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Article

Alkali Etching Hydrochar-Based Adsorbent Preparation Using Chinese Medicine Industry Waste and Its Application in Efficient Removal of Multiple Pollutants

by
Xinyan Zhang
1,*,
Shanshan Liu
1,
Qingyu Qin
2,
Guifang Chen
1 and
Wenlong Wang
1
1
National Engineering Laboratory for Reducing Emissions from Coal Combustion, Engineering Research Center of Environmental Thermal Technology of Ministry of Education, Shandong Key Laboratory of Energy Carbon Reduction and Resource Utilization, School of Energy and Power Engineering, Shandong University, Jinan 250061, China
2
Laboratory of Biomass and Bioprocessing Engineering, College of Engineering, China Agricultural University, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Processes 2023, 11(2), 412; https://doi.org/10.3390/pr11020412
Submission received: 17 November 2022 / Revised: 19 January 2023 / Accepted: 27 January 2023 / Published: 30 January 2023
(This article belongs to the Special Issue Biomass Conversion and Organic Waste Utilization)
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Abstract

:
The annual discharge (6–7 million tons per year) of Chinese medicine industry waste (CMIW) is large and harmful. CMIW with a high moisture content can be effectively treated by hydrothermal carbonization (HTC) technology. Compared with CMIW, the volume and number of pores of the prepared hydrochar increased significantly after alkali etching (AE), and they had abundant oxygen-containing functional groups. These properties provide physical and chemical adsorption sites, improving the adsorbent activity of the alkaline etching of Chinese medicine industry waste hydrochar (AE-CMIW hydrochar). However, few studies have investigated the adsorption of organic dyes and heavy metals in mixed solutions. This study proposed a method of coupling HTC with AE to treat CMIW and explored the potential of AE-CMIW hydrochar to remove metal ions and organic dyes from mixed solution. We analyzed the removal rates of metal ions and organic dyes by the adsorbents and investigated their differences. The results showed that the lead ion, cadmium ion, and methylene blue could be efficiently removed by AE-CMIW hydrochar in a mixed solution, with removal rates of more than 98%, 20–57%, and 60–80%, respectively. The removal rates were different mainly due to the various electrostatic interactions, physical adsorption, differences in the hydrating ion radius of the metal ions, and functional group interactions between the AE-CMIW hydrochar and the lead ion, cadmium ion, and methylene blue. This study provides a technical method for preparing multi-pollutant adsorbents from CMIW, which enables efficient utilization of organic solid waste and achieves the purpose of treating waste with waste.

1. Introduction

The annual output of Chinese medicine industry waste (CMIW), a type of organic solid waste containing many bacteria, is huge [1]. It can reach six to seven million tons per year [2,3,4]. The unreasonable disposal of CMIW causes environmental pollution and ecological damage and threatens human health [5]. The Ministry of Ecology and Environment of the People’s Republic of China promulgated the Law of the People’s Republic of China on the Prevention and Control of Environmental Pollution by Solid Waste, which stated that the prevention and control of environmental pollution by solid waste should adhere to the principles of reduction, recycling and harmless treatment [6]. In the European Union countries, the waste management system presupposes an integrated system of various aspects: social, economic, regulatory, managerial, technical. The new edition of the Environmental Code of the Republic of Kazakhstan on 2 January 2021 indicated that the level of this country’s waste management is closer to international standards [7]. The microbes in CMIW can be killed by heat treatment. The CMIW can be converted into carbon material after high-temperature treatment, which is a method for efficient resource utilization of CMIW. The heat treatment of organic solid waste usually includes pyrolysis, baking, and hydrothermal carbonization (HTC). The moisture content of CMIW is high, and pre-drying is required if CMIW is treated by pyrolysis and baking, which can increase the overall operational cost. HTC is a thermochemical process in a wet medium that can effectively treat organic solid waste with high moisture content, such as CMIW [8,9]. Studies have shown that HTC is a promising technique for converting organic solid waste into hydrochar, which can be used as a fuel, adsorbent, catalyst, and others [10,11,12]. Hydrochar has porous structures and abundant surface functional groups, which also depend on the feed-stock and treatment conditions [13,14]. Studies have shown that hydrochar can be effectively applied to use in wastewater treatment in a wide variety of pollutants including heavy metals and dyes [15,16,17]. Studies have shown that alkali etching (AE) can increase the porosity of hydrochar [18]. This phenomenon happens because the energized element particles (K+ and Na+) of basic activators (e.g., KOH and NaOH) generated by absorbing heat energy can penetrate the carbon structure of the hydrochar and further promote the development of pores [19].
Organic dyes and heavy metals are common toxic and harmful pollutants in the wastewater of the printing, paper-making, leather, textile, and cosmetic industries [20], mainly because many heavy metals (particularly cadmium) are used as mordants in the dyeing process in these industries. Organic dyes have complex and special structural properties and are difficult to degrade using strong oxidants [21]. The accumulation of organic dyes in organisms can easily cause gene mutations and accumulation in the environment, leading to the deterioration of water quality [22]. Heavy metals are highly toxic and carcinogenic [23], can accumulate in living organisms through the food chain, and are difficult to be eliminated from the body through metabolism [24]. These multiple pollutants in wastewater cause great harm to the ecosystem and human health. Therefore, there is an urgent need to develop efficient pollutant removal methods. Removal methods include ultra-filtration, photo-catalytic reduction, reverse osmosis, and adsorption [25,26,27,28,29], among which adsorption is the most convenient, efficient, and low-cost method [30]. Scholars have focused on removing a single pollutant in wastewater using the adsorption method [31,32,33,34]. Little research has been conducted on organic dyes and heavy metals simultaneously. Studies have shown that organic dyes or heavy metals can combine with the functional groups of adsorbents through electrostatic, chelation, and π–π interactions [23,35]. Therefore, competitive adsorption relationships exist between organic dyes and heavy metals in the same environment, leading to challenges in wastewater treatment due to the coexistence of heavy metals and organic dyes. Therefore, exploring an environmentally friendly adsorbent that can efficiently adsorb both organic dyes and heavy metals is necessary.
This study proposed an innovative method of coupling HTC with AE to treat CMIW. After this treatment, the CMIW formed porous structures and rich functional groups, making AE-CMIW hydrochar with both physical and chemical adsorption sites. Thus, CMIW might be advantageous as a raw material for the adsorbents of multiple pollutants. This study investigated the potential of the prepared AE-CMIW hydrochar as an adsorbent for multiple pollutants and the differences in the adsorption of multiple pollutants. The surface morphology and functional groups were analyzed using the Brunauer–Emmet–Teller (BET) model, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The adsorption properties of the AE-CMIW hydrochar on lead ion (Pb2+), cadmium ion (Cd2+) and methylene blue (MB) were measured. Subsequently, the differences in the adsorption characteristics and kinetics of the AE-CMIW hydrochar on multiple pollutants in the mixed solution were investigated. This study made an organic solid waste such as CMIW achieve efficient resource utilization and treat multiple pollutants in wastewater. Therefore, this research is a resource utilization model for organic solid waste treatment.

2. Materials and Methods

2.1. Experimental Material

CMIW was obtained from pharmaceutical factories in Jinan, Shandong Province, China. They consist of a variety of traditional Chinese medicine residues, which mainly included Radix Glycyrrhizae residues, Panax notoginseng residues, Radix Isatidis residues and residues of Huoxiang Zhengqi Liquid. Their main components were crude fibre, crude fat, crude protein, amino acids, polysaccharide, starch, saponins, inorganic elements, total organic matter and total organic carbon. A multiple-component solution was prepared using methylene blue (MB; C16H18N3SCl), Pb(NO3)2 and Cd(NO3)2 4H2O for MB macro-molecules, Pb2+ and Cd2+, respectively. All chemicals were of analytical grade, and no further purification was required. Deionized water was used for solution preparation and dilution to the desired concentration of 100 mg/L to mimic pollutant concentrations in industrial effluents.

2.2. Hydrothermal Carbonization and Alkali Etching

HTC of CMIW was performed using an autoclave reaction kettle (Parr 4848; Parr Instrument Co., Moline, IL, USA). Nitrogen was pumped into the reactor five times to drain air into the kettle. The temperature of the reactor was controlled using a proportional integral derivative (PID) controller [36]. The pressure was generated by self-boosting, and it could reach 2–4 Mpa in the kettle. In the HTC experiments, 100 g of CMIW (~70% w.t.) and 300 mL of deionized water were mixed in a reaction kettle and auto-stirred at a rate of 400 rpm [37]. The temperature in the reaction kettle was set to 200, 230 or 260 °C at the rate of 3–5 °C/min and then maintained at a constant temperature for 1, 2, and 4 h, respectively. After HTC, the hydrochar was separated from the mixture via vacuum filtration. The hydrochar was dried at 40 °C for 24 h for further analysis. Subsequently, a concentration gradient series of aqueous KOH solutions was prepared, and AE was conducted at a 1:15 (w/v) hydrochar/KOH solution ratio. The conditions of HTC coupled with AE are denoted as AEHTC A-B-C in the figures and tables, where A is the temperature (°C), B is the duration (h), and C is the alkali concentration (mol/L).

2.3. Analytical Method

Hydrochar surface morphologies were determined with a ZEISS SUPRA 55 field emission scanning electron microscope (SEM, Carl Zeiss Co., Ltd., Oberkochen, Germany) at 30 kV acceleration voltage. Prior to analysis, the samples were mounted on a stub and sputter coated with gold [38]. The specific surface area (SBET) was calculated using the BET method, whereas the micro-pore and mesopore volumes were calculated using the Barrett–Joyner–Halenda model. The N2 adsorption–desorption isotherms were plotted with a BK100 specific surface area and aperture synchronous analyzer (Beijing Jingwei Gaobo Science and Technology Co. Ltd., Beijing, China) at 77 K and using N2 as the analytical gas. Before the measurement, all samples were degassed at 100 °C for 12 h to remove physisorbed water and impurities. The powder sample was uniformly adhered to the conductive tape. The acceleration voltage of the instrument was 0.02–30 kV, and the amplification factor was 20 K [39]. The surface element compositions of the hydrochar samples were analyzed using a matching Energy Dispersive Spectrometer (EDS, Carl Zeiss Co., Ltd., Oberkochen, Germany). The scans with Cu Kα radiation source were in the range of 10–70 (2θ).
The functional groups were analyzed using a Spectrum 400 Fourier infrared spectrometer (Thermo Nicolet Corp., Waltham, MA, USA). The sample and KBr were dried and mixed in a ratio of 1:100. Then, they were ground with a mortar and mixed thoroughly. The mixture was laminated for infrared spectral analysis. The infrared spectrum range was set as 4000–400 cm−1, and the resolution was set as 4 cm−1. The infrared spectral image of the sample was taken by a Merlin small cold field emission scanning electron microscope at 1 kV [37].
The isopotential point (pHIEP) of the hydrochar in an aqueous environment was tested using a zeta potential analyzer (Malvern Zetasizer Nano ZS90, Arkansas, USA). The pHIEP of hydrochar samples was determined by measuring the initial and final solution pH values. The initial solution pH was varied in the range of 2–10 in intervals of 2 by using 0.1 M HCl and NaOH solutions. The solution (50 mL) with sample (50 mg) was vibrated for 24 h. The pHIEP was obtained by plotting the of initial solution pH against final solution pH [40].
Hydrochar samples (0.5 g) were put into the polyfluoroethylene tube; then, they were digested by microwave digestion (MDS-6C, SINEO) using HNO3-HCl for 1.5 h. The residual concentrations of the heavy metals were determined using inductively coupled plasma–optical emission spectrometry (ICP-OES, Optima 7000D, Waltham, MA, USA). Inductively coupled plasma is a high-frequency electromagnetic field generated by a high-frequency current through an induction coil, which ionizes the working gas (Ar) to form a high temperature plasma with flame discharge. The default flow rate of Ar is 0.8 L/min, and the partial pressure of Ar is maintained between 0.6 and 0.8 Mpa during operation. The maximum temperature of the plasma is 10,000 K. The sample solution passes through the injection capillary and then enters into the atomizer by a peristaltic pump to form aerosol. The high temperature plasma is introduced through the carrier gas and then evaporates, atomizes, excites, ionizes, and produces radiation. The light source passes through the daylighting tube into the slit, the reflector, the prism, the middle step grating and the collimator, and then forms a two-dimensional spectrum. The spectral lines fall on the CID detector of 540 × 540 pixels in the form of light spots, and each spot covers several pixels. A spectrometer measures elemental concentrations by measuring the number of quantal light falling onto a pixel.

2.4. Adsorption Experiment

A mixed solution of MB, Pb2+ and Cd2+ was used to simulate the industrial wastewater. The concentrations of the three solutes were all configured at 100 mg/L, and then, the AE-CMIW hydrochar treated under different conditions was placed in 40 mL of the mixed solution and oscillated at 40 rpm/min in the oscillator for 24 h until the adsorption reached equilibrium. After adsorption, the supernatant was filtered and collected. The absorbance of the filtrate was measured using a UV–vis spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) at a wavelength of 665 nm. Three parallel tests were performed for each group. The removal rates of the three pollutants were calculated using Equation (1)
R = C 0 − C e C 0 × 100
where C0 (mg/L) is the initial concentration, and Ce (mg/L) is the equilibrium of the mixed solution.

2.5. Adsorption Kinetics Experiment

In the adsorption kinetics experiment, 100 mg of the AE-CMIW hydrochar sample was added to separate vials containing 40 mL of a 100 mg/L mixed solution. The vials oscillated at 40 rpm/min for 1, 2, 4, 6, 8, 10, 12, 18, and 24 h. The adsorption capacities for MB and metal ions were measured and calculated. Pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were employed to describe the adsorption behavior of AE-CMIW hydrochar [39]. The PFO and PSO models are expressed by Equations (2) and (3), respectively:
q t = q e ( 1 − exp ( − k 1 t ) )
q t = k 2 q e 2 t 1 + k 2 q e t
where qe (mg/g) is the adsorption capacity at equilibrium, and qt (mg/g) is the adsorption capacity at time t (h). k1 (h−1) and k2 (h−1) represent the kinetic adsorption rate constants of the PFO and PSO models, respectively.

3. Results and Discussion

3.1. Pore Structure Analyses

The pore structure analyses of the AE-CMIW hydrochar are presented in Table 1. Studies have shown that the SBET, pore size and pore volume are the indicators of the available absorption inter-phase on the hydrochar, and they have a significant influence on the adsorptive application and solute binding capacity of hydrochar [15,41]. The SBET and total pore volume (Vtotal) of the AE-CMIW hydrochar increased significantly (p < 0.05) compared with those of CMIW. The SBET increased by approximately two to three times. The total pore volume increased approximately three to four times. The average pore diameter of hydrochar also increased compared to that of CMIW. The increases in SBET and Vtotal indicated that these hydrochars could be used as adsorbents to provide more adsorption sites. With the increase in HTC temperature, the SBET decreased, and the Vtotal and average pore diameter increased. With the increase in HTC duration, the SBET decreased, and the average pore diameter increased. The static adsorption isotherms and pore size distributions of the AE-CMIW hydrochar are shown in Figure 1. The static adsorption and desorption isotherms were H3-type hysteresis rings, which showed that the hydrochars were typical mesoporous material, and the overall hysteresis ring was relatively narrow, indicating that the pore structure of hydrochar was the accumulation of flake particles, with layered structure characteristics [39]. The pore size distribution diagram showed that the pore size of hydrochar was mostly concentrated in the range of 2~10 nm. With the increase in HTC temperature, the SBET decreased. The SBET of AEHTC 200-2-3 was 53.47 m2/g, while that of AEHTC 260-2-3 was 42.61 m2/g. When HTC temperature rose, CMIW had more violent depolymerization reaction. Therefore, a large number of small molecules accumulated and precipitated on the surface of hydrochars, resulting in a part of the pore structure being blocked.

3.2. Surface Microstructures Analyses

Scanning electron microscopy (SEM) images of the CMIW and AE-CMIW hydrochars are shown in Figure 2. The SEM image of CMIW showed a smooth surface. The AE-CMIW hydrochar had rough and uneven surfaces with a coral-like morphology and abundant pores. When the HTC temperature rose, the pressure in the reactor increased accordingly. Then, the organic matter of CMIW would undergo a series of degradation and depolymerization reactions. Part of the organic matter would be converted into gas or tar and precipitated, part would be dissolved in the liquid phase in the form of small organic molecules, and the rest would reaggregate and precipitate on the surface of solid materials. With the assistance of AE, the hydrochars had loose porous surface structure and rough surface morphology [42,43]. After the reactions of HTC coupled with alkaline etching, AE-CMIW hydrochar formed new morphological feature compared with CMIW. The number of pores on the surface of hydrochar increased, and some spherical particles were formed. These pore structures could provide attachment sites for heavy metals or organic dyes. Therefore, these morphological characteristics are conducive to the adsorption of AE-CMIW hydrochar [42].

3.3. Functional Groups Analyses

The surface functional groups of AE-CMIW hydrochar were determined by the FTIR (Figure 3). During the HTC reaction process, hydronium ions could effectively promote the hydrolysis of macromolecular materials to generate various low polymers, which would further decompose into acids, aldehydes and phenolic substances, forming various functional groups on the surface of hydrochar materials. The peaks at 3430 and 1030 cm−1 were attributed to the hydroxyl groups (-OH) in the hydroxyl or carboxyl groups [44,45]. This peak was commonly found in alcohols and acids [46] and could also be characteristic of cellulose [47]. The characteristic peak at 1626 cm−1 was associated with C=O stretching vibration in the carboxyl or carbonyl groups [48]. These peaks were caused by HTC and AE treatment. The fluctuation of 1407 cm−1 was related to O-CH3 in lignin [49]. These results showed that AE-CMIW hydrochar had many oxygen-containing functional groups. They were conducive to the adsorption property of AE-CMIW hydrochar.

3.4. Adsorption Properties Analyses of AE-CMIW Hydrochar for Multiple Pollutants

The CMIW was treated by the technique of HTC coupled with AE. The effects of the AE-CMIW hydrochar on the removal rates of MB, Pb2+, and Cd2+ in the mixed solution are shown in Figure 4. The AE-CMIW hydrochar exhibited adsorption capacities for MB, Pb2+, and Cd2+ in the mixed solution. The surface of the AE-CMIW hydrochar was rough and had abundant pore structures (SEM images), which provided many sites for MB, Pb2+, and Cd2+ to attach to the hydrochar. The SBET data of AE-CMIW hydrochar and the removal rate of MB, Pb2+ and Cd2+ were analyzed and correlated. This has shown that the removal rate of MB was greatly affected by the SBET of AE-CMIW hydrochar, and they showed a relatively positive correlation. However, the removal rates of Pb2+ and Cd2+ were less affected by the changes of SBET. Other factors may mainly affect the adsorption of Pb2+ and Cd2+ by AE-CMIW hydrochar, for example, electrostatic attraction and functional group interactions. FTIR analysis revealed that the surface of the AE-CMIW hydrochar contained abundant oxygen-containing functional groups [50]. These functional groups could form complexes with MB and form coordination with heavy metal ions. In addition, there were π–π stacking interactions, electrostatic interactions, ion exchange, and physical functions between hydrochar and MB [51]. In contrast, the possible mechanisms of Pb2+ and Cd2+ adsorption by hydrochar involved electrostatic attraction, metal–π interactions, ion exchange, precipitation, and complexation [52,53]. In addition, the electrostatic attraction was the main mechanism of Pb2+ adsorption by hydrochar. Taken together, these factors lead to adsorption between the AE-CMIW hydrochar and MB, Pb2+, and Cd2+.
In the mixed solution, the removal rates of MB, Pb2+, and Cd2+ by the AE-CMIW hydrochar were significantly different (p < 0.05). The removal rates of Pb2+ by the AE-CMIW hydrochar were the highest, reaching more than 98%, and their removal rates were stable in the different HTC coupled with AE conditions. With the increase in HTC temperature, the removal rates of MB decreased gradually, and the removal rates of Cd2+ decreased first and then increased. With the increase in HTC time, the removal rates of MB increased first and then decreased, and the removal rates of Cd2+ decreased first and then increased. The removal rates of MB reach 60–80%. In contrast, the removal rates of Cd2+ by the AE-CMIW hydrochar were the lowest (20–57%) among the three. In addition, the removal rates of Cd2+ by different hydrochars were significantly different (p < 0.05). These results indicated varied adsorption strengths of MB, Pb2+ and Cd2+ by the AE-CMIW hydrochar. That is, the interaction force was different, and hydrochar materials have certain selectivity for the adsorption of multiple pollutants. We explained the reasons for the adsorption differences in detail in Section 3.5 and Section 3.6.

3.5. Adsorption Difference Analyses of AE-CMIW Hydrochar for Multiple Pollutants

The adsorption strength of adsorbents is affected by many factors, among which electrostatic attraction, physical adsorption, and functional group interactions (including ion exchange, hydrogen bonding, and π–π stacking interactions) are the main factors [54,55,56]. The pH significantly affects the removal efficiency of adsorbents for dyes and metal ions [57,58]. The pH of the isoelectric point (pHIEP) is the pH at which the net surface charge of the solid material is zero [59]. The pHIEP of the AE-CMIW hydrochar is shown in Figure 5. The pHIEP of AE-CMIW hydrochar was approximately pH 3.63 (p < 0.05). When pH < pHIEP, the surface of the hydrochar was protonated and positively charged. While pH > pHIEP, the surface of the hydrochar was negatively charged, which was conducive to the adsorption of cations [60]. In other words, the AE-CMIW hydrochar was negatively charged at the experimental pH (~6.5) of the adsorbate solutions. Thus, the AE-CMIW hydrochar could attract MB, Pb2+ and Cd2+ with a positive charge by electrostatic interactions. For metal ions, the electronegativities of Pb2+ and Cd2+ were 1.87 and 1.69, respectively; thus, the adsorption of Pb2+ by the AE-CMIW hydrochar was stronger than that of Cd2+. In addition, the hydrated ionic radii of the metal ions affected the rate and amount of metal ions entering the pores of the AE-CMIW hydrochar. The results showed that the hydrated ionic radii of Pb2+ and Cd2+ were 0.401 and 0.426 nm, respectively. Cations with small hydrated ionic radii are more likely to penetrate the pores and channels of the hydrochar. Therefore, the adsorption amount and speed of Pb2+ by the AE-CMIW hydrochar were higher than that of Cd2+. Pb2+ and Cd2+ competed for the adsorption sites on hydrochar, which was mainly determined by the different properties of these metal cations. Metal–π interactions, electrostatic attraction, and precipitation are the mechanisms of Pb2+ adsorption by AE-CMIW hydrochar [61,62,63]. The mechanisms of Cd2+ adsorption by AE-CMIW hydrochar involve electrostatic attraction and ion exchange [64,65], indicating that the adsorption between Pb2+ and AE-CMIW hydrochar was due to both chemical and physical interactions, whereas it was mainly due to the chemical interaction between Cd2+ and AE-CMIW hydrochar. Therefore, the adsorption rate and strength of Pb2+ and Cd2+ by AE-CMIW hydrochar varied, and the removal rates of Pb2+ by the AE-CMIW hydrochar were higher than those of Cd2+.
The distribution of main elements (SEM-EDS images) on the surface and near-surface of the AE-CMIW hydrochar after the adsorption reaction is shown in Table 2. The contents of Pb2+ on the surface and near-surface of AE-CMIW hydrochar were significantly higher than those of Cd2+ (p < 0.05) (Table 2). MB was an organic substance, which could interact with the functional groups of AE-CMIW hydrochar in addition to electrostatic attraction, indicating that MB could combine with AE-CMIW hydrochar in various ways to produce adsorption. In this experiment, electrostatic interaction was the main factor for the adsorption of MB, Pb2+, and Cd2+ by the AE-CMIW hydrochar, followed by the interaction between the functional groups [66]. Therefore, the adsorption order of the three pollutants by the AE-CMIW hydrochar was Pb2+ > MB > Cd2+.

3.6. Adsorption Kinetics Analyses

The adsorption of MB, Pb2+, and Cd2+ by the AE-CMIW hydrochar in the mixed solution was complex. The MB, Pb2+, and Cd2+ kinetics curves are illustrated in Figure 6. The fitting parameters of MB, Pb2+, and Cd2+ adsorption kinetics are shown in Table 3. The PFO kinetic model mainly describes the simple physical adsorption or diffusion of boundary-layer particles. The PSO kinetic model mainly describes chemical surface adsorption, which involves electron sharing or transfer between the adsorbent and adsorbate [67].
By comparing the adsorption kinetics parameters of MB, Pb2+ and Cd2+ on AE-CMIW mixed solution, the following results can be obtained. The adsorption of MB on the AE-CMIW hydrochar was consistent with the PSO kinetic model (Figure 6a,b, Table 3), indicating that the adsorption process was multiple composite adsorptions dominated by chemical adsorption. The adsorption rate of MB by the AE-CMIW hydrochar was high for the first 2.5 h, and then, the adsorption rate decreased. When the adsorption time of MB was 5 h, the adsorption capacity reached 80% of the saturated adsorption capacity, and the highest equilibrium adsorption capacity was 31.1 mg/g (AEHTC 200-2-3 hydrochar). Both the PFO kinetic model and the PSO kinetic model could well fit the adsorption behavior of Pb2+ by the AE-CMIW hydrochar (Figure 6c,d, Table 3), which indicated that the adsorption of Pb2+ by the AE-CMIW hydrochar was the chemical and physical adsorptions. In the mixed solution, Pb2+ was the least affected by the other adsorbates, and its adsorption rates by the AE-CMIW hydrochar were the fastest. Thus, the removal rates of Pb2+ by the AE-CMIW hydrochar were the highest. The adsorption of Pb2+ reached equilibrium for less than 2.5 h, and the equilibrium adsorption capacity was approximately 43 mg/g. The PSO kinetic model could more accurately describe the adsorption of Cd2+ by the AE-CMIW hydrochar (Figure 6e,f, Table 3). The adsorption rates of Cd2+ by AE-CMIW hydrochar were significantly lower than those of Pb2+, and their adsorption equilibriums were reached at approximately 20 h. The equilibrium adsorption capacities of Cd2+ by different AE-CMIW hydrochars were significantly lower than those of Pb2+. Therefore, the removal rates of Cd2+ by AE-CMIW hydrochar were significantly lower than those of Pb2+.

4. Conclusions

This study proposed a method of coupling HTC with AE for treating CMIW. The adsorption characteristics of metal ions and organic dyes by the AE-CMIW hydrochar in the mixed solution were investigated. The reasons for adsorption differences and the adsorption kinetics were analyzed. The conditions of coupling HTC with AE had significant effects on the removal rates of MB and Cd2+ (p < 0.05), while the removal rates of Pb2+ were stable under different conditions. The removal rates of Pb2+, MB, and Cd2+ by AE-CMIW hydrochar were ≥98%, 60–80%, 20–57%, respectively.
According to the pHIEP (pH 3.63), the AE-CMIW hydrochar was negatively charged at experimental pH (~6.5), and then, these hydrochar could attract MB, Pb2+ and Cd2+ with a positive charge by electrostatic interactions. The electronegativity of Pb2+ (1.87) was higher than that of Cd2+ (1.69), indicating that the adsorption of Pb2+ by the AE-CMIW hydrochar was stronger than that of Cd2+. The hydrated ionic radii of Pb2+ (0.401 nm) was smaller than that of Cd2+ (0.426 nm), indicating that the adsorption amount and speed of Pb2+ were higher than those of Cd2+. The AE-CMIW hydrochar had abundant pore structures and functional groups, providing physical and chemical binding sites for MB, Pb2+ and Cd2+ on the absorbents to achieve pollutant removal. The adsorption kinetics analyses showed that the adsorptions of Cd2+ and MB by AE-CMIW hydrochar were consistent with the PSO kinetic model, indicating that their adsorption processes were multiple composite adsorptions dominated by chemical adsorption. The adsorption of Pb2+ conformed to both the PFO and PSO kinetic models, indicating that it was dominated by the chemical and physical adsorptions. Therefore, the removal rate of Pb2+ by the AE-CMIW hydrochar was the highest.

Author Contributions

Conceptualization, X.Z. and W.W.; methodology, S.L., Q.Q. and G.C.; software, S.L.; validation, X.Z. and S.L.; formal analysis, X.Z. and W.W.; investigation, X.Z., S.L., Q.Q. and G.C.; resources, W.W.; data curation, X.Z.; writing—original draft preparation, X.Z., S.L. and W.W.; writing—review and editing, X.Z; visualization, S.L.; supervision, X.Z. and W.W.; project administration, X.Z.; funding acquisition, X.Z and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (51976110), the Postdoctoral Innovation Project of Shandong Province (202101002), and the Major Science and Technology Innovation Project of Tai’an City (2021ZDZX031).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 11 00412 g001 550
Figure 1. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of AE-CMIW hydrochar.
Figure 1. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of AE-CMIW hydrochar.
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Figure 2. Contrast analysis diagrams of surface microstructures of CMIW and AE-CMIW hydrochar.
Figure 2. Contrast analysis diagrams of surface microstructures of CMIW and AE-CMIW hydrochar.
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Figure 3. FTIR spectra of AE-CMIW hydrochar.
Figure 3. FTIR spectra of AE-CMIW hydrochar.
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Figure 4. Effects of AE-CMIW hydrochar on the removal rates of MB, Pb2+, Cd2+ in mixed solution.
Figure 4. Effects of AE-CMIW hydrochar on the removal rates of MB, Pb2+, Cd2+ in mixed solution.
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Figure 5. Plot for the pHIEP determination of AE-CMIW hydrochar.
Figure 5. Plot for the pHIEP determination of AE-CMIW hydrochar.
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Figure 6. Adsorption experimental (symbols) and fitting (lines) kinetics curves of MB (a,b), Pb2+ (c,d), Cd2+ (e,f) on AE-CMIW hydrochar in mixed solution.
Figure 6. Adsorption experimental (symbols) and fitting (lines) kinetics curves of MB (a,b), Pb2+ (c,d), Cd2+ (e,f) on AE-CMIW hydrochar in mixed solution.
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Table
Table 1. Porous structure properties of CMIW and AE-CMIW hydrochar.
Table 1. Porous structure properties of CMIW and AE-CMIW hydrochar.
SampleSBET (m2/g)Vtotal (cm3/g)Average Pore Diameter (nm)
CMIW19.121 ± 0.05 f0.055 ± 0.01 c11.445 ± 0.11 bc
AEHTC 200-2-353.466 ± 0.04 c0.213 ± 0.03 b14.729 ± 0.15 d
AEHTC 230-1-344.633 ± 0.06 e0.225 ± 0.02 b18.437 ± 0.13 bc
AEHTC 230-2-343.381 ± 0.07 b0.226 ± 0.02 b18.977 ± 0.10 bc
AEHTC 230-4-342.023 ± 0.05 a0.225 ± 0.04 a19.527 ± 0.09 a
AEHTC 260-2-342.611 ± 0.06 d0.236 ± 0.05 a20.056 ± 0.12 bc
a–f Means followed by different superscripts in the same column are significantly different at p < 0.05. a–f Means followed by same superscripts in the same column are not significantly different at p > 0.05.
Table
Table 2. Distribution of main elements on the surface and near-surface of AE-CMIW hydrochar after the adsorption reaction.
Table 2. Distribution of main elements on the surface and near-surface of AE-CMIW hydrochar after the adsorption reaction.
Element Contents (w.t.%)COFeCdPb
AEHTC 200-2-311.87 ± 0.01 a31.31 ± 0.02 a35.92 ± 0.01 a3.20 ± 0.07 a17.69 ± 0.02 a
AEHTC 230-1-35.03 ± 0.05 b22.69 ± 0.03 b42.38 ± 0.04 b2.54 ± 0.04 b27.37 ± 0.05 b
AEHTC 230-2-35.95 ± 0.02 a24.56 ± 0.05 a41.44 ± 0.06 a2.17 ± 0.02 a25.89 ± 0.04 a
AEHTC 230-4-36.44 ± 0.01 b24.73 ± 0.03 b44.78 ± 0.01 b5.21 ± 0.01 b18.84 ± 0.05 b
AEHTC 260-2-37.90 ± 0.01 a27.82 ± 0.02 a39.13 ± 0.02 a4.59 ± 0.01 a20.55 ± 0.02 a
The figures are the mean value ± standard deviation in the table. a–b Means followed by different superscripts in the same column are significantly different at p < 0.05. a–b Means followed by same superscripts in the same column are not significantly different at p > 0.05.
Table
Table 3. Adsorption kinetics parameters of MB, Pb2+, Cd2+ on AE-CMIW hydrochar in mixed solution.
Table 3. Adsorption kinetics parameters of MB, Pb2+, Cd2+ on AE-CMIW hydrochar in mixed solution.
SamplePseudo-First-OrderPseudo-Second-Order
qe (mg/g)k1 (h−1)R2qe (mg/g)k2 (g/mg · h)R2
AEHTC 200-2-3MB29.4711.1090.98631.0760.0670.995
Pb2+42.7383.7720.99942.9220.8580.999
Cd2+27.8230.5100.93931.2070.0220.983
AEHTC 230-1-3MB27.2321.3280.99628.3800.1000.997
Pb2+43.0283.3210.99943.2550.6220.999
Cd2+19.6610.4930.92422.1220.0300.975
AEHTC 230-2-3MB28.9491.0560.98730.6220.0630.996
Pb2+42.4653.2380.99842.7860.4900.999
Cd2+24.1690.2670.92427.9490.0130.959
AEHTC 230-4-3MB28.6381.1490.96630.4700.1040.988
Pb2+42.8602.6260.99943.3150.2990.999
Cd2+16.6620.5240.86618.7600.0380.941
AEHTC 260-2-3MB26.4821.0970.95328.2040.0660.985
Pb2+42.4212.2530.99743.2230.1780.999
Cd2+14.8890.8880.90717.0590.0250.959
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Zhang, X.; Liu, S.; Qin, Q.; Chen, G.; Wang, W. Alkali Etching Hydrochar-Based Adsorbent Preparation Using Chinese Medicine Industry Waste and Its Application in Efficient Removal of Multiple Pollutants. Processes 2023, 11, 412. https://doi.org/10.3390/pr11020412

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Zhang X, Liu S, Qin Q, Chen G, Wang W. Alkali Etching Hydrochar-Based Adsorbent Preparation Using Chinese Medicine Industry Waste and Its Application in Efficient Removal of Multiple Pollutants. Processes. 2023; 11(2):412. https://doi.org/10.3390/pr11020412

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Zhang, Xinyan, Shanshan Liu, Qingyu Qin, Guifang Chen, and Wenlong Wang. 2023. "Alkali Etching Hydrochar-Based Adsorbent Preparation Using Chinese Medicine Industry Waste and Its Application in Efficient Removal of Multiple Pollutants" Processes 11, no. 2: 412. https://doi.org/10.3390/pr11020412

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