Potassium permanganate modification of hydrochar enhances sorption of Pb(II), Cu(II), and Cd(II)

Abstract

Hydrochars formed by hydrothermal carbonization of hickory wood, bamboo, and wheat straw at 200 °C were modified by potassium permanganate (KMnO₄) for the sorption of Pb(II), Cd(II), and Cu(II). The wheat straw hydrochar (WSHyC) modified with 0.2 M KMnO₄ resulted in the most promising adsorbent (WSHyC-0.2KMnO₄). Characterization of WSHyC and WSHyC-0.2KMnO₄ revealed that the modified hydrochar features large specific surface area, rich of surface oxygenic functional groups (OCFG), and a significant amount of MnOx micro-particles. Batch adsorption experiments indicated that the adsorption rate by WSHyC-0.2KMnO₄ was faster than for WSHyC, attaining equilibrium after around 5 h. The optimum adsorption capacity (Langmuir) of Pb(II), Cd(II), and Cu(II) by WSHyC-0.2KMnO₄ was 189.24, 29.06 and 32.68 mg/g, respectively, 12 ~ 17 times greater than by WSHyC. The significantly enhanced heavy metal adsorption can be attributable to the increased OCFG and MnOx microparticles on the surface, thereby promoting ion exchange, electrostatic interactions, and complexation mechanisms.

Graphical Abstract

1. Introduction

Heavy metal pollution remains among the most pressing global issues affecting human health and the environment, especially in aquatic systems receiving heavy metal-contaminated wastewater (Liu et al., 2020b). For instance, heavy metals are polluting about 40% of surface water on the Earth (Unnikrishnan et al., 2021). Lead, cadmium, and copper are commonly found toxic heavy metals in wastewater, mining sites, and aquatic sediments (Ajiboye et al., 2021). They are not easily broken down in the environment and can remain present for extended periods in living organisms. Ingestion of water and food containing elevated levels of these heavy metals is a major exposure route for human beings, causing detrimental health problems (Khalid et al., 2021). Developing innovative water and wastewater treatment technology to reduce the negative effects of these harmful metal ions on the environment and human health continues to draw widespread interests from the environmental community.

Several environmental treatment technologies have been applied to remediate soil and water resources contaminated with heavy metals. Some of the common ones include chlorination (Fraissler et al., 2009), chemical extraction (Stylianou et al., 2007), ion exchange (Vilensky et al., 2002), membrane separation (Abu Qdais & Moussa, 2004), and adsorption (Hu et al., 2011). Among those, adsorption is widely applied for its simplicity and efficiency. The low cost and environment-friendly nature of biochar, a black carbon derived from biomass pyrolysis, makes it an outstanding carbon-based adsorbent that has been studied extensively (Qiu et al., 2021). Existing research has emphasized on biological, physical, and chemical modifications to functionalize pristine biochar to extend its environmental applications for environmental remediation of targeted pollutants including heavy metals. A fundamental goal achieved with biochar modification is the improved surface or pore structure and increased surface functional groups of biochar to raise its adsorption ability (Wang & Wang, 2019). In contrast, the pristine biochar has a restricted amount of functional groups and is less stable (Gong et al., 2022). While the specific surface area (SSA) with microporous volume of modified biochar may not be as high as that of activated carbon, rich functional groups (e.g., –OH, –COOH, -C=O, and –NH₂) on biochar surface render high adsorptive affinity to heavy metals and other contaminants.

Hydrothermal carbonization (HTC), a form of aqueous carbonization, has been used to convert biomass into a special biochar, called hydrochar. Hydrochar is typically processed at a relatively mild temperature of around 200 °C and under the presence of aqueous solution. Thus, production of hydrochar has lower energy consumption than for biochar or other carbon-based adsorbents (Zhang et al., 2019). Additionally, hydrochar has a low ash content and a unique chemical structure with abundant oxygen containing functional groups (OCFG) on the surface of a carbon skeleton. In general, pristine hydrochar does not provide sufficient adsorption sites for contaminants because of its limited SSA and pore volume (He et al., 2021). Therefore, surface functionalization is critical for improving hydrochar’s adsorption capacity. Surface functionalization of hydrochar can be achieved through various methods, including chemical modifications with oxidizing agents, such as potassium permanganate (KMnO₄) and hydrogen peroxide (H₂O₂), or physical activation methods such as carbonization or gasification. Hydrochar can attain improved adsorption capability by developing hierarchical porous structures through surface functionalization. The introduction of OCFG onto the hydrochar surface can also enhance its affinity for contaminants. Moreover, the presence of OCFG can provide active sites for complexation and chelation with metal ions, which can effectively remove pollutants in aqueous solution.

Potassium permanganate (KMnO₄), a commonly used oxidizing agent, has been widely used in engineered biomass/biochar to remove heavy metals and other contaminants from water bodies (Xu et al., 2022). Oxidization with KMnO₄ creates a hierarchical porous structure of biochar, resulting in a remarkable increase in the OCFG on carbon surface (Zheng et al., 2021). Furthermore, KMnO₄ oxidization introduces MnOx to biochar surface, and heavy metal ions can combine with MnOx, thereby significantly improving the removal of heavy metals (Liu & Zhang, 2022). Selection of a suitable KMnO₄ concentration is a crucial factor in the modification process. In previous studies, KMnO₄ concentrations in the range of 0.05 to 0.5 M were applied (Zhang et al., 2020). The concentrations showed varying degrees in improvement of the adsorption efficiency; yet an optimal concentration has not been identified due to differences in experimental conditions, especially the effects of feedstock types. Furthermore, current research on the usage of KMnO₄-modification focuses on biochars. Comparable research on heavy metal adsorption by modified hydrochar is still limited. Therefore, exploring the application of KMnO₄ modification on hydrochar has practical implications for improving heavy metal removal efficiency as an alternative of biochar.

In this work, hickory chips (HC), bamboo (BB), and wheat straw (WS) were hydrothermally carbonized and the resulting hydrochar (HCHyC, BBHyC, and WSHyC) was treated with different concentrations of KMnO₄ to enhance the adsorption of heavy metal ions Pb²⁺, Cd²⁺ and Cu²⁺. The objectives were: (1) to evaluate the efficacy of KMnO₄ oxidation with regards to feedstock types and KMnO₄ concentrations; (2) to study the impact of KMnO₄ modification on the physicochemical characteristics of the hydrochar; and (3) to explore the kinetics and equilibrium of heavy metal sorption by KMnO₄-modified hydrochar. The sorption kinetics and isotherms were described using dynamical and isothermal equilibrium models.

2. Materials and methods

2.1. Materials procurement and collection

The hickory chips (HC), bamboo (BB), and wheat straw (WS) used to prepare the hydrochar were obtained locally from Gainesville, Florida. The analytical grade chemicals used in this study, including Pb(NO₃)₂, Cd(NO₃)₂·4H₂O, Cu(NO₃)₂·3H₂O, and KMnO₄, were ordered from Fisher Scientific. Deionized (DI) water (0.055μS/cm) was utilized for rinsing hydrochar samples and preparing solutions.

2.2. Preparation of sorbents

The cleaned and air-dried HC, BB and WS were milled to a granularity of 0.5–1 mm. Respectively, 30 g of HC, BB, and WS were combined with 250 mL of DI water and sealed in a 500 mL hydrothermal reactor, which was then heated to 200 °C for 4 h. The resulting mixture was vacuum filtered to separate the products, washed using deionized water, and then desiccated at 80 °C for 24 h. The end-product was named hickory chips hydrochar (HCHyC), bamboo hydrochar (BBHyC), and wheat straw hydrochar (WSHyC).

To identify the optimal concentration of KMnO₄ in this research, we developed the following modification methods. First, 3 g of HCHyC, BBHyC, and WSHyC was immersed in 50 mL of 0.05, 0.1, 0.2, and 0.5 M KMnO₄ solution for 2 h at ambient temperature, respectively. Then, the hydrothermal carbon was washed using DI water and dried at 60°C for 24 h. Finally, 50 mg of hydrochar samples were mixed with 50 mL of 1 M Pb(II), 1 M Cd(II) and 3 M Cu(II) heavy metal solutions and batch adsorption experiments were carried out on a platform shaker at 230 rpm for 24 h. The mixture was filtered through a Fisherbrand^® 0.22 μm nylon membrane and the concentration of Pb(II), Cd(II), and Cu(II) in the filter liquor was measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, Spectro Blue, Germany).

The 0.2 M KMnO₄ concentration and WSHyC were selected for the following experimentation based on considerations of three factors including the improvement in adsorption capacity, cost (the amount of KMnO₄ used), and efficiency (the time needed to prepare the KMnO₄ solution). This modified wheat straw hydrochar samples were labeled as WSHyC-0.2KMnO₄.

2.3. Characterization of hydrochar

The following measurements were used to characterize selected hydrochar samples pre- and post- KMnO₄ modification. The Bruauer - Emmet - Teller (BET) method was used for the analysis of SSA. The configuration of surface was analyzed by scanning electron microscopy (SEM, Hitachi SU8020, Japan) and energy spectrometry (EDS). The changes of functional groups were captured using Fourier transform infrared (FTIR, Nichlet IS 10, USA) spectroscopy and X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi, USA). The crystalline manganese minerals were studied by X-ray diffraction (XRD, BRUCKER D8 ADVANCE, Germany).

2.4. Heavy metals sorption characteristics

Laboratory experiments were performed to compare the kinetics and isotherms of Pb(II), Cd(II), and Cu(II) adsorption by WSHyC and WSHyC-0.2KMnO₄. Sorption kinetics were examined by adding 50 mg of adsorbent to 50 mL of 200 mg/L Pb(II), 90 mg/L Cd(II), and 90 mg/L Cu(II). Specimens were then shaken at 230 rpm for 5 min to 48 h on a shaker. The mixture was filtered through a Fisherbrand^® 0.22 μm nylon membrane and the concentration of Pb²⁺, Cd²⁺ and Cu²⁺ in the solution was measured using inductively coupled plasma emission spectrometry (ICP-OES, Spectro Blue, Germany).

For sorption isotherms, 50 mg of sorbent was applied to 50 mL of Pb(II), Cd(II), and Cu(II) solutions at room temperature with starting concentrations ranging from 20 to 500 mg/L of Pb(II), 10 to 100 mg/L of Cd(II), and 10 to 100 mg/L Cu(II), respectively. Samples were then shaken at 230 rpm for 24 h on a shaker. The mixture was then filtered as described above and metal concentrations were measured. Each adsorption experiment was conducted in duplicate, with the average value reported.

3. Results and discussion

3.1. Effects of feedstock type and KMnO₄ concentration

As depicted in Fig. 1, the type of biomass feedstock (hickory chips, bamboo, wheat straw) and the KMnO₄ modification concentration both had significant effects on heavy metal adsorption. For each feedstock, heavy metal removal increased with increasing KMnO₄ modification concentration. This suggests that as the KMnO₄ concentration increased, the modified hydrochar gained more cavities, larger porosity, more manganese oxides and OCFG to sorb heavy metals (Shyam et al., 2022, Tran et al., 2022). Nevertheless, the best increase of heavy metal adsorption per unit KMnO₄ concentration was reached at 0.2 M. This is especially so for adsorption of Pb(II) and Cu(II). Considering the cost for scaling up (amount of KMnO₄ and ease of operation) and improvement in adsorption efficiency, 0.2 M was chosen as the optimal KMnO₄ modification concentration for hydrochar.

Fig. 1.

Effect of feedstock and KMnO4 modification concentrations on heavy metal adsorption by hydrochar (a) Pb, (b) Cd, and (c) Cu. Error bars are standard deviation from the mean of three replicates.

Hydrochar’s adsorption capability for heavy metals is also dependent on the feedstock material. For a given concentration, wheat straw (WS), which is an agricultural biomass, generally sorbed more heavy metals than hickory chips (HC) and bamboo (BB), both are woody biomass. This is likely because wheat straw, being herbaceous, allowed for production of more refined hydrochar than woody feedstock. Wheat straw contains up to 50% cellulose and hemicellulose, and only 10–20% lignin. The lignin content in HC and BB is about 30%, much higher than in WS. Additionally, previous research has demonstrated that agricultural biomass has excellent hydrothermal carbonization capability (Wang et al., 2022). Therefore, the sorption performance of WSHyC-0.2KMnO₄ were further examined in subsequent experiments.

3.2. Characterization of hydrochar

3.2.1. SEM and surface area

The SSA of WSHyC-0.2KMnO₄ (15.1 m²/g) is about 8 times larger than that of the pristine WSHyC (1.8 m²/g). This can be explained by the SEM-EDX analysis which reveals the different surface morphology and elemental composition between WSHyC and WSHyC-0.2KMnO₄ (see supplementary materials). The morphology of WSHyC still preserves the original structure of wheat straw to a certain extent. After modification with KMnO₄, the surface texture and pores of WSHyC were disrupted, thus facilitating the generation of large specific surface areas (from 1.8 to 15.1 m²/g). The specific surface area directly or indirectly affects the active adsorption sites on the surface of the hydrochar and thus its adsorption performance on contaminants.

The hydrothermal reaction in disrupting the lignocellulose structure also promotes the incorporation of fine granular nanoparticles present on the modified hydrochar (see supplementary materials). The EDS spectra exhibit a Mn peak in WSHyC-0.2KMnO₄ with Mn content increasing in hydrochar surface from 0.5% in WSHyC to 26.4% after modification (see supplementary materials). This is further confirmed by elemental mapping (see supplementary materials) which shows that KMnO₄ modification increased O and Mn while reducing C in the modified hydrochar. Our results are consistent with previous research that MnOx particles were introduced onto the surface of modified biochar by the KMnO₄ oxidation process (Bian et al., 2021).

3.2.2. FTIR

FTIR spectroscopic analysis revealed the difference in functional groups between WSHyC and WSHyC-0.2KMnO₄ (see supplementary materials). The peaks of WSHyC and WSHyC-0.2KMnO₄ at 3373 cm⁻¹ and 2907 cm⁻¹ are correlated with the extension vibration of –OH and alkane group C-H. The peak of WSHyC-0.2KMnO₄ at 1713 cm⁻¹, which is not as strong in WSHyC, can be attributed to the carboxyl functional group –COOH (Zhang et al., 2021). The vibrational signals at 1503 cm⁻¹ to 1105 cm⁻¹ correspond to the stretching of C-O-C/C-O and HO-C=O (Song et al., 2019). The modified hydrochar exhibits two additional peaks at 618 cm⁻¹ and 451 cm⁻¹, which can be ascribed to the tension of the Mn-O bond. The overall result indicates that KMnO₄ modification was effective in producing additional OCFG (Ahmed et al., 2016), which provide active sorption sites for immobilization of heavy metals.

3.2.3. XPS

Using XPS, changes in surface elemental composition and bonding state of hydrochar before and after modification were investigated. Specifically, a decrease in C1s from 73.63% in WSHyC to 52.4% in WSHyC-0.2KMnO₄ was noted (see supplementary materials). In contrast, the O1s increased from 23.99% in WSHyC to 38.23% in WSHyC-0.2KMnO₄ and Mn2p from 0.27% in WSHyC to 7.71% in WSHyC-0.2KMnO₄. This change is caused by the addition of MnOx particles and additional OCFG to the modified hydrochar’s surface.

Also note the C1s spectrum displays three spikes at 284.8, 286.0, and 288.5 eV, and they correspond to C=C/C-C, C-O, and O-C=O groups, respectively (Fig. 2). After modification with KMnO₄, C=C/C-C group drooped (from 60% to 54%) while O-C=O groups raised (from 6% to 13%). C-O groups remained about the same. The O1s spectrum of WSHyC-0.2 KMnO₄ shows three peaks, with the C-O and C=O bonds identified at 533.5 and 531.5 eV, respectively, and a new spike at the bound energy of 530.1 eV (Fig. 2). The new peak represents the Mn-O bond, which accounts for 32% (Zhang et al., 2022). The Mn 2p _3/2 and Mn 2p _1/2 peaks in the Mn2p spectrum were resolved in the WSHyC-0.2 KMnO₄ sample (Fig. 2), with two peaks at binding energy of about 642.27 (63%) and 653.76 (37%) eV. The inter-peak distance between the Mn 2p _1/2 peak and the Mn 2p _3/2 peak was 11.49 eV. Consistent with earlier research, Mn³⁺ and Mn⁴⁺ were the oxidation states expressed by Mn (Fu et al., 2022).

Fig. 2.

XPS spectra of C1s, O1s Mn2p of WSHyC and WSHyC-0.2KMnO_4.

3.2.4. XRD

The crystallographic structure of WSHyC and WSHyC-0.2KMnO₄ is presented by the XRD pattern (see supplementary materials). The WSHyC spectrum exhibited three peaks at 15.96°, 22.59°, and 34.58°, indicating the cellulose in the original wheat straw was not completely carbonized during the hydrothermal carbonization process (Li et al., 2021b). However, after modification, these cellulose crystal peaks disappeared, while a broad diffraction peak near 19.76° and a relatively weak diffraction peak near 36.83° appeared (Li et al., 2020). The peak at 19.76° is known to be attributed to the α-MnO₂ crystal structure, while the peak appearing at 36.83° may be concerned with Mn₃O₄ (Wang et al., 2015). This outcome confirms that the KMnO₄ modification process produced MnOx crystals.

In summary, the SEM-EDS and XPS analyses consistently showed the existence of Mn-O bonds and OCFG. The FTIR spectroscopy confirmed the stretching of Mn-O bonds and an increase in O containing groups. The XRD analysis detected MnOx crystals. All considered, KMnO₄ modification in this study successfully loaded Mn oxides and OCFG on the surface of WSHyC-0.2KMnO_4.

3.3. Heavy metals sorption

3.3.1. Adsorption kinetics

The kinetics of Pb(II), Cd(II), and Cu(II) adsorption onto WSHyC and WSHyC-0.2KMnO₄ consisted of two stages (Fig. 3). The initial stage is the rapid adsorption with>90% of the adsorption accomplished in the first 1–2 h. The second stage is the slowing adsorption, lasting for about 5 h until equilibrium is reached. Heavy metal adsorption by WSHyC-0.2KMnO₄ was much higher and faster than that of WSHyC. To better understand the adsorption kinetics, experimental data were fitted to the pseudo-first-order, pseudo-second-order, and Ritchie models. Among these three dynamical models, the pseudo-second order and Ritchie models generally fit the data better than the pseudo-first-order model, with the exception of Pb (II), for which all three models fit well (Table 1). Also note that WSHyC-0.2KMnO₄ generally had a better fit than WSHyC except for Pb (II). The adsorption kinetics result suggests that heavy metal adsorption by WSHyC-0.2KMnO₄ may involve various mechanisms, including static attraction, ion-exchange and complexation on surfaces (Liu et al., 2020a). The increased Mn oxides and OCFG on the modified hydrochar offer more sorption sites for heavy metals (Sudibyo et al., 2022), as indicated by much larger q_e values than for WSHyC in the kinetic models (Table 1).

Fig. 3.

Sorption kinetics data and fitted models of Pb (II), Cd (II), and Cu(II) onto WSHyC and WSHyC-0.2KMnO_4.

Table 1.

Best-fit parameters of various adsorption kinetic and isotherm models for Pb(II), Cd(II), and Cu(II) adsorption onto WSHyC and WSHyC-0.2KMnO₄.

Adsorbate	Kinetic and isotherm models	Adsorbent	Parameter-1	Parameter-2	Parameter-3	R 2
Pb(II)	Pseudo first order	WSHyC	k1 = 0.697	qe = 15.273	–	0.998
		WSHyC 0.2KMnO4	k1 = 5.479	qe = 115.471	–	0.947
	Pseudo second order	WSHyC	k2 = 0.005	qe = 16.550	–	0.989
		WSHyC 0.2KMnO4	k2 = 0.079	qe = 119.669	–	0.984
	Ritchie	WSHyC	kn = 0.116	qe = 15.422	N = 1.194	0.999
		WSHyC 0.2KMnO4	kn = 0.478	qe = 117.576	N = 1.573	0.985
	Langmuir	WSHyC	K = 0.027	Smax = 15.189	–	0.961
		WSHyC 0.2KMnO4	K = 2.378	Smax = 189.237	–	0.961
	Freundlich	WSHyC	Kf = 2.529	n = 0.295	–	0.843
		WSHyC 0.2KMnO4	Kf = 82.768	n = 0.171	–	0.857
Cd(II)	Pseudo first order	WSHyC	k1 = 5.729	qe = 1.787	–	0.873
		WSHyC 0.2KMnO4	k1 = 7.508	qe = 28.845	–	0.966
	Pseudo second order	WSHyC	k2 = 5.267	qe = 1.853	–	0.926
		WSHyC 0.2KMnO4	k2 = 0.427	qe = 29.845	–	0.954
	Ritchie	WSHyC	kn = 5.186	qe = 1.861	N = 2.075	0.927
		WSHyC 0.2KMnO4	kn = 2.551	qe = 29.145	N = 1.379	0.974
	Langmuir	WSHyC	K = 0.128	Smax = 1.696	–	0.943
		WSHyC 0.2KMnO4	K = 17.031	Smax = 29.058	–	0.969
	Freundlich	WSHyC	Kf = 0.346	n = 0.408	–	0.858
		WSHyC 0.2KMnO4	Kf = 25.131	n = 0.213	–	0.844
Cu(II)	Pseudo first order	WSHyC	k1 = 4.482	qe = 3.147	–	0.870
		WSHyC 0.2KMnO4	k1 = 7.704	qe = 20.112	–	0.885
	Pseudo second order	WSHyC	k2 = 2.633	qe = 3.239	–	0.892
		WSHyC 0.2KMnO4	k2 = 0.607	qe = 20.872	–	0.958
	Ritchie	WSHyC	kn = 3.314	qe = 3.191	N = 1.627	0.899
		WSHyC 0.2KMnO4	kn = 0.379	qe = 21.061	N = 2.181	0.960
	Langmuir	WSHyC	K = 0.111	Smax = 2.610	–	0.956
		WSHyC 0.2KMnO4	K = 1.879	Smax = 32.678	–	0.994
	Freundlich	WSHyC	Kf = 0.451	n = 0.449	–	0.875
		WSHyC 0.2KMnO4	Kf = 18.158	n = 0.376	–	0.938

3.3.2. Adsorption isotherms

The Langmuir and Freundlich models were applied to characterize the equilibrium sorption data on WSHyC and WSHyC-0.2KMnO₄ (Fig. 4). The R² values of the Langmuir model are generally larger than for the Freundlich model (Table 1). The excellent fit between data and Langmuir model indicates that WSHyC-0.2KMnO₄ is chemically homogeneous. In other words, the reactive groups (functional groups and MnOx) are homogeneously distributed over the surface of hydrochar. The Langmuir maximum adsorption volume of WSHyC-0.2KMnO₄ was 189.24 mg/g for Pb(II), 29.06 mg/g for Cd(II), and 32.68 mg/g for Cu(II), about 12.46, 17.09 and 12.52 times higher than for the pristine WSHyC, respectively. The significantly improved adsorption capacity is associated with the greater number of MnOx and OCFG on the hydrochar surface in association with the modification. As a result, heavy metal adsorption is greatly enhanced (Qi et al., 2022).

Fig. 4.

Sorption isotherm data and fitted models of Pb (II), Cd (II), and Cu(II) onto WSHyC and WSHyC-0.2KMnO_4.

The adsorption efficiency of Pb(II), Cd(II), and Cu(II) by WSHyC-0.2KMnO₄ was compared with the values reported in the literature for hydrochars or biochars. It is noteworthy that the Pb(II) adsorption capacity of WSHyC-0.2KMnO₄ (189.24 mg/g) is greater than that of most other carbon-based adsorbents reported in the literature, such as Zn-Fe modified kiwi branch biochar (161.29 mg/g) (Tan et al., 2022), iron oxides decorated bamboo hydrochar (153.85 mg/g) (Hu et al., 2022), sulfide-modified magnetic pinecone hydrochar (149.33 mg/g) (Zhang et al., 2022), H₂O₂ modified hydrochar (92.80 mg/g) (Xia et al., 2019), KOH modified sugarcane bagasse hydrochar (90.1 mg/g) (Malool et al., 2021), and citric acid-sewage sludge hydrochar (60.88 mg/g) (Huang et al., 2022). The KMnO₄ modified hydrochar in this research exhibited superb Pb (II) adsorption capacity, only exceeded by two biochars: Zn–Al LDH modified sludge biochar (226.1 mg/g) (Cheng et al., 2022) and 650 °C wood ear mushroom sticks biochar (234.2 mg/g) (Ji et al., 2022); both may potentially have a high phosphate content in feedstock.

Similar observations about the comparative absorption ability were also true for Cd (II) and Cu (II), whereby the absorption ability of WSHyC-0.2KMnO₄ resides on the high end of the spectrum. When compared to other hydrochar adsorbents reported in the literature, WSHyC-0.2KMnO₄ demonstrates a Cd(II) adsorption capacity (29.06 mg/g) that surpasses the majority of adsorbents, including microbial aging hydrochar (20.18 mg/g) (Hua et al., 2020), HNO₃ modified hydrochar (19.99 mg/g) (Li et al., 2021a), and anaerobic fermentation modified poplar sawdust hydrochar (16.68 mg/g) (Fu et al., 2021). With a Cu(II) adsorption capacity of 32.68 mg/g, the WSHyC-0.2KMnO₄ also exceeds the majority of other adsorbents, for instance, 210 °C microalgae hydrochar (14.96 mg/g) (Saber et al., 2018), cattle manure biochar (14.7 mg/g) (Wang et al., 2020), and willow wood biochar (12.2 mg/g) (Wang et al., 2020). This comparative analysis indicates that the KMnO₄ modified hydrochar is competitive among bio-sorbents for heavy metal adsorption.

3.3.3. Sorption mechanisms

Hydrochar characterization and batch adsorption experiments support that treatment with KMnO₄ effectively increases the specific surface area of hydrochar and facilitates thermochemical conversion with incorporation of MnOx nanoparticles and OCFG (primarily carboxyl and hydroxyl groups) on hydrochar surface. The specific surface area is a direct indicator of active adsorption sites on the surface of hydrochar and thus its adsorption performance. While the disrupted lignocellulose structure under hydrothermal reaction facilitates the generation of large specific surface areas, KMnO₄ modification further produces nanoscale MnOx particles which have a high surface area due to their superior polycrystalline structure. Furthermore, as a metal catalyst, MnOx can improve the selectivity and removal efficiency of contaminants. Thus, the porous hydrochar with micro/nano MnOx has enhanced heavy metal removal performance (Liu et al., 2019, Song et al., 2014). Overall, the MnOx nanoparticles and OCFG species on hydrochar surface engage in physiochemical interactions with heavy metals in aqueous solutions, thereby promoting the adsorption of Pb(II), Cd(II), and Cu(II) through mechanisms involving ion exchange, complexation, and electrostatic interactions.

3.4. Practical implications

The modified hydrochar (WSHyC-0.2KMnO₄) has great potential in water treatment and environmental remediation applications. In addition to adsorption of heavy metals, it can serve as an adsorbent to remove other contaminants from water such as organic pollutants and dyes (Cheng et al., 2021, Kou et al., 2021). The porous structure of the modified hydrochar provides a large surface area for adsorption and filtration processes (Wu et al., 2017); thus it can also be used as a filtration medium for the removal of particulate matter, suspended solids, and even microorganisms. Moreover, WSHyC-0.2KMnO₄ can be applied to contaminated soils to immobilize contaminants, improve soil structure, and support microbial activity for degradation of organic pollutants (Xu et al., 2022).

With real-world applications, it is important to assess the cost-effectiveness and feasibility of the production process and scale-up. Several points are worth noting from this perspective. First, wheat straw, the biomass feedstock used to produce hydrochar, is an agricultural by-product, which would otherwise be burned after harvesting and releases carbon dioxide. Utilizing wheat straw for hydrochar production provides a great zero-waste and carbon neutral option while providing farmers with a source of income. Second, the hydrothermal carbonization process is typically produced at mild temperatures around 200 °C with a residence time of 4 h. Compared to conventional methods for preparing biochar, hydrochar production is more convenient and less energy intensive, offering higher cost-effectiveness in the production process. Third, the efficiency of scaling up the modification process needs to consider the required dosage, availability, and cost of KMnO₄. The 0.2 M KMnO₄ concentration is selected based on considerations of three factors including the improvement in adsorption capacity, cost (the amount of KMnO₄ used), and efficiency (the time needed to prepare the KMnO₄ solution). By considering these factors and comparing with other treatment options, the modified hydrochar can be employed in practical applications as a low-cost sorbent, offering the needed economic viability for potential implementation in real-world scenarios. Further research is needed to evaluate the regeneration and reusability of the modified hydrochar regarding its sustainability and long-term viability.

4. Conclusions

This study demonstrated that wheat straw hydrochar modfied by 0.2 M KMnO₄ is an excellent sorbent for effective immobilization of Pb(II), Cd(II) and Cu(II). The modified hydrochar had SSA eight times higher than the pristine hydrochar with increased OCFG (e.g., hydroxyl and carboxyl) and MnOx on the surface. The sorption capacity of heavy metals was greatly improved (>12 times higher) compared with the pristine hydrochar and other adsorbents. The modified hydrochar can be employed in practical applications as a low-cost sorbent.

Data availability

Data will be made available on request.