Removal of Trace Cu²⁺ from Water by Thermo-Modified Micron Bamboo Charcoal and the Effects of Dosage

Abstract

Chronic copper intoxication via drinking water induces diseases and physiological toxicity. Bamboo charcoal has been applied in the treatment of copper (Cu²⁺) in water. However, the adsorption by micron bamboo charcoal (MBC) of trace Cu²⁺ in tap drinking water and the underlying factors behind it have not been sufficiently reported. In this study, to improve the adsorption by MBC of trace levels of Cu²⁺ in drinking water, MBC was thermo-modified and characterized. Through batch experiments, the adsorption equilibrium was analyzed, and isotherm models were simulated. The removal rates and the optimization were investigated through a general full factorial design including the thermo-modified temperature (MT), initial concentration (C₀), and dosage. The results indicated that the thermo-modification significantly improved the removal by MBC of Cu²⁺ at trace level C₀. The satisfactorily low level of 0.12 ± 0.01 mg⋅L⁻¹ was achieved in the range of C₀ from 0.5 to 2.0 mg⋅L⁻¹ within the short contact time of 0.5 h. The processes conformed to the Freundlich and Langmuir adsorption isothermal models at a C₀ lower than 4.0 mg⋅L⁻¹ and higher than 8.0 mg⋅L⁻¹. The correlation between C₀ and dosage played an important role in the removal of Cu²⁺. This work proposes the application of the ecofriendly material MBC and an optimization mode in the removal of trace Cu²⁺ from tap drinking water. It is also revealed that the positive and negative correlation and the “critical point” of the removal rate with dosage depend on the initial concentrations.

1. Introduction

Up to 3.0 mg⋅L⁻¹ of copper ions (Cu²⁺) in drinking water has been reported to induce cell death, neurodegeneration, cognitive warning, and/or b-amyloid deposition, though Cu²⁺ is essential to human health [1,2,3]. The bioaccumulation of copper via drinking water has raised concerns. The residue of Cu²⁺ can be from the source water or the pipeline [3,4]. Most countries’ threshold for the standard of Cu²⁺ is at the level of 1.0~1.3 mg⋅L⁻¹ in drinking water. Rigorous limitations on copper are also required because of sensitive demographics in the population, especially infants and children [3,5]. It is considered that a higher intake of copper than the prescribed limit can cause serious health problems [6,7,8].

Adsorption has been developed to effectively remove heavy metals from water, especially for small-volume treatment. Mesoporous MgO sheets [9], modified sludge [10], co-immobilization between fungi and graphene oxide [11], and bioadsorbents [12] represent techniques that have been applied to remove Cu²⁺ from water. However, drinking water treatment requires strict consideration of the potential risks from materials prepared using unsafe stock [13,14].

Bamboo charcoal is a highly safe and green byproduct of health products, especially in Asia. It shows excellent adsorption of metal ions, recovering or removing them from water [15,16,17]. The particle size of bamboo charcoal is an important factor in its application, since small particles have a large specific surface area, which is advantageous in terms of adsorption capabilities [18,19,20]. Nano-size bamboo charcoal exhibits advanced adsorption capacities and adsorption forces due to its hydrophobic surface [21,22]. However, additional adhesiveness is required to avoid the nano-size resulting in the outflow of particles from the filter [22,23,24]. The potential risks of the extra chemicals needed for assembly have to be evaluated in drinking water treatment [25,26]. Furthermore, unlike nano-materials prepared by chemical reactions, nano-size bamboo can only be produced by crushing conventionally sized particles, which is not economical. Compared with nano-size particles, the micron size of bamboo charcoal is seldom considered in applications.

The adsorption isotherm is usually utilized to simulate the adsorption process and indicate the adsorption mechanisms at a constant temperature, especially the interaction between the adsorbent and the adsorbate, through evaluating the equilibrium data and the adsorption properties. It is important for scientists to comprehend the equilibrium to anticipate the adsorption mechanisms. Moreover, the application parameter in engineering can be acquired from the adsorption isotherms in batch experiments [27]. The adsorption of contaminants at low concentrations in water typically conforms to the two-parameter adsorption isothermal models of Langmuir and Freundlich. Most studies on copper adsorption in water have modeled the Langmuir and Freundlich adsorption isotherms to obtain two important parameters. However, there have been few further recommendations regarding engineering guidance.

Factors affecting removal have been investigated in the adsorption of Cu²⁺. The initial concentrations (C₀) limit the treatment of Cu²⁺ in drinking water because the adsorptions are difficult to initiate when the element is present at a low level in water [28,29,30]. The designed C₀ levels are from 1.3 to 5 mg⋅L⁻¹ or even higher than 30.0 mg⋅L⁻¹ in most reports (

Table 1. Comparison of the removal by MBC with the typical materials reported.

Adsorbent			Adsorption			Results
Material	Scale/μm	Stocks	Dosage/g⋅L−1	C0/mg⋅L−1	Contact Time/h	qe/ mg⋅g −1	Removal Rate/E, %	Residues/mg⋅L−1
[35] CBC-2/pH = 2.1	600~2000	Bamboo charcoal	2.0	10.0	2.0	10.0	~100.0	/
			0.4	10.0	24.0	0.5~1.7
[36] Modified biochar/unadjusted pH		Biochar (sludge + coconut shell sawdust)/modified by MnO	1.0 (optimal)	3.5	2.0	3.5~3.6 (pH= 4.0)	90.0	0.45
[11] Microbeadsorbents/pH= 3.0~5.0	1000~2300	Metal-resistant adapted fungus/graphene oxide/activated carbon	20	100	1.67	1.10~1.55	87~94	6~13
[30] Palm oil mill sludge/pH = 2.0~8.0	500~2000	Sludge wastes	0.3	200	1.0	14.84	~80.0	~40.0
			0.5	50	1.5	5.72	~39	~30.5
[12] Tip-tea-bag/pH = 6.5~7.5	48	Tea biotanic adsorbents	2.0	10.0	0.25	/	91.2 *	0.88
[37] Bamboo sawdust/pH = 3.5~8.5	100~200	Fresh bamboo	10, 20, 30	10.0~25.0	4.0 (20 °C)	/	~70	3.0
[31] Nanocomposite beads/pH= 4.0~6.0	/	Sawdust chitosan	20	25–200	1.11	1.16–6.25	86.20 (C0= 50.0 mg⋅L−1)	6.9
[9] Microsized MgO mosaics/pH= 4.0	5~15	Cetyltrimethylammonium bromide/Mg(NO3)2/(NH4)2CO3	1.25	1.0	4.0	233	83.0	0.17

* Azadirachta indica [12].

). Moreover, the dosage has been discussed extensively in reports about the effects on the removal rate and dynamic and isothermal models of adsorption [30,31,32]. However, the effects of the dosage combined with C₀ on the removal rate have not been sufficiently reported. Meanwhile, most reports have optimized adsorption at a pH lower than 6.0, because copper is easily adsorbed in acidic aqueous solutions [33,34]. To meet the necessary standards, further understanding of the optimization is required for the treatment of drinking water.

In this work, micron bamboo charcoal was prepared and thermo-modified in a limited oxygen atmosphere to improve the low concentration of Cu²⁺ adsorption in drinking water. The properties of thermo-modified MBC were characterized. A batch experiment was conducted by general full factorial design (GFD). Three factors, namely, the modified temperature (MT), initial concentration (C₀), and dosage, from five to seven levels, were evaluated in the treatments. The adsorption equilibrium was observed, and the isothermal adsorptions of Langmuir and Freundlich were modeled to guide the application. This work analyzes the results of adsorptions and modeling, and discusses the factors affecting the removal rates. The aim is to suggest applications and the optimization of the ecofriendly material MBC in the treatment of low-level Cu²⁺ in tap drinking water.

2. Materials and Methods

2.1. Chemicals and Materials

Chemicals, including copper chloride (CuCl₂), nitric acid (HNO₃), ethylenediaminetetraacetic acid disodium salt (C₁₀H₁₄N₂Na₂O₈), ammonium citrate (C₆H₅O₇ (NH₄)₃), sodium diethyldithiocarbamate (C₂H₅)₂NCSSNa), ammonia (NH₃·H₂O), and ammonium chloride (NH₄Cl) were analytically pure (Tianjin Chemical Reagent Co. Ltd., Tianjin, China). All solutions were prepared from chemicals dissolved with deionized water. Bamboo charcoal materials with a conventional size of 2~5 mm were purchased from Anji County (Huzhou, China). Filter membrane (0.45 µm, RMF50C1, Nanjing, China).

2.2. Methods

2.2.1. MBC Preparation, Thermo-Modification, and Characterization

The purchased bamboo charcoal with a size of 2~5 mm was grounded and screened by a sieves with 200 meshes (75 µm screen diameter) to obtain particles with a size lower than 75 µm. Then, the screened particles were suspended in water and filtrated through 0.45 µm film to obtain particles with sizes from 0.45 µm to 75 µm. The particles were dried in the ambient air to prepare MBC.

MBC was thermo-modified at temperatures of 60, 80, 100, 200, 300, and 400 °C, respectively, in a Tube furnace (BTF-1200C, Best Equipment Ltd., Hefei, China.) with limited oxygen for 120 min [38,39].

The thermo-modified MBC samples were characterized through Fourier transform infrared spectroscopy (FTIR, Nicolet iS50, Thermo Fisher Co., Waltham, MA, USA) tested in the range of 500~4000 cm^–1 wavenumber in attenuated reflectance mode, with a scanning electron microscope (SEM, S-4800, Hitachi Ltd., Tokyo, Japan) and an analyzer of surface area and porosity (BET, Tristar II 3020, Micromeritics, Norcros, GA, USA). The zero-charge point (pH_pzc) was determined by referring to the study of Hu, H. et al. [40] and Rong, Z. et al. [41].

2.2.2. Batch Experiments

The level of Cu²⁺ in water was investigated. The qualities of the tap drinking water in the domestic supply system were given (Table S1). The concentration of Cu²⁺ in the bottled drinking water was determined (Table S2). The sewage water was also assessed to determine the local level of Cu (Table S3).

Batch experiments were designed through GFD to observe the process of adsorption of Cu²⁺. Three independent factors, MT, C₀, and dosage, were set at 5~7 levels (Table S4; Figure S1). In total, there were 210 tests with two replicates, i.e., 420 runs in total, which were grouped by MT. There were two responses for the removal rate (E) and equilibrium adsorption capacity (q_e).

MBC was added into TDW fortified with CuCl₂ to prepare the suspension solutions of 100 mL in conical flasks of 250 mL. The flasks were shaken at the temperature of 18 ± 1 °C and the speed of 180 rpm in a temperature-controlled oscillator. After the initial and final pH were measured, the supernatant was filtered through a 0.45 µm filter disk. Then, the concentration of Cu²⁺ was detected by an atomic absorption spectrometer (AAS, type AA-400, Perkin-Elmer Corp., Waltham, MA, USA) at 324.7 nm using an air–acetylene flame. The removal rate (E) and equilibrium adsorption capacity (qe) were calculated according to Equation (1) and Equation (2), respectively.

$$\mathrm{E}={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_0}}\times 100$$

(1)

In Equation (1), C₀—initial concentration (mg⋅L⁻¹); C_e—equilibrium concentration (mg⋅L^{− 1}).

$$\mathrm{q}\mathrm{e}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{\mathrm{m}}}$$

(2)

In Equation (2), V—volume of the aqueous (L); m—weight of adsorbent (g).

2.2.3. Adsorption Equilibrium, Isothermal Models, and Desorption

The sorption test was carried out for 32 h to observe the equilibration.

The independent desorption tests were conducted. The supernatant was filtrated from those samples with the highest sorption loading. The filtrated solids were added into conical flask of 100 mL of TDW. The flasks were shaken for 500 min to observe the release of Cu²⁺ into water.

The Langmuir (L-tyl) and Freundlich (F-tyl) models (Equation (3) and (4)) were applied to simulate the adsorption. There were 76 simulations in working adsorptions from 210 tests.

$$\mathrm{q}\mathrm{e}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{K}}_{\mathrm{L}}\mathrm{C}\mathrm{e}}{1+{\mathrm{K}}_{\mathrm{L}}\mathrm{C}\mathrm{e}}(\mathrm{L}-\mathrm{t}\mathrm{y}\mathrm{l})}$$

(3)

In Equation (3), q_max—the maximum adsorption capacity (mg⋅g ⁻¹); K_L— Langmuir’s affinity constant.

$$\mathrm{q}\mathrm{e}={\mathrm{K}}_{\mathrm{f}}·{\mathrm{C}\mathrm{e}}^{\frac{1}{\mathrm{n}}}(\mathrm{F}-\mathrm{t}\mathrm{y}\mathrm{l})$$

(4)

In Equation (4), K_f—the constant related to adsorption capacity. n—the constant associated with the intensity of adsorption [42].

3. Results and Discussion

3.1. Characterization

MBC was characterized through SEM, FTIR, BET, and pH_pzc. SEM showed that the inner construction of MBC was modified under different temperatures (Figure 1A). It was obvious that micropores were enlarged and were regular at 200 °C compared with at 80 °C. Meanwhile, the micropore cell walls were deformed, and the surface gaps increased significantly. The voids collapsed, exposing the interior at 300 and 400 °C. Some coke-like substances were observed on the internal side of MBC with MT at 400 °C. The enlarged and deformed micropores might expose more activate points. BET showed that the pore size, micropore volume, total pore volume, and specific surface area (SSA) changed (

Table 2. BET data on the thermo-modified MBC.

MT/°C	Pore Size/nm	SSA/m2⋅g−1	Micropore Volume/cm3⋅g−1	Total Pore Volume/cm3⋅g−1
80	4.31	29.56	0.005	0.036
100	4.32	29.01	0.005	0.035
200	4.92	31.28	0.008	0.038
400	5.09	60.06	0.013	0.056

). The increased SSA indicated that the adsorption performance might be improved. FTIR detection showed that the aromatic carboxylic acid amines, phenols, and aliphatic cyclic ethers were the majority groups. After thermal modification, especially with the temperature higher than 200 °C, some aromatic rings broke and isomerized, forming trans-alkyl compounds, while the group of aliphatic cyclic ethers remained (Figure 1B). The altered or broken functional groups might increase the possibility of MBC combining with pollutants. The value of point of zero charge (pH_pzc) of MBC is generally in the alkaline range (Figure 1C). Thermo-modification increased the value of pH_pzc from 8.5 (MT at 60 °C) to 11.0 (MT at 400 °C). The physicochemical characteristic changes might alter the adsorption properties of MBC.

3.2. Adsorption Equilibrium, Isothermal Models, and Factors

3.2.1. Adsorption Equilibrium

The adsorption of MBC to Cu²⁺ in TDW was rapid and reached the first equilibrium in 0.5 h (Figure 2A). Only 5~10% of Cu²⁺ was released by MBC with MT from 60 to 300 °C in the later observation. After the slight release, the second equilibrium was observed in 4 h (Figure 2B). The second equilibrium could be maintained for 28 h at least (Figure 2C). For the adsorption of MBC with MT at 400 °C, the release of Cu²⁺ was not observed. This indicated that MBC samples at 400 °C possessed greater adsorption capabilities and stronger combination force than the other MBC samples.

The results of the desorption tests indicated that no Cu²⁺ was desorbed into TDW in 500 min when the process was not interfered with by chemicals.

The maximum removal rate (E_max) grouped by MT was from 76.38 ± 3.64 to 92.69 ± 0.52% (

Table 3. The highest removal rate (E_max) and the lowest residues (C_min) grouped by thermo-modified temperature (MT).

MT/°C	Max. Removal Rates (Emax)				Min. Residues (Cmin)
	Values/%	C0/mg⋅L−1	AR/g⋅L−1	qe/mg⋅g−1	Values/mg⋅L−1	C0/mg⋅L−1	AR/g⋅L−1	qe/mg⋅g−1
60	86.98 ± 0.14	8.0	0.8	8.70	0.41 ± 0.04	0.5	0.04	2.17
80	76.38 ± 3.64	8.0	2.0	3.06	0.66 ± 0.06	1.0	0.04	8.52
100	81.43 ± 0.74	8.0	0.8	8.14	0.38 ± 0.02	0.5	0.04	2.86
200	89.51 ± 1.35	4.0	0.04	89.51	0.12 ± 0.01	1.0	0.04	22.11
300	92.34 ± 1.05	2.0	0.04	46.17	0.16 ± 0.01	2.0	0.04	46.17
400	92.69 ± 0.52	2.0	0.04	46.34	0.12 ± 0.01	1.0	0.04	21.94

). MBC achieved the optimal removal rate of 92.69 ± 0.52% at C₀ of 2.0 mg⋅L⁻¹ and adsorption equilibrium capacities of 46.34 ± 0.98 mg⋅g ⁻¹ at a dosage of 0.04 g⋅L⁻¹. The lowest residual of Cu²⁺ (C_e) in all treatments was 0.12 ± 0.01 mg⋅L⁻¹. The residual levels were satisfied to the standards of 1.0~1.3 mg⋅L⁻¹ that are used in most countries [3,43] and the rigorous threshold of 0.17~0.30 mg⋅L⁻¹ set by California Environmental Protection Agency [43]. It was observed that MBC at a MT lower than 200 °C adsorbed with removal rates less than 25.0% when C₀ of Cu²⁺ was at 0.5 mg⋅L⁻¹. Compared with the materials reported in Cu removal, the lowest residual of 0.12 ± 0.01 mg⋅L⁻¹ indicated that MBC with MT higher than 200 °C was superior in the treatment of drinking water and was competitive with the materials.

Sustainable materials and methods are the focus of future research to remove Cu²⁺ in water. One of the major obstacles in the adsorption methods is reported to be the high contact time needed for the application [44]. Neisan et al. modified activated carbon derived from orange peel and date seeds to remove copper from simulant wastewater and evaluated the efficiency [45]. The optimum conditions required the long contact time of 3.0~3.6 h to achieve satisfactory treatment with residues of 0.02 and 0.56 mg⋅L⁻¹. Dalgic et al. modified kapok fibers (Ceiba pentandra) by adipic dihydrazide and oxalic dihydrazide for the removal of copper ions from aqueous solutions through adsorption [46]. Also, the adsorbents removed about 62.0% of Cu²⁺ with 6.0 h of contact time. With the same size, the MBC in this work required a shorter adsorption time (0.5 h) than the reported materials did and achieved the same level of residues [9,12,36].

Meanwhile, the initial concentrations, i.e., C₀, are usually designed to be higher than 10.0 mg⋅L⁻¹ and seldom lower than 3.0 mg⋅L⁻¹ in the tests reported. The thermo-modified MBC in the work was used at the initial concentration lower than 2.0 mg⋅L⁻¹, and the performance was at a similar level to that achieved by artificial synthesis materials [9].

3.2.2. Isothermal Adsorption Models

Figure 3 indicates the isothermal adsorptions simulated by the Langmuir (L-tyl) (Figure 3A) and Freundlich (F-tyl) models (Figure 3B). The goodness of fit (R²) in the high-C₀ groups of 12.0 and 24.0 mg⋅L⁻¹ verified that the adsorptions conformed to both models (

Table 4. Working adsorptions and the goodness-of-fit (R²) values simulated by the L-tyl and F-tyl models.

	L-tyl						F-tyl
MT/°C C0/mg·L−1	60	80	100	200	300	400	60	80	100	200	300	400
24	0.962	0.934	0. 922	0.903	0.93	0.996	0.863	0.814	0.706	0.741	0.714	0.787
12	0.968	0.982	0.9421	0.753	0.761	0.863	0.792	0.7	0.847	0.928	0.865	0.95
8	~	0.985	~	~	~	0.979	0.842	0.796	~	~	~	0.849
4	~	~	~	0.844	0.86	~	~	0.828	~	0.744	0.839	0.702
2	~	0.873	0.767	~	~	0.793	0.946	0.879	0.819	0.834	0.99	0.848
1	NW	NW	~	~	~	~	0.892	NW	0.876	0.866	0.958	0.845
0.5	NW	NW	NW	~	~	~	NW	NW	NW	0.945	0.995	0.898

~—the value is less than 0.7. NW—adsorption not working.

), corresponding to the results previously reported [30]. In the two groups, the average R² was 0.911 ± 0.059 for L-tyl and 0.808 ± 0.042 for F-tyl, while the most R² indicated that the adsorptions conformed to the F-tyl model at the low C₀ level of 0.5, 1.0, and 2.0 mg⋅L⁻¹.

It was noted that both the L-tyl and F-tyl models simulated the adsorptions poorly when C₀ was at 4 or 8 mg⋅L⁻¹. The adsorption isotherm illustrates the equilibrium performance of adsorbents at the constant temperature. The results can be applied to plan the adsorption process in industry. Furthermore, the significance of the adsorption isotherm is also a source of the necessary information for substance adsorption equilibrium in industrial processes [27]. Al-Ghouti et al. reviewed the adsorption isotherm for the process of heavy metal removal by charcoals including biocharcoals and pointed out that most processes conform to L-tyl and F-tyl [27]. The modeling results herein did not conform to L-tyl or F-tyl, which implied that the adsorption process should be further considered in terms of the parameters of C₀ and the dosage involved in models.

3.2.3. Removal Rates and Factors

Temperature of thermo-modification. The thermo-modification with a high temperature improved the MBC, enabling an excellent removal rate at low C₀ levels. The MBC with or without thermo-modification performed the adsorption at C₀ at a level higher than 2.0 mg⋅L⁻¹. When C₀ was at 0.5 mg⋅L⁻¹, the MBC in the groups with a MT from 60 to 200 °C did not work or adsorbed weakly (dosage at 0.4 g⋅L⁻¹). However, the removal rate and the performances were stably increased when the MBC had a MT of 300 and 400 °C. This indicated that MT played an important role in the low-level C₀ removal of Cu²⁺. Sustainable and cost-effective materials and methods are the keys of future research to remove Cu²⁺ [44]. Biocharcoal absorbents as ecofriendly materials are the focus when it comes to adsorption in water. Physical or chemical modifications are applied in the preparation of biocharcoal absorbents to improve their adsorption performance [36,45]. There are no extra elements that need to be utilized in the physical modification of calcination, which is considered to lower the risk of secondary pollution. Meanwhile, the thermo-modified MBC can be applied directly in water without treatment to avoid hazards resulting from modification.

Thermo-modification enlarged the important adsorption characteristic of the SSA. Naef A et al. reviewed biocharcoal and its modifications and reported that the SSA ranged from lower than 1.0 m²⋅g⁻¹ (a straw-based absorbent) [47] to 708.1 m²⋅g⁻¹ (potassium-hydroxide-activated charcoal) [48]. MBC possessed a larger SSA (60 m²⋅g⁻¹) at 400 °C than at the lower temperature of 200 °C, which might result in the adsorption of Cu²⁺ at a low C₀ level. The activation points can significantly improve adsorption. Through pyrolysis, the thermo-modification also increased the activation point that chemo-modifications have to implement in reactions with extra chemicals.

Dosage and initial concentration. Figure 4 showed the effects of the relationship between C₀ and dosages on the removal rates. The removal rates (E) displayed correlations with the dosage depending on C₀ (Figure 4A). When C₀ was higher than 8.0 mg⋅L⁻¹ or lower than 4.0 mg⋅L⁻¹, the removal rates showed positive or negative correlations with the dosage (Figure 4B). The adsorption process also conformed to the isothermal models of L-tyl and F-tyl well.

When C₀ was at 4.0 and 8.0 mg⋅L⁻¹, the critical point of removal rate indicated the maximum dosage of adsorbents applied in adsorption (Figure 4B). For the two groups, the relationship between the removal rate and the dosage regressed to the parabola well (

Table 5. The parabolic regressions for the correlation of the removal rate with AR (C₀ = 8.0 mg⋅L⁻¹).

MT/°C	Regression Equation of Parabola (y = ax2 + bx + c)			R2	Vertex *1 /%	Emax. Exp. *2 /%
	a	b	c
100	−9.19 ± 6.20	14.55 ± 13.50	71.00 ± 4.28	0.7079	76.33	81.75
200	−7.47 ± 1.476	18.34 ± 3.22	71.86 ± 1.02	0.9492	83.12	88.82
300	−5.27 ± 0.60	10.22 ± 1.31	84.68 ± 0.41	0.9777	89.63	89.16

*¹—the average maximum values of the calculated removal rate. *²—the average maximum values of the experimental removal rate.

). The critical point was at the vertex of the parabola. Also, the adsorption of C₀ at 4.0 and 8.0 mg⋅L⁻¹ was poorly simulated by the adsorption isotherm of L-tyl and F-tyl. The adsorption equilibrium quantities of Cu per gram (q_e) of MBC represented one of two parameters of the isothermal models of L-tyl and F-tyl [27], and the relationship between the dosage and C₀ influenced the values of q_e. We deduced that this was the reason why the processes were simulated unsuccessfully by the L-tyl and F-tyl isothermal models.

Figure 4B also indicates that the critical point, as well as the correlations of the removal rate with the dosage, were independent of the MT. This implies that the thermo-modification of MBC has little effect on the trend in removal rate change when the dosage and the initial concentrations are considered simultaneously.

The correlation of the removal rates with dosage under a certain initial concentrations should be considered in the industrial application of MBC. Though most reports indicate that the removal rates of Cu²⁺ are positive related with C₀ [30], the effects of dosages have not been discussed comprehensively, especially at a low level of C₀. The critical point disclosed in the work indicated a specific threshold or turning point in the adsorption of MBC from a high C₀ level of Cu²⁺ to a low one, where a significant change or transition in the removal trend occurred. In a certain concentration range of Cu²⁺ in drinking water, the decision-making and optimizing processes or designs for the dosage of MBC to adsorb crucially rely on this point. Identifying and understanding critical points could enable us to predict and control the behavior and performance of adsorption, which would lead to improvements in the efficiency, safety, and overall effectiveness of engineered systems of MBC adsorbing Cu²⁺.

The values of q_e were positively correlated with C₀ (shown by ‘A1’) but negatively correlated with dosage (shown by ‘A2’) (Figure 4C). For instance, q_e was 8.67 ± 0.52 mg⋅g⁻¹ at C₀ = 0.5 mg⋅g⁻¹ but 419.56 ± 13 mg⋅g⁻¹ at 24.0 mg⋅g⁻¹ of C₀, when MT was at 400 °C and the dosage was at 0.4 g⋅L⁻¹. Moreover, 419.56 ± 13 mg⋅g⁻¹ of q_e indicated the competitive saturated adsorption capacities of MBC. Increasing the dosage was obviously disadvantageous to qe at the same level of C₀ and especially at a low C₀ level. The effects of agglomeration have recently been focused on in material engineering [49,50]. Negative correlations with dosages were considered to be influenced by material agglomeration. Though q_e was affected by dosages negatively, the adsorption capacities of MBC were satisfactory for application with low-level C₀ in water.

Value of pH. The effects of pH illustrated that the adsorptions were basically steady in the pH range 4.0~8.0 (Figure S2). The optimal pH was 2.0, which matched reports by Wang [35]. Though pH plays an important role in the removal of Cu²⁺ from water [10,11,32], the matrix of TDW requires that the effluent be neutral. In addition, the extra process of pH adjusting will cost more and might lead to safety concerns. Thus, treatment with a regular pH for TDW is suggested.

3.3. Mechanism for MBC Adsorption

In this work, MBC without or with low-temperature thermo-modification was observed not to adsorb Cu²⁺ at a C₀ level lower than 0.5~1.0 mg⋅g⁻¹. However, MBC with a thermo-modification temperature higher than 200 °C achieved this adsorption. We deduced the mechanisms that the thermo-modification altered the microstructure of the MBC. The characteristic analysis indicated that the micropore volume and the total pore volume increased and the SSA of the MBC was enlarged. Thus, the modifications with high temperatures exposed more activate points to combine with Cu²⁺ than those with low temperatures (Figure 5A).

Furthermore, thermo-modification induced functional group cleavage, increasing the anions of HO- or COO- on the surface. The exposed anions chelated Cu²⁺, separating copper from water to remove it. Meanwhile, the anions might raise the value of pH_pzc. If the pH value of TDW is lower than the pH_zpc of MBC, the thermo-modified MBC could be advantageous to adsorb the positive-charge ions of Cu through electrostatic attraction [25,51]. MBC with high-temperature modification performed stronger electrostatics adsorption onto Cu²⁺ than that with low-temperature modification [51].

The adsorption of Cu²⁺ by bamboo charcoal in water has been a focus in recent years. The primary mechanism behind this is that bamboo charcoal possesses functional groups impregnated easily with copper [39], a large adsorption capacity [18], and electrostatic attraction [25,51]. Thus, we deduced the mechanism as follows: through reconstructing the microstructure to expose the activated points and increasing the activated points and electrostatic adsorption, thermo-modification with a temperature higher than 200 °C improved MBC’s ability to adsorb Cu²⁺ at a low C₀ level in TDW.

It is hard to independently identify the roles of each interaction force in the engineering application. The fine particles can be aggregated in water, resulting in significant effects on the interior structure and porosity of the aggregate morphology (Figure 5B), the properties of which intensively impact the adsorption of particles to pollutants in water [52]. The critical point of the removal rate affected by the dosages and C₀ was considered to be a critical state of the force of balance between particles of MBC aggregation and dispersion and ions of Cu²⁺.

4. Conclusions

We reported the adsorption by MBC of trace Cu²⁺ in TDW. Thermo-modified MBC adsorption achieved a satisfactory level of Cu²⁺ adsorption according to the standards for TDW. Thermo-modification improved the inner structure and changed the functional groups and the electrostatic condition on the surface of MBC. This improvement allowed MBC to perform excellently at the low initial concentration of 0.5 mg⋅L⁻¹. MBC adsorbed Cu²⁺ rapidly in 0.5 h and acquired the lowest residues of 0.12 ± 0.01 mg⋅L⁻¹ in water. The correlation of the removal rate with the dosage was dependent on the initial concentration, which was implied by the simulation of the L-tyl and F-tyl isothermal models and confirmed through the analysis of multi-level factors in the initial concentration and dosage. The critical point between the positive and negative correlation of the removal rate with the dosage at the initial concentration of 4.0 and 8.0 mg⋅L⁻¹ might indicate the critical state of the interaction force between the aggregated or dispersed MBC and the ions of Cu²⁺. The critical point is beneficial in the design of dosage applications in engineering. This study presents an eco-friendly absorbent of MBC through efficient and sustainable modifications to rapidly remove the low-level Cu²⁺ from TDW, and also proposes the optimization of application parameters after thoroughly investigating the influencing factors and simulating the isothermal adsorption models.