Granulated Activated Carbon as an Efficient Adsorbent for the Removal of Organic Matter from Water ^†

Abstract

This study examined the characteristics of granulated activated carbon (GAC) as an adsorbent for the removal of organic matter from the surface water of the Jala River. The adsorbent was characterized by XRD, FTIR, Raman spectroscopy, BET and SEM/EDS methods, while a detailed physicochemical characterization was performed for the water sample. The adsorption process was carried out under the following laboratory conditions: T(water) = 25 °C, individual doses of GAC off 1, 2 and 4 g/L, stirring speed of 200 rpm and time of 60 min. The research showed that GAC has good structural and textural morphological characteristics and that it can be successfully applied to remove organic matter from water (70.53%) using the lowest dose.

1. Introduction

Adsorbents are materials that have the ability to collect molecules of other substances (adsorbates) on their surface [1]. Depending on the type of bond between the adsorbent and the adsorbate, physical and chemical adsorption are distinguished, with the former being achieved through physical interactions such as Van der Waals forces [2] and the latter through chemical bonding [3]. The phenomenon of adsorption is widely used in various applications, such as water purification [4] and air purification [5], in the chemical [6] and pharmaceutical [7] industries, in catalysis [8] and in medicine [9]. The most common adsorbents are natural minerals [10], carbon materials [11] and synthetic materials [12]. Among the key characteristics considered for the selection of an appropriate adsorbent are a high specific surface area [13] and porosity [14], selectivity [15] and regenerability [16]. Compared to synthetic adsorbents, natural ones have advantages in terms of availability, low cost and environmental friendliness [17].

Carbon-based adsorbents provide benefits such as high stability and inexpensive precursors, despite their relatively low adsorption capacity and limited selectivity [18]. Activated carbon is one of the most commonly used adsorbents [19].

The basic forms of activated carbon are granulated activated carbon (GAC) and powdered activated carbon (PAC). Both types of activated carbon have a high surface area and porosity, but their particle size differs in such a way that GAC typically ranges from 0.2 to 5 mm in diameter [20], while PAC has much finer particles, usually less than 100 μm [21].

This study examined the characteristics of granulated activated carbon (GAC) as an adsorbent for the removal of organic matter from surface water.

2. Materials and Methods

In the experimental part of this research, the following materials were used:

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Granulated activated carbon (GAC) from coconut shell (NSF International, Germany), characterized by a developed porous structure, a satisfactory participation of micro and meso-pores and a large active surface, which provided good adsorption, chemisorption and catalytic characteristics;

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Water samples (WS) taken from the river “Jala” in the city area of Tuzla;

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Other chemicals and reagents of laboratory grade, which were required for the characterization of the GAC and water samples.

The following methods were used to characterize the granulated activated carbon (GAC): X-ray diffractometry (XRD), Fourier transform infrared spectrophotometry (FTIR), Raman spectrophotometry, the low-temperature nitrogen adsorption method (BET) and energy dispersive scanning electron microscopy by spectrophotometry (SEM/EDS). XRD was performed on a multipurpose Rigaku Ultima IV X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) (sample in horizontal position) of a parfocal Bragg–Brentano geometry using a D/teX Ultra 250 strip detector in the 1D standard mode with a CuKα1,2 (λ = 1.54178 Á) radiation source (U = 40 kV, I = 30 mA) with the Rigaku PDXL 2.0 software being implemented (with the ICDD PDF-2 2016 database). A low background silicon sample holder was used. XRD data were collected in the angular interval 2–50° 2θ, with a step of 0.05° and a time constant of 1.5 s per step.

Examination by the FTIR method included sample preparation (spraying in agate avan) and spectroscopic imaging. The tested sample of granulated activated carbon (GAC) was recorded on a Thermo Scientific Nicolet 6700 (Thermo Fisher Scientific, Waltham, MA, USA) infrared spectrophotometer. The range of wave numbers was 4000–500 cm⁻¹.

The micro-Raman spectra were recorded using XploRA ONE™ (Horiba Jobin Yvon, Paris, France) Raman microscope equipped with an Nd/YAG (Carl Zeiss Meditec, Jena, Germany) laser with an excitation wavelength (λ_exc) of 532 nm. The sample was placed on an X-Y motorized sample stage. The scattered light was analyzed using the spectrograph with a grating of 2400 lines/mm, with a resolution of 3 cm⁻¹. The spectra were recorded in the wavenumber interval from 4000 to 500 cm⁻¹.

The textural characteristics of the granulated activated carbon (GAC) were determined using an Anton Paar-NOVAtouch LX2 (Anton Paar GmbH, Graz, Austria) gas sorption analyzer with the following parameters: adsorbent nitrogen (N₂) and a temperature of −195.8 °C.

Scanning electron microscopy (SEM) was performed using a JEOL-JSM-6460LV (JEOL Ltd., Tokyo, Japan) electron microscope (Japan) at a resolution of 3–4 nm and a 500–3000 times magnification. The samples were sputtered with gold on a BAL-TEC SCD 005 device (Bal-Tec AG, Pfäffikon, Switzerland), with a current of 30 mA, from a distance of 50 mm for 80 s. Analysis of the chemical composition of the material by EDS was performed using an energy dispersive spectrometer with a Noran System Six 200 analyzer (Thermo Fisher Scientific, Waltham, MA, USA) (detection of elements Z ≥ 5, detection limit of ~0.1% m/m, resolution of 126 eV).

The adsorption properties of the granular activated carbon were additionally tested in the laboratory, by performing adsorption of organic matter from the water samples of the Jala River. The physicochemical analysis of the water before the adsorption treatment included the determination of pH and electrical conductivity by potentiometric methods, the content of organic matter, iron, manganese and chloride by volumetric methods and ammoniacal nitrate and nitrite nitrogen by spectrophotometric methods.

The adsorption procedure was carried out at a temperature ≈ 25 °C, by adding determined doses of GAC (0.5, 1.0 and 2.0 g/L) to glass beakers with 500 mL of sampled water, after which the suspensions were mixed using a magnetic stirrer at a specified speed (200 rpm) and time (60 min). After the adsorption treatment, the water samples were filtered on a blue filter strip, and then the content of organic matter was determined in the filtrate. The efficiency of removing organic matter from water (E_om) was calculated using the following relation:

E_om (%) = (OM_b − OM_a)/OM_b

(1)

where OM_b and OM_a are the contents of organic matter in the water before and after the adsorption.

3. Results and Discussion

Figure 1 shows the X-ray diffractogram of the examined sample of granulated activated carbon (GAC). The diffractogram of the sample shows two broad peaks in the angular interval ~20–30° 2θ and ~40–50° 2θ, characteristic of the amorphous nature of activated carbon.

The FTIR spectrum of the activated carbon, shown in Figure 2, is characterized by a broad vibrational band in the wavenumber region from 1500 to 500 cm⁻¹. The broad peak indicates the amorphous nature of activated carbon. This region contains bands characteristic for aromatic carbon–carbon stretching vibration (~1500 cm⁻¹) and C-O symmetric and asymmetric stretching vibration of the -C-O-C- ring (~1150 cm⁻¹) [22]. The sharp bands at 668 and 597 cm⁻¹ can be assigned to the out-of-plane C-H bending mode [23]. The broad region at 600–680 cm⁻¹ can be associated with the coupled vibrations of C-H bending in the carbon structure and physiosorbed CO₂ [24,25].

The Raman spectrum of the activated carbon is shown in Figure 3. The two intense peaks at 1338 cm⁻¹ and 1597 cm⁻¹ are peaks characteristic for carbon structures. The peak at 1597 cm⁻¹, denoted as the G peak, is characteristic for graphite’s structure (sp²-bonded carbon atoms in a hexagonal graphite lattice), while the peak at 1338 cm⁻¹ is denoted as the D peak and corresponds to a disordered graphite structure (presence of lattice defects, vacancies, disordered arrangement or impurities in the material). The measured intensity ratio of the discussed peaks I_D/I_G is 0.92 and is used to estimate disorder in the structure. A higher I_D/I_G ratio indicates greater disorder, while a lower ratio shows a higher ordering of a graphitic carbon structure. The low-intensity bands at 2670 cm⁻¹, 2920 cm⁻¹ and 3200 cm⁻¹ correspond to overtones 2D, D + G and 2G, respectively, originating from the disordering of the activated carbon structure [26,27].

Figure 4 shows the adsorption–desorption isotherm for a powder sample of granulated activated carbon (GAC), which is characterized by a H4 type of hysteresis loop, typical for materials with micropores in the shape of cracks and splits. The specific surface area of this sample is exceptionally high (941.6 m²/g). Taking into consideration the appearance of the obtained hysteresis, it can be suggested that the high surface area is the consequence of the micropores that are mainly present in this sample.

Figure 5 shows the distribution of the pore sizes for values < 10 nm, obtained from the desorption branch of the isotherm developed by Barret, Jouner and Halenda (BJH). According to the pore diameter distribution, the highest portion of pores is located in the micro domain (below 2 nm), with only one extremely low portion of mesopores between 3 and 5 nm. The average pore diameter is around 1 nm, and a total pore volume accounting for 0.47 cm³/g comprises the volume of pores below 10 nm, since the volume of the pores with a diameter up to 40 nm is negligible.

Figure 6 shows SEM micrographs of the tested granulated activated carbon (GAC) with different magnifications (500, 2000 and 3000 times). Visible macropores and cracks in image (a) suggest a highly porous material. In image (b), the pore walls appear more defined, and there are signs of small particles or debris, possibly related to activation or adsorption processes, while image (c) emphasizes the presence of micro- and mesopores, crucial for adsorption efficiency.

The micro-elemental (EDS) analysis (Figure 7), given in weight and atomic percentages, shows the dominant presence of carbon (C), 96.68% wt, i.e., 99.46% at, in the tested sample of granulated activated carbon (GAC).

The obtained results of the physicochemical analysis of raw water (

Table 1. Results of physicochemical characteristics of raw water (PV).

Parameter	Value
pH	7.6
Electrical conductivity (µS/cm)	465
Organic matter as KMnO4 consumption (mg/L)	118
Ammoniacal nitrogen (mg/L)	2.28
Nitrate nitrogen (mg/L)	0.42
Nitrite nitrogen (mg/L)	0.102
Iron (mg/L)	0.00
Manganese (mg/L)	0.02
Chlorides (mg/L)	41.00

) show slight deviations in quality parameters compared to the earlier analysis [28], but their values are still typical for urban surface waters, especially in terms of pH [29] and electrical conductivity [30].

Table 2. Effect of adsorbent dose on organic matter removal.

Dose of Adsorbent (g/L)	Consumption of KMnO4 (mL)	Content of Organic Matter in Treated Water (mg/L)	Organic Matter Removal Efficiency (%)
1	5.5	34.77	70.53
2	8.0	50.57	57.14
4	12.5	79.02	33.03

shows the results of adsorption of organic matter from the water with different doses of granular activated carbon, with the highest adsorption efficiency achieved at a dose of 1 g/L (70.33%), which is half the dose of GAC previously used to remove organic matter from water under the same conditions [31]. However, although the aforementioned reference work reported that the adsorption efficiency increased with increasing the adsorbent dose, in the present research, this was not the case, i.e., increasing the dose from 1 to 4 mg/L was accompanied by a decreasing trend in efficiency, from 70.53 to 33.03%. This can be explained by the accumulation of GAC particles in the water as a consequence of higher doses, which reduces the available surface area and their adsorption efficiency.

4. Conclusions

In this paper, the characterization of granulated coconut shell activated carbon was investigated using modern methods of analysis: XRD, FTIR, Raman spectrophotometry, BET and SEM/EDS. The tests using XRD and FTIR methods showed that the GAC has an amorphous structure, and the results of the BET method showed that the tested material has a large specific surface area (941.6 m²/g), which is a consequence of the presence of mostly micropores (below 2 nm) and a slightly smaller proportion of mesopores (from 3–5 nm).

The average pore diameter is about 1 nm, and the total pore volume of 0.47 cm³/g includes the pore volume below 10 nm, since the pore volume up to 40 nm in diameter is negligible. The hysteresis loop is of type H4, which is typical for materials with micropores in the form of cracks and splits. The morphological characteristics were determined by the SEM/EDS method, and from the micrographs of the GAC recorded at different magnifications, it was possible to see the forms of micropores, mesopores and cracks, which are indicative of good adsorption characteristics. Micro-elemental (EDS) analysis showed that there is a dominant proportion of carbon (96.68% wt, i.e., 99.46% at.) in the GAC sample. Under the given materials and research conditions, the optimal dosage of 1 g/L GAC for organic matter removal was determined, achieving an efficiency of 70–71%. Increasing the adsorbent dosage does not improve the adsorption efficiency and even leads to a decrease in performance. Therefore, the optimal dosage must be carefully determined to avoid unnecessary use of adsorbent while maximizing removal efficiency. Further tests should include removal of not only organic compounds but also heavy metals and other impurities in surface waters, as well as in various wastewaters.