLM.

Thermodynamic studies
The temperature impact on the adsorptions of PA and MF on WTC and ATC was investigated, and for a better understanding of the adsorption behaviors of PA and MF on WTC and ATC, the thermodynamics of this process was investigated. The thermodynamic parameters, including the enthalpy (ΔH o ), entropy (ΔS o ), and Gibbs free energy (ΔG o ), were evaluated (Eqs. 12, 13).  Fig. 6. The ideal gas constant (R = 0.0081345 kJ mol −1 ) was applied in all calculations, and the thermodynamic parameters were monitored in Table 2. The negative ΔH o values indicated the exothermic nature of the adsorption of PA and MF on both sorbents meaning that the adsorption efficiency may be enhanced by lowering the solution's temperature. Besides, a ΔH o of less than 40 kJ mol indicates a physisorption process; conversely, it is considered chemisorption when having a ΔH o of 80-450 kJ/mol [81][82][83] . The adsorption spontaneity was inferred from the negative ΔG o values for both pollutants on WTA and ATC 84 ; this finding was supported by the negative ΔS o values 85 . Also, the adsorption appeared to go less spontaneous as the temperature increased. Nevertheless, the obtained ΔG o values (less than 20 kJ mol −1 ) indicated that the adsorption process is a physisorption 67,68,83,86-88 . Application to natural water samples. Tap water (TW) was collected from the chemistry Lab at Riyadh, Kingdom of Saudi Arabia (KSA). The Red Sea water sample (RSW) was picked from the coast of Jeddah City-KSA, while the groundwater (GW) sample was collected from the Sudair area (150 km North Riyadh-KSA). All samples were collected during the summer season, the total dissolved salts for TW, GW, and RSW was 0.2, 1.1 and 32.5 g L −1 , respectively; and the total hardness was 146.2, 751.8 and 7293 mg L −1 , respectively. The applicability of WTC and ATC for the removal of MF and PA from TW, GW, and RSW was explored. A concentration of 1, 5, and 10 mg L −1 were prepared for each drug in 250 mL of each water sample. The optimized conditions were applied, and the removal of PA and MF by WTC and ATC (S. 3). WTC and ATC were found to be efficient for the removal of PA and MF from the three samples. Besides, a promising result has emerged during this application that the total dissolved solids (TDS) of RSW, which was measured by a conductivity-meter (Jennway 6605), decreased significantly after being in contact with the sorbents revealing a desalination capacity of 1200 and 1500 mg g −1 for WTC and ATC, respectively.

Conclusion
This work demonstrates the optimization of adsorption of PA and MF by WTC and ATC from aqueous solution. The optimized conditions were applied to real environmental water samples. The adsorption isotherms data fitted the Freundlich model, and the adsorption processes obeyed the pseudo-second-order with a better regression parameter with the NLPSO. The maximum adsorption capacity ranged between 20.4 and 30.5 mg g −1 . The application of both sorbents for fast removal of PA and MF from contaminated water is recommended. Considering the almost-zero-cost sorbent, the relatively short intake time and the desalination effect.

Experimental
Materials. NC was collected from the Eldoushain area, Nile River state, Sudan, hydrochloric acid 37% was purchased from Sharlau-Spain. The active pharmaceutical ingredients MF and PA were purchased from Ranbaxy Laboratories Limited, India. The washing of sorbent and the preparation of all solutions were performed using distilled water.
Preparation of WTC and ATC . The NC was crushed in a porcelain mortar and milled by ball milling at 500 rpm for 30 min, the ball: clay mass was 1:1. 10 g of NC were stirred with 200 mL distilled water for 1 h, filtered through Buchner system, washed by 1 L distilled water, dried at 120 °C for 2 h, cooled in a desiccator, and kept in a polypropylene container. 10 g of NC were stirred with 200 mL of diluted hydrochloric acid (1:1), filtered through Buchner system, washed by distilled water (to pH = 7), dried at 120 °C for 2 h, cooled in a desiccator, and kept in a polypropylene container.
Characterization of WTC and ATC . WTC and ATC were characterized using scanning electron-energy dispersive X-ray technique (SEM-EDS), model JSM-IT300. The WTC and ATC crystalline structures were examined by X-Ray diffractometer, Bruker, D8 Advance; Billerica, MA, USA. The functional groups for both sorbents were monitored using Bruker TENSOR-FTIR spectrophotometer (Germany). Characteristics of pores (i.e., pore diameter and volume) and specific Brunauer-Emmett-Teller (BET) surface area were evaluated based on N 2 adsorption-desorption isotherms at 77 K using an ASAP 2020 Micromeritics analyzer. The surface area, pore size and pore diameter were calculated from the Brunauer-Emmett-Teller (BET) equation 89 14) and (15).
where C o is the initial fed concentration, C t is the unabsorbed concentration, q t is the adsorbed amount (mg g −1 ), V is the volume of the solution (L), and M is the mass of sorbent (g). Each experiment was carried three times, and the average was calculated.