Tea waste-supported hydrated manganese dioxide (HMO) for enhanced removal of typical toxic metal ions from water

Abstract

Tea waste (TW) was modified by depositing hydrated manganese oxide (HMO) onto it through in situ precipitation and a novel hybrid bio-adsorbent, namely HMO-TW, was obtained. The successful deposition of HMO in/on tea waste was confirmed by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) analysis. The removal of four typical heavy metals (i.e., Pb(II), Cd(II), Cu(II), Zn(II)) by HMO-TW was pH-dependent, and higher pH favored the sorption at the tested pHs of 2–7. HMO-TW showed excellent sorption selectivity toward all four metal ions, and the removal efficiency of target metal ions was sustained at 30%–90% even in the presence of 50 times higher competing Ca(II) and Mg(II) ions. Sorption isotherms of four metal ions by HMO-TW are all well represented by the Freundlich model, and the maximum experimental sorption capacities of Pb(II), Cd(II), Cu(II), Zn(II) were 174.3, 78.38, 54.38 and 37.5 mg g⁻¹, respectively. Compared to the unmodified tea waste, the sorption capacities and selectivity of Pb(II), Cd(II) and Cu(II) onto HMO-TW improved significantly. The sorption process reached equilibrium within 200 min, and the kinetics could be well fitted by a pseudo-second order model. Fixed-bed column sorption results further showed that the bed volume (BV) of C_e/C₀ reaching 0.5 for Pb(II), Cd(II), Cu(II), Zn(II) were 1170, 1130, 820 and 1450 BV, respectively. In addition, the exhausted HMO-TW can be effectively regenerated using a 0.5 M HCl solution. All results reported herein validate that HMO-TW is a promising sorbent for practical treatment of heavy metal contaminated water.

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Introduction

Contamination of heavy metals has attracted increasing attention due to their high toxicity, widespread presence in many kinds of environmental medium and potential bio-accumulation in food chain.¹ Even at extremely low concentrations, heavy metals can cause many harmful effects towards public health and aquatic life.² Unfortunately, heavy metals are inevitable by-products of rapid economic development. Owing to high malignancy of heavy metals, stringent regulations have been set up to control their maximum residual concentrations in all industrial effluents of China. For example, the Chinese government has set the maximum residual concentrations of Pb(II), Cd(II), Cu(II) and Zn(II) in effluents as 1.0, 0.1, 0.5, and 2.0 mg L⁻¹, respectively.³ Therefore, it is of great importance and urgency to develop efficient and cost-effective technologies for removal of toxic heavy metals from wastewater.

The primary methods for removing heavy metals from water/wastewater include chemical precipitation, adsorption, coagulation, and membrane filtration.^4,5 Among above-mentioned technologies, adsorption has been generally regarded as one of the most promising technologies.⁶ The development of adsorbent is the key for adsorption technology, and numerous new types of adsorbents like polymer, chelates, and composite have been constantly developed in recent years.^7–10 Meanwhile, many ordinary and original bio-adsorbents have gradually come into the vision of researchers because of their salient advantages such as easy availability, cheapness and low pollution. For example, wastes like walnut shells,¹¹ tea leaves,¹² rice hulls,¹³ bamboo,¹⁴ corn cobs¹⁵ and eggshell membrane^16–18 with reasonable numbers of carboxyl, phenolic hydroxyl and amine groups that can bind to metal ions have been used for toxic metal capture, and some relatively satisfactory results were obtained. Unfortunately, a lot of fatal defects such as low sorption capacity and poor sorption selectivity have seriously limited their further application. The sorption behavior of Pb(II), Cd(II), Cu(II) onto original tea waste (TW) was systematically investigated in our previous research,¹⁹ and the sorption capacities and selectivity of these metals were not very satisfactory. Therefore, it is very imperative to improve the performance of tea waste through some modifications in order to promote its practical application.

Hydrous manganese oxides (HMO) have always been considered to be one of the most efficacious adsorbents for heavy metal removal due to its specific and strong binding to target metal ion.^20,21 However, HMO is still far from practical application in fixed-bed and flow-through systems because of excessive pressure drop and difficulty of solid–liquid separation caused by its fine or ultrafine particle size. Some host materials such as activated carbon,^20,22 zeolite,²³ and porous polymer²⁴ have been selected to synthesize various hybrid adsorbents to overcome this technical barrier and these hybrid sorbents usually have good performance toward heavy metal removal. However, the high cost of these supporting materials makes their application in real world wastewater treatment unaffordable. Therefore, efficient and low-cost supporting materials are still in great demand to facilitate the application of HMO in wastewater treatment.

In this study, we prepared a novel composite sorbent (HMO-TW) by depositing HMO onto TW to maximally utilize the benefits of both adsorbents for heavy metal removal. We anticipate that TW would be an effective and cost-efficient support for highly dispersed HMO. HMO, on the other hand, provides the composite good selectivity for heavy metal ions against competing cations commonly existing in water (e.g., Ca(II) and Mg(II)). Sorption behaviors of four typical toxic metal ions, Pb(II), Cd(II), Cu(II) and Zn(II), in aqueous solution were investigated by evaluating the influence of solution pH, coexist competing ions, temperature and contact time. Column experiments were performed to simulate the real situation treatment process. In addition, the regeneration of HMO-TW was also evaluated.

Experimental

1. Chemicals

All chemicals used in the current study are analytical grade (AR) or better and were purchased from Guoyao Reagent Station (Anhui, China). Stock solutions containing 1 g L⁻¹ of the target heavy metals were prepared by dissolving their corresponding nitrates with ultrapure water (resistivity > 18.2 MΩ cm⁻¹), which were further diluted prior to use.

2. Preparation of HMO-TW

The TW was collected from tea plants located in Green tea region in Huangshan, China. The pretreatment of TW had been described in our previous research.¹⁹ HMO-TW was prepared by in situ precipitation and the procedures are as following: Firstly, 10.0 g TW after pretreatment was added into 1 mol L⁻¹ MnSO₄·H₂O solution and stirred continuously at 303 K for 24 h. Secondly, the Mn(II)-loaded tea waste was filtered from solution and then dried under vacuum at 333 K for 2 h. After this, the solid was added into a binary NaClO–NaOH solution with alkalinity of 10% and active chlorine of 13%, and stirred continuously at 303 K for 24 h to oxidize Mn(II) to Mn(IV) and precipitate.²⁴ Finally, the obtained hybrid materials were washed with 0.1 M HCl and ultrapure water until neutral pH and followed by desiccation under vacuum at 333 K till reaching a constant weight. The final products were stored in sealed glass bottles for future use.

3. Batch sorption experiments

Batch sorption experiments were carried out using traditional bottle-point method in 250 mL Erlenmeyer flasks. The detailed experimental information is described in following sections.

3.1 Effect of solution pH on heavy metal adsorption. 20.0 mg of HMO-TW particles were dispersed into 100 mL Erlenmeyer flasks containing 50 mL solution of either Pb(II) (40 mg L⁻¹), Cd(II) (20 mg L⁻¹), Cu(II) (10 mg L⁻¹) or Zn(II) (10 mg L⁻¹). A 1.0 M HCl or NaOH solution was used to adjust the desired solution pH to 2–7 throughout the experiments. A thermostatic orbit incubator shaker (New Brunswick Scientific Co. Inc.) was used to shake the stoppered flasks at 120 rpm and 303 K for 24 h, which was sufficient to reach adsorption equilibrium according to preliminary kinetic tests. Finally, the supernatant was sampled and analyzed for equilibrium solution pH and concentration of target metal ions. The amount of adsorbed heavy metals onto HMO-TW was calculated based on mass balance between initial and finial metal ion concentrations.

3.2 Competitive sorption test. For the competitive sorption tests, the initial concentrations of heavy metals were 40 mg L⁻¹ for Pb(II), 20 mg L⁻¹ for Cd(II), 10 mg L⁻¹ for Cu(II) and 5 mg L⁻¹ for Zn(II), respectively. Other conditions were basically same as the above-mentioned pH effect experiments except certain amount of representative competitive ions (Ca(II) and Mg(II)) were introduced into the test solution.

3.3 Sorption isotherm. For sorption isotherm tests, 20 mg HMO-TW was dispersed into 100 mL flasks containing 50 mL solution of various contents of metal ions. The experimental temperature was set at 298 K. Langmuir and Freundlich equations²⁵ were applied to fit the sorption data from the isotherm tests:where q_e is the amount of adsorbed metal at equilibrium (mg g⁻¹); C_e is the concentration of the metal ion in solution at equilibrium (mg L⁻¹); q_m is the maximum adsorption capacity of metal on HMO-TW (mg g⁻¹); and K_L (L mg⁻¹) is a binding constant, and both K_F and n are the Freundlich constants.

3.4 Adsorption kinetics. For kinetic tests, 200 mg HMO-TW was added into 1000 mL Erlenmeyer flasks containing 500 mL solution of either 70 mg L⁻¹ of Pb(II), 25 mg L⁻¹ of Cd(II), 30 mg L⁻¹ of Cu(II) or 10 mg L⁻¹ of Zn(II). Aliquots of 0.5 mL were sampled at various time intervals, e.g., every 5 min for 0–30 min, every 10 min for 30–150 min, every 20 min for 150–210 min and every 30 min for 210–360 min. The sorption capacities of heavy metals versus time constituted the kinetic curve.

The pseudo-second-order equation²⁶ was used to fit the kinetic data:

where q_e and q_t are sorption capacities of heavy metal (mg g⁻¹) at equilibrium and at time t, respectively and k₂ is second-order sorption rate constant.

4. Fixed-bed column sorption and desorption

In column experiments, a 5 mL aliquot of wet HMO-TW particles was filled into a polyethylene column (12 mm diameter and 130 mm length) equipped with a water bath to maintain a desired temperature. A Lange-580 pump (Baoding, China) was used to ensure a constant flow rate. Simulated wastewater containing given concentration of target metal ions was supplied as influent. A 0.5 M HCl solution was used to regenerate the exhausted HMO-TW after sorption. Detailed operation conditions (e.g., superficial liquid velocity (SLV) and empty bed contact time (EBCT)) are depicted in the related figure captions.

5. Analyses

Metal ion concentration in aqueous phase was determined using a TAS-990 flame atom adsorption spectrophotometer (Persee inc., China), and the absorbance of all samples was tested in triplicate. An atom fluorescence spectrophotometer (AF-640) (Ruili Co., Ltd) was also used to determine metal ion content, if lower than 1 mg L⁻¹. The amount of loaded HMO in/on tea waste was calculated using Mn content in aqueous phase after HNO₃–HClO₄ digestion process. FT-IR spectra of HMO-TW before and after metal sorption were obtained using a Nicolet 380 FT-IR spectrometer (USA) with a pellet of powered potassium bromide and adsorbent in the range of 400–4000 cm⁻¹. Crystallinity of HMO-TW was determined by means of X-ray diffraction analysis with a step size of 0.02° (D8 ADVANCE, Germany). The surface morphology of the host tea waste and HMO-TW was observed using a scanning electron microscope (S-3400N, Japan) and a transmission electron microscopy (JEM-2000EX). The zeta potential of HMO-TW was measured using a Malvern Zetasizer Nano ZS90. N₂ adsorption–desorption tests onto HMO-TW particles were carried out at 77 K to obtain specific surface area based on BET model and pore size distribution based on BJH model using a Micromeritics ASAP2020 (USA).

Results and discussion

1. Characterization of the adsorbents

The successful deposition of nano-sized HMO particles on the surface of tea waste is clearly shown in a TEM image of HMO-TW (Fig. 1a). The sizes of the highly dispersed HMO particles are typically less than 5 nm. The surface morphology of TW was obviously different before and after HMO loading according to SEM images of TW and HMO-TW depicted in Fig. S1.† Specifically, the pores located in host tea waste surface were wide and sparse, while the pores became tiny and dense after HMO particles loading. FT-IR spectra of the original tea waste and HMO-TW are depicted in Fig. 1b. Obvious sorption peaks of 1623, 915 cm⁻¹ assigned to Mn–OH group^27,28 are observed in the FT-IR spectra of HMO-TW but not TW, which indicates that HMO particles had been successfully loaded onto TW. The zeta potential of HMO-TW (Fig. 1c) was always negative in the tested pH range of 2–10, indicating HMO-TW can strongly attract cations. The broad and weak diffraction peaks in X-ray diffraction pattern of HMO-TW (Fig. S2†) are similar to those of δ-MnO₂,²⁹ which suggests that most of the loaded HMO is amorphous in nature. Some major physicochemical properties of the host TW and HMO-TW are listed in Table 1. The loaded HMO amount on TW was 4.82 (w/w)% in Mn mass, while no Mn was detected in the supporting TW. The larger BET specific surface area of HMO-TW (5.92 m² g⁻¹) compared to that of host TW (0.86 m² g⁻¹) should result from the large surface area of HMO, which was reported to be 100.5 m² g⁻¹ (ref. 28) (note that the HMO content is only 4.86% as Mn in HMO-TW).

Fig. 1 Physical characterization of TW and HMO-TW: (a) TEM of HMO-TW; (b) FT-IR spectrum of TW and HMO-TW; and (c) zeta potential of HMO-TW at 298 K.

Table 1 Major physicochemical properties of the host tea waste and HMO-TW

Designation	Tea waste	HMO-TW
Functional group	–COOH, –NH2, –OH	–COOH, –NH2, –OH and HMO
Particles size (mm)	0.25–0.88	0.25–0.88
BET surface area (m^2 g^−1)	0.86	5.92
Average pore diameter (nm)	3.62	1.03
Mn content (%)	0	4.82

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2. Effect of solution pH

Solution pH is a significant environmental factor that usually influences the sorption process by altering surface properties of the sorbent and the speciation of the sorbate. Therefore, removal of Pb(II), Cd(II), Cu(II) and Zn(II) by HMO-TW at different solution pH was conducted, and the effect results are shown in Fig. 2. The removal efficiency of heavy metals increased with the increase of solution pH. More specifically, the removal of heavy metals almost linearly increased at acidic pH range 2–5, and became more gradual at the near neutral pH range 5–7. The above pH effect results are consistent to sorption of many heavy metals onto other adsorbents,^15,28,30 which can be explained by the exchange of H in functional groups of sorbent with target metal ions. In our previous research, we elucidated the ion exchange process between the functional groups, e.g. carboxyl and phenolic hydroxyl groups, on TW and the target metal ions.¹⁹ For the as-prepared HMO-TW, the loaded HMO would result in the same pH effect by forming inner-sphere complexes with metal ions through^31,32

–Mn–OH– + M²⁺ ⇄ –Mn–O–M⁺ + H⁺ (4)

–Mn–(OH)₂– + M²⁺ ⇄ –Mn–O–M + 2H⁺ (5)

Fig. 2 Influence of pH on sorption of Pb(II), Cd(II), Cu(II) and Zn(II) by HMO-TW. Conditions: C₀ (Pb(II)) = 40 mg L⁻¹; C₀ (Cd(II)) = 20 mg L⁻¹; C₀ (Cu(II)) = 10 mg L⁻¹; C₀ (Zn(II)) = 10 mg L⁻¹; sorbent dose = 0.4 g L⁻¹; temperature = 298 K.

Obviously, more H⁺, namely, lower solution pH, is unfavorable for heavy metal sorption onto loaded HMO, while higher pH promotes the above reactions. The inner-sphere complexes between Mn–OH and heavy metals had been extensively researched using EXAFS.^33,34 The combined effects of HMO and host TW led to the pH-dependent plots depicted in Fig. 2. Besides, the contents of Mn released into solution after Pb(II) adsorption at different pHs (2–7) were analyzed, and no Mn leaching was detected (data not shown). Thus, HMO-TW could be an ideal sorbent for heavy metal removal from slightly acidic wastewater (e.g., pH 4–7) in terms of both material stability and removal efficiency.

3. Effect of coexisting ions

The effects of co-existing alkali and alkaline-earth metal cations on Pb(II), Cd(II), Cu(II) and Zn(II) adsorption by HMO-TW were conducted, and the results are shown in Fig. 3. Here, two divalent cations with low hydration energy, Ca(II) (−1656 kJ mol⁻¹) and Mg(II) (−2049 kJ mol⁻¹),³⁵ were selected as representative co-ions because of their stronger competition for heavy metal adsorption than Na(I) or K(I). The addition of Ca(II) and Mg(II) resulted in decline of the sorption of four heavy metals onto HMO-TW to various degrees. Specifically, only slight drop of removal efficiency (<25%) (i.e., good selectivity) was observed for Pb(II) and Cu(II) in the presence of co-ions at 0 to 50 times higher concentrations. However, greater impacts with drop of removal efficiency up to 60% were observed for Cd(II) and Zn(II).

Fig. 3 Influence of Ca(II) (a) and Mg(II) (b) on sorption of Pb(II), Cd(II), Cu(II) and Zn(II) by HMO-TW. Conditions: C₀ (Pb(II)) = 40 mg L⁻¹; C₀ (Cd(II)) = 20 mg L⁻¹; C₀ (Cu(II)) = 10 mg L⁻¹; C₀ (Zn(II)) = 5 mg L⁻¹; sorbent dose = 0.4 g L⁻¹; pH = 5.5 ± 0.2; temperature = 298 K.

The sorption selectivity order of Pb(II) > Cd(II) > Zn(II) can be explained by two mechanisms, ion exchange with the host TW and inner-sphere complexation with the loaded HMO. Generally, the hydration energy of metal ions is related to their ion exchange ability, and lower hydration energy favors ion exchange.³⁶ Thus, Pb(II) with lower hydration energy (−1504 kJ mol⁻¹) compared to that of Cd(II) (−1708 kJ mol⁻¹) and Zn(II) (−1992 kJ mol⁻¹)³⁵ had the most ion exchange with TW. As for the inner-sphere complexation with HMO, it can be illustrated by the hard and soft acids and bases (HSAB) principle.³⁷ Here, the target heavy metal ions act as Lewis acids while HMO as a Lewis base. We note that the softness of Pb(II) is higher than that of Cd(II) and Zn(II) as a result of their different ionic radius (0.118 nm for Pb(II), 0.095 nm for Cd(II) and 0.075 nm for Zn(II)). Since high softness favors Lewis acid–base interaction, the adsorption of Pb(II) to HMO was the strongest (i.e., best selectivity).

The stronger adsorption selectivity of Cu(II) than Cd(II) and Zn(II) is somewhat unexpected since Cu(II) has the highest hydration energy (−2030 kJ mol⁻¹) and the smallest ionic radius (0.073 nm), which suggest the lowest ion exchange and weakest HMO-Cu(II) interaction. The results can be attributed to the amine groups on host TW, which form strong, specific coordination with Cu(II).^38,39 This also explains that the sorption of Cu(II) onto TW was higher than Cd(II) although Cu(II) has much higher hydration energy in our previous study.¹⁹

Since TW does not have satisfactory selectivity for metal ion adsorption, the good sorption selectivity of HMO-TW is mainly from the HMO on TW, which is known to have strong sorption specificity for heavy metals. Nonetheless, the functional group rich TW can facilitate the sorption selectivity through the Donnan membrane effect⁴⁰ caused by the attraction of positively charge metal ions by non-diffusible, negatively charged carboxyl groups on TW. The Donnan membrane effect is illustrated in Fig. S3.† The Ca(II) had greater influence than Mg(II) on sorption of all four heavy metals by HMO-TW as expected due to its lower Gibbs free energy of hydration.³⁶

The distribution coefficient K_d (L g⁻¹) is then introduced to quantify the sorption selectivity of four metal ions onto HMO-TW⁴¹

where C₀ (mg L⁻¹) is the aqueous initial heavy metal ion concentration, C_e is the aqueous heavy metal ion concentration at equilibrium (mg L⁻¹), V (mL) is the volume of the solution, and m (g) is the mass of the adsorbent. Table 2 summarizes the K_d values for four heavy metal ions in the presence of different levels of Ca(II) and Mg(II). The trend of K_d value variation is in good agreement with the curves in Fig. 3.

Table 2 Distribution coefficients (K_d) of Pb(II), Cd(II), Cu(II) and Zn(II) for HMO-TW in the presence of different levels of Ca(II) and Mg(II)

Competing ions	Heavy metals	Kd (mL g^−1) at different initial Ca(II) or Mg(II)/M(II) ratios in solution (mol/mol)
		5	10	15	20	50
Ca(II)	Pb(II)	23 644	25 048	16 105	15 967	12 260
	Cd(II)	4799	3263	2589	2024	1224
	Cu(II)	15 681	14 167	8968	8661	5325
	Zn(II)	4238	2935	2336	1737	1400
Mg(II)	Pb(II)	30 126	24 418	18 959	15 013	73 039
	Cd(II)	11 105	4122	3616	2628	1468
	Cu(II)	26 740	18 333	12 293	12 652	8736
	Zn(II)	13 028	6241	6091	5751	3221

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4. Sorption isotherms and kinetic

The sorption isotherms of Pb(II), Cd(II), Cu(II) and Zn(II) by HMO-TW were investigated at 298 K, and the results are illustrated in Fig. 4. The sorption capacities of four metal ions all increased with the increase of C_e as well as C₀ (Fig. S4†). Subsequently, the four isotherms are fitted using both the Freundlich and Langmuir models. Generally, the Langmuir model refers to a monolayer adsorption onto homogeneous surface with no interactions between the adsorbed molecules, and the Freundlich model is an empirical equation usually used to describe chemisorption on heterogeneous surface.⁴² The detailed fitted parameters are listed in Table 3. As shown, the sorption isotherms of four heavy metals can be better fitted by the Freundlich model than the Langmuir one, with related coefficients R² larger than 0.962. The better fitting of Freundlich model indicates a chemisorption process, and the effective binding sites on HMO-TW surface for four metal ions are heterogeneous. The strong interaction (e.g., inner-sphere complexes) between heavy metal ions and HMO clearly contributes to the heterogeneous chemisorption process. The experimental sorption capacities for Pb(II), Cd(II), Cu(II) and Zn(II) were 174.3, 78.38, 54.38 and 37.5 mg g⁻¹, respectively. The capacities for Pb(II), Cd(II) and Cu(II) are much higher than the calculated q_m values of the host TW, which were 26.98 mg g⁻¹ for Pb(II), 15.04 mg g⁻¹ for Cd(II) and 18.54 mg g⁻¹ for Cu(II).¹⁹

Fig. 4 Sorption isotherms of Pb(II), Cd(II), Cu(II) and Zn(II) onto HMO-TW. Conditions: sorbent dose = 0.4 g L⁻¹; pH = 5.5 ± 0.2; temperature = 298 K.

Table 3 Isotherm constants for uptakes of Pb(II), Cd(II), Cu(II) and Zn(II) onto HMO-TW at 298 K

Heavy metals	Freundlich model			Langmuir model
	KF (mg^1−n L^n g^−1)	n	R^2	KL (L mg^−1)	qm (mg g^−1)	R^2
Pb(II)	101.18	0.17	0.974	0.99	172.50	0.882
Cd(II)	47.19	0.13	0.979	1.55	72.88	0.807
Cu(II)	28.15	0.19	0.962	1.08	52.70	0.887
Zn(II)	19.92	0.16	0.988	3.27	32.37	0.756

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It is worth noting that the utilization efficiency of unit mass Mn is improved for around 5 times by loading HMO onto tea waste. The Pb(II), Cd(II) and Zn(II) sorption capacities of unit mass Mn for HMO-TW were 3616.2, 1626.1 and 778 mg g⁻¹, respectively, and for pure HMO were 710.1, 304.1 and 117.7 mg g⁻¹, respectively.²⁸ The improvement of Mn utilization efficiency can be attributed to the dispersion of HMO nanoparticles (<5 nm based on TEM) on TW increased accessible binding sites of HMO. In addition, the sorption capacities of Pb(II), Cd(II), Cu(II) and Zn(II) by HMO-TW and other previously reported materials are compared in Table 5. The sorption capacities of four heavy metals onto HMO-TW are better than most previously reported sorbents with low-cost merits. It is also noteworthy that HMO-TW is more applicable for heavy metal removal from lightly polluted water base on the results depicted in Fig. S4.† The removal efficiency of four toxic metals decreased with the increase of initial metal ion concentration, which can be explained by the diminished opportunities of target metal ions binding to sorption sites on HMO-TW at higher initial concentrations.

Table 4 Kinetic parameters of Pb(II), Cd(II), Cu(II) and Zn(II) removal by HMO-TW at 298 K

Heavy metals	Pseudo-second-order			Experimental values
	k2, 10^−2 min^−1	qe, mg g^−1	R^2	qe, mg g^−1
Pb(II)	1.80	110.3	0.957	118.9
Cd(II)	1.41	37.7	0.994	38.4
Cu(II)	2.38	28.2	0.991	29.0
Zn(II)	1.00	17.3	0.993	18.0

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Table 5 Comparison of the sorption capacities for Pb(II), Cd(II), Cu(II) and Zn(II) onto various adsorbents^a

Sorbent	Metal ions	Conditions	Sorption capacity (mg g^−1)	Ref.
a NA: not available.
HMO-TW	Pb(II)	pH 5.5, 298 K	174.3	Present study
The host tea waste	Pb(II)	pH 5.0, 298 K	33.49	19
Wheat bran	Pb(II)	NA	63.9	43
SD-SNTs	Pb(II)	pH 7.0, 303 K	112.36	44
HMO-D001	Pb(II)	pH 4.4, 298 K	395	24
Manganese oxide-coated carbon nanotubes	Pb(II)	pH 5.0, 323 K	78.74	45
Manganese oxide coated sand	Pb(II)	pH 4.0, 318 K	1.9	46
HMO-TW	Pb(II)	pH 5.5, 298 K	78.38	Present study
The host tea waste	Cd(II)	pH 5.5, 298 K	15.04	19
Wheat bran	Cd(II)	NA	24.2	43
Lewatit CNP 80	Cd(II)	pH 8, 298 K	4.48	47
Graphene oxide membranes	Cd(II)	pH 5.8, 303 K	83.8	48
Moss	Cd(II)	pH 5.0, 303 K	29.0	49
Phosphorylated PAN-based nanofiber	Cd(II)	NA	37.3	50
HMO-TW	Cd(II)	pH 5.5, 298 K	54.38	Present study
The host tea waste	Cu(II)	pH 5.0, 298 K	18.54	19
Wheat bran	Cu(II)	NA	14.5	43
Graphene oxide membranes	Cu(II)	pH 5.8, 303 K	72.6	48
Manganese oxide coated sand	Cu(II)	pH 4.0, 318 K	0.48	46
Lewatit CNP 80	Cu(II)	pH 8, 298 K	10.24	47
HMO-TW	Zn(II)	pH 5.5, 298 K	37.5	Present study
Moss	Zn(II)	pH 5.0, 303 K	15.0	49
Lewatit CNP 80	Zn(II)	pH 8, 298 K	20.15	47
Tree fern	Zn(II)	NA	7.58	51

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The sorption kinetics of the four heavy metal ions onto HMO-TW was also investigated (Fig. 5). The sorption process was very quick by HMO-TW in the initial 100 min then slowed down to approach sorption equilibrium within 200 min. The kinetic data of all heavy metal ions were well fitted by the pseudo-second-order model, and the fitting parameters are listed in Table 4. The calculated q_e values based on pseudo-second-order model (110.3 mg g⁻¹ for Pb(II), 37.7 mg g⁻¹ for Cd(II), 28.2 mg g⁻¹ for Cu(II), 17.3 mg g⁻¹ for Zn(II)) are close to the experimental data (119.28 mg g⁻¹ for Pb(II), 38.45 mg g⁻¹ for Cd(II), 29.35 mg g⁻¹ for Cu(II), 18.03 mg g⁻¹ for Zn(II)). The sorption of heavy metal ions by HMO-TW was relatively slow compared to host TW, which may be caused by the pore blockage effect as a result of HMO loading.

Fig. 5 Sorption kinetics of Pb(II), Cd(II), Cu(II) and Zn(II) onto HMO-TW. Conditions: C₀ (Pb(II)) = 70 mg L⁻¹; C₀ (Cd(II)) = 25 mg L⁻¹; C₀ (Cu(II)) = 30 mg L⁻¹; C₀ (Zn(II)) = 10 mg L⁻¹; sorbent dose = 0.4 g L⁻¹; pH = 5.5 ± 0.2; temperature = 298 K.

5. Fix-bed column sorption and regeneration

Fix-bed column sorption trail is an essential step for evaluating the engineering application potential of a given sorbent. The complete effluent history of the column sorption tests of HMO-TW is described in Fig. 6. The bed volume (BV) of C_e/C₀ reaching 0.5 for Pb(II), Cd(II), Cu(II) and Zn(II) are around 1170, 1130, 820 and 1450 BV, respectively. Note that almost no target heavy metal ions was detected in the effluent until a sudden increase after 830 BV for Pb(II), 400 BV for Cd(II), 650 BV for Cu(II) and 950 BV for Zn(II). The excellent results of all four heavy metal adsorption by HMO-TW in the column tests, especially in the early stage, imply that HMO-TW has great potential for practical wastewater treatment in flow-through systems.

Fig. 6 Breakthrough curves of Pb(II), Cd(II), Cu(II) and Zn(II) in a column system filled with 5 mL wet HMO-TW. Conditions: influent concentration (C_in) of Pb(II) = 9.0 mg L⁻¹; C_in (Cd(II)) = 1.5 mg L⁻¹; C_in (Cu(II)) = 2.7 mg L⁻¹; C_in (Zn(II)) = 1.1 mg L⁻¹.

The exhausted HMO-TW was able to be regenerated using a 0.5 M HCl solution at 298 K as shown in Fig. 7. The adsorbed heavy metals were completely regenerated after only 6 BV of HCl solution flushing, and the regeneration efficiency was larger than 95%. In addition, only negligible Mn was detected in the regeneration solution.

Fig. 7 The cumulative desorption curves of the exhausted HMO-TW in column flushed with 0.5 M HCl.

Conclusions

A novel bio-adsorbent HMO-TW was successfully prepared by impregnating hydrated manganese oxide (HMO) onto tea waste. Sorption of Pb(II), Cd(II), Cu(II) and Zn(II) onto HMO-TW was observed to be pH-dependent due to a combined effect induced by the ion-exchange with the host TW and the inner-sphere complexation with impregnated HMO. Even in the presence of competing ions at 50 times higher concentration, HMO-TW maintained excellent sorption selectivity toward four heavy metal ions due to the specific binding between heavy metal ions and the loaded HMO and Donnan membrane effect. Besides, the sorption of four metal ions onto HMO-TW can be well fitted by the Freundlich model and a pseudo-second-order model. Satisfactory results of column sorption and regeneration tests further show that HMO-TW is a promising sorbent for removing heavy metals from contaminated water in engineering settings.

Acknowledgements

This work has been supported by the Science Foundation for Excellent Youth Scholars of Universities and Colleges of Anhui Province (2013SQRL091ZD), Innovative Training of College Students of Huangshan University (AH2014103753123) and the National Natural Science Foundation of China (No. 51308312).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16556c

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