Preparation and performance of surface-modified adsorbent materials from discarded traditional chinese medicine residues

Waste Chinese medicine residue was used as a raw material and pretreated with sodium hydroxide and hydrogen peroxide, followed by chemical modification with a silane to prepare an inexpensive and highly efficient hydrophobic biobased adsorbent material. The adsorbent was characterized with SEM, TGA and FTIR analyses. The adsorption capacities and wettabilities of the Chinese medicine residue were analyzed before and after surface modification to explore the adsorption performance and surface modification mechanism of the material. The results showed that the modified Chinese medicine residue was rough and uniformly modified, with successfully grafted hydrophobic functional groups, and it had added adsorption sites, exhibiting good hydrophobicities and oleophilicities. The contact angles between the modified materials and water reached 127°, and the modified Chinese medicine residue had an adsorption capacity of 51.7 mg g−1 for organic compounds, which was a significant improvement over the original waste Chinese medicine residue. The adsorption kinetics were best described with the pseudosecond-order kinetic model, which exhibited a higher linear correlation and was closer to the measured adsorption equilibrium value based on chemical adsorption. This study demonstrated a novel use of waste Chinese medicine residues for environmental remediation.


Introduction
Chinese herbal medicines are drugs used to prevent or treat diseases, according to the clinical practices of traditional Chinese medicine.This includes Chinese medicinal materials, Chinese herbal decoctions, and Chinese patent medicines, which have been in use for over 2,000 years and are still widely used in China and Southeast Asia [1].Chinese patent medicine has gained popularity owing to the wide range of diseases it treats with limited side effects, leading to increased demand in the pharmaceutical industry, especially in China.As a result, the annual residual emission of traditional Chinese medicine in China has reached 60-70 million tons [2,3].Despite the benefits of Chinese herbal medicines, traditional disposal methods such as incineration, stacking, and landfilling have caused serious environmental pollution and resource waste [4,5] .Similar to crop straw and green manure, traditional Chinese medicine residues are solid plant substances produced after extraction of the pharmaceutical ingredients from herb materials.Waste Chinese medicines are rich in cellulose, hemicellulose, lignin, and others, which makes them promising raw materials for energy, material, and chemical production [6,7].Therefore, new treatment methods for effectively managing waste Chinese medicine residues and improving resource utilization have become research priorities.The goal is to turn waste into protection by reducing environmental pollution and avoiding resource waste.In recent years, waste Chinese medicine residues with cellulose structures, have shown certain advantages as natural biologically-based adsorption materials and received significant attention in industrial and environmental protection research.Many studies have analyzed the structures of Chinese medicine residues, selected suitable surface modifiers, and altered the surface structures of Chinese medicine residues for use in the treatment of heavy metal water pollution [8][9][10].Based on the characteristics of Chinese medicine residues, appropriate processes have been identified for modification of waste Chinese medicine residues to treat antibiotic contamination [11][12][13][14][15] and common water pollution [16][17][18], with promising application prospects.Potential modification sites include the many hydrophilic groups present on the surfaces of waste Chinese medicine residues.However, the process of grafting hydrophobic and hydrophilic groups through surface modification to achieve high adsorption efficiencies and selective adsorption in the treatment of organic water pollution requires further study.
To expand the possible uses of waste traditional Chinese medicine dregs, this study utilized these dregs as raw materials for grafting of hydrophobic groups onto their surfaces by selecting appropriate surface modifiers.Through adhesion testing, it proved possible to determine the adsorption capacities, wettabilities, and kinetic adsorption mechanisms of the materials for organic pollutants.The purpose of this research was to prepare biobased adsorbent materials with highly selective adsorption properties for treatment of the organic pollutants in water, which would solve the problem of treating waste Chinese medicine residues and demonstrate the remarkable potential of the absorbent for remediating organic water pollution.

Materials
The waste Chinese medicine residues used in this study were obtained from a pharmaceutical factory in Guiyang, China.The material mainly comprised pharmaceutical residues extracted from common Chinese herbal medicines, including Lysimachia christinae Hance, Dandelion, Salvia miltiorrhiza, Bupleurum, Semen cassiae, Dendrobium, and other Chinese herbs.Prior to use, the material underwent pretreatment with sodium hydroxide (NaOH) and hydrogen peroxide (H 2 O 2 ), which were purchased from Aladdin Reagent (Shanghai) Co., Ltd.Silane, silicon dioxide, and hydrochloric acid (HCl) were employed as modifying agents and were purchased from Sinopharm Shanghai Co., Ltd Ethanol (CH 3 CH 2 OH, which was chemically pure) was purchased from Guangdong Guanghua Chemical Co., Ltd Toluene, Sudan-III, xylene, and petroleum ether were purchased from Tianjin Kemeiou Chemical Reagent Co., Ltd.All chemicals used in this study were of analytical grade and used as received without further purification.

Preparation of the materials
To prepare the materials, the waste Chinese medicine residue was crushed and dried.Five grams of the waste Chinese medicine residue was then placed into a 1 l three-necked flask and mixed with 800 ml of a 1.5 g l −1 sodium hydroxide solution.The mixture was stirred and placed in a water bath magnetic stirring pot at 95 °C.Then, 2 ml of hydrogen peroxide was added dropwise and stirred magnetically for 1 h.Once the reaction was over, the mixture was allowed to cool at room temperature, and the pH was adjusted to neutral with a 0.1 mol l −1 diluted hydrochloric acid solution.The pretreated Chinese medicine residue was then separated with vacuum filtration.The filtered Chinese medicine residue was dried in a vacuum drying oven at 50 °C for 6 h to obtain the pretreated Chinese medicinal residue [19].
To modify the surface of the pretreated Chinese medicinal residue, 0.50 g of the residue was placed into a beaker filled with 80 ml of anhydrous ethanol, 0.10 g of silica, and 3 ml of a 0.1 mol l −1 sodium hydroxide solution at room temperature.After stirring for 3 min, 4 ml of vinyltriethoxylsilane was added dropwise, stirred together and then subjected to ultrasonic treatment for 30 min.The mixture was transferred to a Teflon-lined stainless steel reaction kettle and reacted in a muffle furnace at 105 °C for 1.5 h.After sufficient reaction time, the mixture was allowed to cool to room temperature and dried in an oven after extraction and filtration to obtain the surface-modified waste Chinese medicine residue adsorption material.

Analytical methods
The waste Chinese medicine residue was modified through silanization and metal spraying.The surface morphology of the modified Chinese medicine residue was characterized with an FEI QUANTA-200 scanning electron microscope, and the surface elements were analyzed at the corresponding positions.The surface structures of the Chinese herb residue were analyzed before and after modification with a TENSOR2 infrared spectrometer.The thermal stability of the Chinese herb residue was characterized with a TG209F1 thermogravimetric system under heated air; the temperature rose from 25 °C to 900 °C at a rate of 10 °C min −1 .The wetting abilities of the Chinese pharmaceutical residue before and after modification were characterized with a JC96-JC2000C wetting angle tester.
The adsorption capacities of the materials were characterized by effective adsorption of different organic compounds.Table 1 shows the parameters for these organic compounds at room temperature.The desired amount of the modified Chinese medicine residue adsorption material was weighed and immersed in each organic solution for 10 min to achieve adsorption equilibrium.The difference in the material mass before and after adsorption was calculated to determine the adsorption efficiency, and the value was measured several times and averaged.The adsorption efficiency of the material was calculated with the following formula: Where W 2 is the weight of the Chinese medicine residue when adsorption equilibrium was reached, and W 1 is the weight of the original Chinese medicine residue before adsorption.

Analyses of morphologies
Figure 1 shows the microtissue morphologies of the discarded Chinese medicine residue before and after surface modification.The surface morphology in figure 1(a) appeared smooth and nonporous, with a layer of lipids wrapped on the outside and a layer of solid lignin present on the surface of the cellulose.However, after surface silanization of the adsorption material, the interlayer structure on the surface of the treated waste Chinese medicine residue was controllable and provided more active sites for subsequent grafting.This change can be seen clearly in figure 1(b), which shows the rougher surface structure with visible holes and cracks, implying that silicon covered the surface of the Chinese medicine residue.As shown in figure 2, the surface of the modified Chinese medicine residue displayed an obvious absorption peak for Si.The presence of silicon distributed evenly on the surface of the modified adsorbent indicated that surface silanization of the waste Chinese medicine residue was uniform.The figure shows some inhomogeneities due to the increased surface roughness of the modified Chinese medicine residue, which is consistent with the report of Zhang et al [21].

Infrared spectroscopy
Figure 3 displays the infrared spectra of the waste Chinese medicine residue before and after surface modification.Both samples exhibited a clear absorption peak in the high-frequency area of 3485 cm −1 , which was caused by bending vibrations of the hydroxyl groups (-OH) [22,23].However, the strength of this hydroxyl peak diminished significantly after surface modification of the waste Chinese residue.This reduction was due to reactions of the reagent used for silanization of the waste Chinese medicine residue material, which was pretreated to expose numerous hydroxyl groups on its surface.The end result was the formation of the -Si-O-Si-bonds on the surface [24]: the hydrophobic groups were connected, forming a thin layer of polysiloxane.
Consequently, the number of hydrophilic groups on the surface of the material decreased significantly, making it hydrophobic.New absorption peaks appeared at 2925 cm −1 and 1470 cm −1 for the waste Chinese medicine residue after surface modification, and these peaks resulted from tensile vibrations of the silanized groups leading to in-plane deformations.Moreover, a peak at 673 cm −1 was evident for -Si-C-vibrations [25].The presence of these infrared peaks indicated that the surface of the modified waste Chinese medicine residue had many hydrophobic groups successfully grafted, which resulted in a significant reduction in the number of hydrophilic hydroxyl groups (-OH) resulting from silanization.

Thermogravimetric analyses
Figure 4 shows the thermal weight analyses conducted on the waste Chinese medicine residue before and after surface modification to confirm the thermal stability.The TGA curves indicated a slight mass loss below 200 °C caused by continuous volatilization of the water, hydrocarbons, and other volatile organic compounds  contained within the Chinese medicine residue material [26].The waste Chinese medicine residue material, composed primarily of plant-based cellulose tissue, showed a pyrolysis temperature of 310 °C-380 °C.
To determine the thermal stability of the Chinese medicine residue before and after modification, the temperature T at which 10% mass loss occurred and the corresponding temperature T dm for the maximum decomposition rate were selected for comparison.The TGA curve showed that after surface modification, T 10 had increased from 195.6 °C to 230.4 °C, while T dm increased from 324.4 °C to 331.5 °C.These results suggested that the thermal stability of the waste Chinese medicine residue material was improved after surface modification.At 600 °C, the mass remained relatively constant, with some sample residue remaining.The presence of minerals in the Chinese medicine residue and silicon-containing substances such as polysiloxane, which results from the silanization treatment, may have accounted for this observation.

Adsorption kinetics
To characterize the adsorption capacities of the raw and modified Chinese medicine residue over time, adsorption experiments were carried out with organic matter.Figure 5 contains curves showing the adsorption capacity over time within 15-105 min for the Chinese medicine residue before and after modification.The adsorption efficiency of the modified Chinese medicine residue was obviously higher than that of the original Chinese medicine residue.Moreover, the curves revealed that the amount of organic matter adsorbed by the Chinese medicine residue material after surface modification increased rapidly between 15 min and 105 min.Subsequently, the amount adsorbed increased more gradually.The adsorption rate was fast and reached equilibrium at 105 min, making it ideal for treatment of organic water pollutants.The maximum adsorption capacity of the modified Chinese medicine residue was 36.924mg g −1 , after which it stabilized and showed only small fluctuations.This could be due to reversible adsorption and desorption [27].
To investigate the adsorption rate and principles of the modified Chinese medicine residue, two different kinetic equations were used to fit the adsorption data, the pseudofirst-order and pseudosecond-order equations.Figure 6 displays the fitted curves.The Lagrange pseudofirst-order kinetic model (equation ( 1)) is mainly used with solid-liquid adsorption [28,29] and emphasizes the linear relationship between the adsorption rate and the reactant concentration, as expressed by: On the other hand, the pseudosecond-order kinetic equation (equation ( 2)) describes chemisorption, in which electrons are shared and exchanged between the adsorbent and adsorbate [30][31][32]: where t is the adsorption time in minutes, q e is the amount adsorbed at equilibrium in mg/g, and q t is the amount adsorbed at time t in mg/g.The fitted slope and intercept provided the q , e k , 1 and k 2 vlues.The fitted curves for ( ) q q ln e t and t q t versus t are shown in figure 6.
According to the fitted adsorption curves, the pseudofirst-order kinetic model had a fitting equation of y = −0.0504x+ 3.7858 and an R 2 value of 0.7375.Meanwhile, the pseudosecond-order kinetic model had a fitting equation of y = 0.0228x + 0.4045 with an R 2 value of 0.9885.Table 2 displays the data obtained from fitting the experimental data for adsorption by the modified Chinese medicine residue over time (as shown in figure 6) with the pseudofirst-order and pseudosecond-order equations.
The fitting results indicated that the pseudosecond-order kinetic equation exhibited a better linear correlation than the pseudofirst-order kinetic equation, as reflected by an R 2 value of 0.9885.Furthermore, the theoretical saturated adsorption quantity q e obtained with the pseudosecond-order kinetic equation was closer to the measured value than the quasifirst-order kinetic equation.These results demonstrate that the quasisecond-order kinetic model provided a better description for the adsorption of organic compounds by the modified Chinese medicinal residue, which primarily involved chemical adsorption [33,34].

Wettability analyses
To analyze the wettability of the modified waste Chinese medicine residue, we measured the wetting angle between the material and water, as shown in figure 7. The original waste Chinese medicine residue exhibited good wettability, with a wetting angle of 61.4°.After half a minute, the wetting angle decreased rapidly, indicating Table 2. Adsorption kinetic parameters for adsorption of organic matter onto the modified Chinese medicine residues.

Pseudofirst-order kinetic model
Pseudosecond-order kinetic model hydrophilic characteristics (figures 7(a), (b)).After the surface modification, the wetting angle of the waste Chinese medicine residue increased to 127°, demonstrating hydrophobicity, as confirmed by the stable and sustained wetting angle.To verify the adsorption efficacy of the waste Chinese medicine residue, Sudan IIIstained organic matter and blue ink-stained water were used, as revealed in figures 7(c), (f).The original waste Chinese residue exhibited efficient adsorption of both organic matter and water.In contrast, the modified waste Chinese residue adsorption material demonstrated poor wettability with water, a sustained large wetting angle, and rapid and complete organic matter absorption, reflecting hydrophobicity and oil affinity.Surface silanization was feasible and highlighted the potential for use of the modified waste Chinese medicine residue as an adsorbent in managing petroleum leakage and chemical pollution.The waste Chinese medicine residue displayed hydrophobicity and lipophilicity after surface modification.To verify selective adsorption of the modified waste for treating organic pollutants in water and ensure optimal adsorption efficiency, we used a specific amount of surface-modified waste Chinese medicine residue to adsorb organic matter present on a water surface, as presented in figure 8.The modified waste Chinese medicine residue was distributed evenly on the surface of the oil-containing water.After slight stirring, as shown in figure 8(c), the modified waste Chinese medicine residue material adsorbed the oil present on the water surface and progressively accumulated and diminished it.After aggregation of the adsorbed waste Chinese medicine residue material, the organic pollutant on the water surface was adequately absorbed and collected with a leakage net, as depicted in figure 8(d), confirming selective adsorption with high efficiency.

Adsorption capacity tests
To verify the ability of the modified waste Chinese medicine residue to adsorb organic matter, we used petroleum ether, xylene, edible oil, and machine oil as the organic materials.The silanized and untreated waste  Chinese medicine residues were immersed and used to adsorb the organic pollutants, and the adsorption rates of the materials were measured upon reaching adsorption equilibrium, as seen in figure 9.The surface modification significantly increased the adsorption capacity of the waste Chinese medicine residue and the saturated adsorption capacity of the modified residue was twice that of the original residue.The oil adsorption capacity of the modified Chinese medicine residue was 51.7 mg g −1 , which was a notable improvement over that of the original waste Chinese medicine residue.

Mechanism for the surface modification
The mechanism for formation of the modified Chinese medicine residue material with excellent hydrophobicity and lipophilicity is shown in figure 10.The Chinese medicine residue contained significant amounts of cellulose and other components [35], and the surface also contained hydrophilic functional groups, such as hydroxyls (-OH), which showed a certain capacity for adsorption both water and organic substances.
However, in the presence of oil and water, the water and oil competed for the surface sites of the Chinese medicine residue, resulting in poor adsorption of organic substances.Due to the stability of the cellulose surface, it was difficult for the vinyltriethoxysilane to contact the active hydroxyl groups on the surface of the Chinese medicine residue, leading to reduced accessibility for surface modifications and significantly impairing modification of the Chinese medicine residue material surface.Therefore, sodium hydroxide and hydrogen peroxide were used for activation pretreatments of the Chinese medicine residue surface prior to surface modification, which exposed many hydroxyl (-OH) groups on the surface of the cellulose and provided more sites for surface silanization.Additionally, the silane coupling agent was easily hydrolyzed to form silanols, and  the hydroxyl groups in the silanols are highly active and readily form strong covalent bonds with hydroxyl groups on the surfaces of metals or inorganic materials.These moieties reacted readily with ethoxy groups (-O-C 2 H 5 ) of VTES in the silanization reaction.This process, along with self-polymerization of the silane (Reaction in figure 10(a)), generated polysiloxanes on the surface of the Chinese medicine residue, greatly reducing the number of exposed hydrophilic functional groups and resulting in the formation of a hydrophobic surface.As a result, the modified Chinese medicine residue exhibited excellent selective adsorption of the organic compounds polluting water.

Conclusion
In summary, we developed a new organic adsorbent material through simple surface silanization with a biodegradable Chinese medicine residue as the starting material.
The resulting modified Chinese medicine residue absorbent showed excellent hydrophobicity, with a water wetting angle of 127°and stability, which was due to the removal of hydrophilic groups, successful grafting of hydrophobic groups on the surface and increased roughness of the material surface.The prepared absorbent adsorbed various organic liquids with a maximum adsorption capacity of 51.7 mg g −1 for motor oil.In addition, the material selectively adsorbed the oil from an oil-water mixture at room temperature.With some advantages, such as easy applicability, excellent phase selectivity, short absorption times, and a green preparation process, the modified Chinese medicine residue overcame the difficulty of treating waste Chinese medicine residue and demonstrated its remarkable potential as an effective absorbent for organic water pollutants.

Figure 1 .
Figure 1.SEM images of Chinese medicine residues before and after modification: (a)-(b) original Chinese medicine residues; (c)-(d) Chinese medicine residues after modification.

Figure 2 .
Figure 2. Elements present in Chinese medicine residues after modification: (a) EDS analysis and (b) elemental maps for C, O and Si.

Figure 3 .
Figure 3. FT-IR spectra of Chinese medicine residue samples before and after modification.

Figure 4 .
Figure 4. TG curves for the Chinese medicine residues before and after modification.

Figure 5 .
Figure 5. Adsorption dynamics of Chinese medicine residues before and after modification.

Figure 6 .
Figure 6.Fits of the adsorption kinetic date for modified Chinese medicine residues.

Figure 7 .
Figure 7. Hydrophobic and oleophilic properties of the original Chinese medicine residue (a)-(c) and modified Chinese medicine residue (d)-(f).

Figure 8 .
Figure 8. Adsorption of oil pollution by the modified Chinese medicine residue: (a) Macro morphology of the modified Chinese medicine residues , (b) oil-water mixture, (c) adsorbed material and (d) the material separated after adsorption.

Figure 9 .
Figure 9. Adsorption of different organic compounds on Chinese medicine residues before and after modification.

Figure 10 .
Figure 10.Mechanism for functionalization of the modified Chinese medicine residue.