Adsorption Kinetics of N-Doped Carbon Modified with Magnetite for Hexavalent Chromium ^†

Abstract

Chromium six valence (Cr(VI)) is a hazardous heavy metal that tends to bioaccumulate in aquatic species. Adsorption is preferred used in Cr(VI) treatment due to its operational simplicity, low cost, environmental sustainability, and high efficiency. This research aimed to investigate the kinetics of substantial Cr(VI) adsorption using Magnetite-modified N-doped adsorbent. The research started with the synthesis of adsorbents, proceeded to the test of adsorption under varying contact times, and finished with an analysis of adsorption kinetics. The results indicated that the magnetite-modified N-doped adsorbent successfully adsorbed Cr(VI) at a rate of 0.1336 ${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$, with an equilibrium adsorption capacity of 158.73 mg/g. The kinetics of this adsorption follows to a pseudo second-order model.

1. Introduction

Chromium is a heavy metal that tends to bioaccumulate in aquatic creatures. Chromium exists as Cr(III), an essential component for humans, and Cr(VI), which is toxic and causes diarrhea, skin irritation, and liver disease [1]. Cr(VI) consists of several ionic species that are affected by pH, including HCrO₄⁻, Cr₂O₇²⁻, and Cr₂O₄²⁻ [2,3]. At pH 6, the predominant form of Cr(VI) is HCrO₄⁻ ion [3]. Technologies for treatment of Cr(VI) metal ions consist of photocatalysis, electrochemistry, adsorption, solvent extraction, membrane separation, and ion exchange. Adsorption is preferred because of its operational simplicity, low cost, environmental sustainability, high efficiency, and its effectiveness in eliminating pollutants in water [1].

Carbon-based materials perform as effective, low-cost, easy to use and environmentally friendly adsorbents for treatment of contaminants in aqueous solutions [4,5]. Furthermore, carbon materials with magnetic modifications are more advantageous for usage as adsorbents, since these modifications facilitate the separation process of adsorbents from remaining adsorbate solutions [6]. In a prior study, Popovic et al. reported the application of magnetite-modified carbon adsorbent for the removal of the heavy metal Cr(VI), given an adsorption capacity of 62.9 mg/g for the magnetite-modified lignin-based carbon adsorbent [7].

Our study reported the preparation of N-doped carbon-based on cellulose from palm empty fruit bunches, modified with magnetite from a Fe(III) precursor solution and treated with pyrolysis at 700 °C. Adsorption kinetics were evaluated by batch system adsorption at pH 6, varying the time of contact between the adsorbent and adsorbate for determining the best appropriate adsorption kinetics model.

2. Experimental Method

2.1. Materials

Materials NaOH, NH₃ 25%, FeCl₃. 6H₂O, K₂Cr₂O₇, KHC₈H₄O₄, and CH₃CH₂OH 99.9% were purchased from Merck (Darmstadt, Germany) with a purity of 99.9%. Palm empty fruit bunches were purchased from PT Polytech Indonesia (South Tangerang, Indonesia) with containing of 49% cellulose. Urea was purchased from PT Petrokimia Gresik. H₂O₂ (50%) was purchased from CV Hekto Gilang Meraki (Malang, Indonesia). Demineralized water was utilized for the synthesis procedure and the performance adsorption test.

2.2. Synthesis Procedure for Adsorbent Material

The synthesis method for the magnetite-modified N-doped adsorbent was based on prior research [6]. In briefly, 20 g of palm empty fruit bunches was carried out alkaline digestion by reflux at 100 °C for 2 h, utilizing a 17% NaOH solution at a ratio of 1 g for 20 mL. After that, the pulp was taken through to filtration and rinsing. The pulp was dried in an oven at a temperature of 101 °C for 4 h. The dry pulp was bleached using 3% H₂O₂ at pH 9 and a temperature of 90 °C for 1 h, with a ratio of 1 g for 20 mL. The cellulose pulp was subsequently rinsed with water adjusted to a neutral pH.

1 g of cellulose pulp was mixed with 1 g of NaOH, 4 g of urea, and 7.8 mL of 25% NH₃ until a homogenous mixture, followed by a gelation process at −12 °C for 24 h. Subsequently, 15 mL of 96% ethanol was added into the resultant gel for the coagulation phase for 24 h. It was followed by impregnation with 20 mL of FeCl₃ solution containing 5% Fe for 24 h. The subsequent step involved solvent exchange. The resulting gel was separated from the solvent and subsequently frozen to −20 °C for 24 h, followed by freeze-drying for 24 h at −40 °C. The cellulose aerogel was generated using a pyrolysis procedure during 2 h to yield magnetite-modified nitrogen-doped carbon. The materials were further characterized using X-ray diffraction (XRD; X’pert Pro, PANalytical, Almelo, The Netherlands) within an angular range of 2θ = 20–80° and analyzed by scanning electron microscopy (SEM; FlexSEM 1000, Hitachi, Tokyo, Japan).

2.3. Procedure for Analyzing Adsorption Performance

10 mg of adsorbent was placed in a 50 mL plastic container. Subsequently, 20 mL of a 200 ppm Cr(VI) solution was prepared using K₂Cr₂O₇ as the precursor. The adsorption process in a batch system was conducted with contact times of 15, 30, 60, 90, and 120 min at a pH of 6. The filtrate was analyzed using an atomic absorption spectrophotometer (AAS, PinAAcle 900T, Perkin Elmer, Shelton, CT, USA) to determine the quantity of adsorbed Cr(VI) ions. The quantity of Cr(VI) adsorbed was evaluated using the following formula [8,9]:

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

(1)

Adsorption kinetics analysis using the following formula [8,9]:

$$\mathrm{first}-\mathrm{order} \mathrm{pseudo}: \ln { (q}_e- q_t)=\ln q_e- k_{1}t$$

(2)

$$\sec \mathrm{ond}-\mathrm{order} \mathrm{pseudo} {\displaystyle \frac{\mathrm{t}}{q_t}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{k_2{q_e}^2}}+{\displaystyle \frac{1}{q_e}}t$$

(3)

Furthermore, adsorption kinetics are possible via the diffusion of the adsorbate into the adsorbent’s pores, as described by the Weber and Morris intra-particle diffusion model [8]:

$$q_t=K_{i }{t}^{1/2}+c$$

(4)

3. Result and Discussion

3.1. Characteristics of Magnetite Modified N-Doped Carbon

The N-doped carbon material modified with magnetite was characterized by X-ray diffraction (XRD), as illustrated in Figure 1. Symbol C with inverted triangle in black as carbon notation and symbol M with inverted triangle in red as Magnetite notation. N-doped carbon material treated with magnetite has a diffraction peak (0 0 2) at an angle of 26°, corresponding to carbon material, based on JCPDS card 75-1621 [5,6,10]. Furthermore, there is also a characteristic peak of magnetite material (Fe₃O₄) at the peak of 2 θ = 30°, 36°, 43°, 54°, 57°, and 63° that each of the Crystal plane of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) based on JCPDS of Fe₃O₄ card 19-0629 [6,11]. In addition, the N-doped carbon material modified with synthesized magnetite has a porous structure as shown in Figure 2.

3.2. Kinetics of Cr (VI) Adsorption by Magnetite-Modified N-Doped Carbon

The adsorption of Cr (VI) using a magnetite-modified N-doped carbon adsorbent was conducted at pH 6 and a Cr(VI) concentration of 200 ppm, with contact time varying from 15 to 120 min. The results of the adsorption kinetics analysis of Cr(VI) using magnetite-modified N-doped carbon adsorbent are shown in Figure 3 and

Table 1. Kinetic parameters of Cr(VI) adsorption with magnetite-modified N-doped carbon adsorbent.

Kinetic Adsorption	Parameters	Value
first-order pseudo	${\mathrm{q}}_{\mathrm{e}} \left(\mathrm{m}\mathrm{g}/\mathrm{g}\right)$	16.04
	${\mathrm{k}}_1\left({\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}\right)$	0.0185
	${\mathrm{R}}^2$	0.88914
second-order pseudo	${\mathrm{q}}_{\mathrm{e}}(\mathrm{m}\mathrm{g}/\mathrm{g})$	158.73
	${\mathrm{k}}_2\left({\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}\right)$	0.1336
	${\mathrm{R}}^2$	0.98769
intra-particle diffusion model	${\mathrm{K}}_1 \left(\mathrm{m}\mathrm{g}/(\mathrm{g}·{\mathrm{m}\mathrm{i}\mathrm{n}}^{1/2}\right)$	0.1025
	${\mathrm{R}}^2$	0.62339

. The results indicated that the adsorption of Cr (VI) using magnetite-modified N-doped carbon adsorbent conformed to pseudo second-order kinetics, with a R² value of 0.98769, an adsorption rate of 0.13363 ${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$), and an equilibrium adsorption capacity of 158.73 mg/g.

Second-order pseudo adsorption kinetics indicates that the interaction is a chemical adsorption process, potentially involving electrostatic interactions between a magnetite-modified N-doped carbon adsorbent and Cr(VI) ion species as the adsorbate. At pH 6, the amine group ($-$N-H) on the magnetite-modified N-doped carbon can exist as a positively charged group ($-$N-H₂)⁺ [12], although the Cr(VI) ion exists as HCrO₄⁻ [3]. Therefore, the ammonium group ($-$N-H₂)⁺, which carries a positive charge from the adsorbent, is capable of participating in electrostatic chemical interactions with negatively charged Cr(VI) ions in the HCrO₄⁻ species [12].

4. Conclusions

Magnetite-modified N-doped carbon material has been successfully made from empty oil palm bunches and a Fe(III) precursor derived from FeCl₃ through freeze-drying and pyrolysis. The adsorption of Cr (VI) using magnetite-modified N-doped carbon adsorbent adheres to pseudo-second-order reaction kinetics. The R² value is 0.98769, indicating an adsorption rate of 0.1336 ${\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$ and an equilibrium adsorption capacity of 158.73 mg/g.