Efficiency of KOH-activated carbon for removal of heavy metal pollution from water of carbon for removal of

The study to reduce heavy metals pollution from water using the KOH-activated carbon was studied the factors affecting the adsorption capacities of Cu(II) and Pb(II), in particular, initial metals 12 concentration, pH of the solution, and contact time in static conditions. Using X-ray photoelectron spectroscopy and FTIR analysis to determine the elemental composition and surface functional groups of the activated carbon surface, the presence of oxygen-related functional groups was 15 observed. The maximum adsorption capacities were 135.8 mg g -1 and 31.0 mg g -1 for removal of lead and copper solutions with the initial concentration of 300 mg L -1 of metal at 318 K, respectively. The removal percentage was found to be higher for Pb (II) when compared with Cu (II). 18


INTRODUCTION
Most heavy metals pollutants are released from metallurgical, oil refining, petrochemical, fertilizers/pesticides production [1], electrochemical [2], leather and ceramic industries, and mining, particularly during gold mining activity of extracted ore [3]. These 24 contaminants are a major cause of surface and groundwater pollution. The heavy metal pollutants are cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni), lead (Pb), arsenic (As), copper (Cu), and zinc (Zn) are commonly associated with toxicity problems. Mining and metallurgical industries 27 have been discharged toxic metals into the air, water and soil [4]. High levels of lead toxicity can lead to cognitive disorders, behavioral disorders, kidney damage and anemia [5]. Copper is relatively less toxic than lead, but excessive copper concentrations can lead to weakness, lethargy 30 and anorexia, as well as damage to the gas-trointestinal tract [6]. In Mongolia, several studies have been investigated heavy metal pollution of groundwater, stream and soil around the mining industrial area, such as copper-molybdenum and gold mining factors [7, 33 8, 9]. Although heavy metals are often highly toxic, the recycling of some rare precious metals from contaminated waters is also a rationale for saving natural resources and protecting the environment [10]. At the same time, the removal of heavy metal pollutants from contaminated soil and natural 36 water is an important task to prevent the ecosystem from severe environmental damage. There are several methods for the removal of heavy metal pollutants from water media, in particular biological, chemical, and physical methods [11]. However, all of them have advantages and 39 disadvantages, depending on their cost and disposal problems. Nowadays many techniques open up to remove these pollutants from water, such as chemical oxidation, coagulation, membrane separation, electrochemical and anaerobic microbial degradation, ion exchange, irradiation and 42 adsorption. Among all of the methods adsorption has been allowed due to its cheapness and high quality of the treated wastewaters by well designated adsorption processes. Adsorption by activated carbon method is the most convenient method for regeneration after the removal process of 45 pollutants. Its main advantage is the low operation cost of removal processes [12]. Furthermore, activated carbon is a well-known and popular adsorbent that has high porosity and a large surface area compared with other sorbent materials. Various pores with large surface area are dominant in 48 removing impurity molecules. The adsorption method used by activated carbon is an integral part of alternative water purification methods, which allows for more economical cleanup opportunities. Because activated carbon is 51 dense and resistant to chemical interactions, it is structurally less damaged when reused. Therefore, this study to conduct on the adsorption and separation of heavy metal ions from the aquatic media by natural coal derived activated carbons.

Materials:
The activated carbon (PMAC3) was prepared from semi-anthracite of the Tukhum deposit is located in East-Gobi of Mongolia by chemical activation with the mass ratio of activation 57 agent (KOH) of 3:1 dry mixed directly in our previous study. Chemical activation was performed up to 400 o C with a heating rate of 5 o C/min for 2 h to remove volatile compounds and the temperature continuously increased to 750 o C and the sample powders were activated at 750 o C for 2 h [13]. 60 The proximate and ultimate analyses of the initial sample were performed used with the Elemental analyzer (Flash 2000) are shown in Table 1. 25 mL of heavy metal solutions with different initial concentrations (5 -300 mg/L) in static conditions. The pH of the solution was adjusted with 1 mol/L HNO3 or NaOH. The flasks with the sample were placed in an automatic shaker at a constant temperature (298, 308, and 318 K) and 72 agitation speed of 150 rpm for 120 min. The samples were filtered after the adsorption process, and the residual concentration of metal ions was analyzed by UV-spectrophotometry and ICP-OES (Inductively coupled plasma -optical emission spectrometry). 75 The physical properties of activated carbons, such as the specific surface area, total pore volume, and pore size distributions were determined using N2-adsorption data. The surface chemical property of activated carbons was analyzed by X-ray photoelectron spectroscopy (Multilab 2000, 78 Thermoelectron Corp., England). The removal of heavy metals using the activated carbons was carried out in static conditions and the factors affecting the adsorption capacities of KOH-activated carbon (PMAC3) for copper and 81 lead were investigated in detail. The adsorbed amount of metal ions qt (mg/g) were calculated as follows:

Mongolian Journal of Chemistry
The heavy metals removal percentages were calculated as follows: Where, 0 and (mg/L) are the initial and equilibrium concentration of the metal ion. V (L) is a volume of the metal solution, and m (g) is the mass of activated carbon [14,15]. All adsorption data were optimized with the Langmuir, Freundlich, and Sips models to determine 90 the effects of temperature and initial concentration.  The physical characteristics of activated carbon were estimated by the nitrogen adsorption and 99 desorption data at 77 K using an adsorption analyzer (Micrometrics ASAP 2020, USA). The specific surface area (SBET) of activated carbon was calculated by the Brunauer-Emmett-Teller (BET) method. The pore width was determined using the Density functional theory (DFT). The total pore 102 volume (Vtotal), the micropore volume (Vmicro), and the mesopore volume (Vmeso) were estimated using Horvath & Kawazoe (HK) and the Barrett-Joyner-Halenda (BJH) methods. The average pore size (DAPD) was calculated by expression: (4Vt/SBET) using the BET surface area (SBET) and total 105 pore volume (Vt) from the BET [13] results. The nitrogen adsorption and desorption isotherm curve and DFT pore size distribution of PMAC3 at 77 K are shown in Fig. 1. The nitrogen adsorption isotherm shows typical three steps with the 108 increase in relative pressure. The first step is a steeply increasing region at low relative pressures less than 0.02, which stands for the adsorption or condensation in small micropores. Then the adsorption amount slowly increases with relative pressure without any notable hysteresis which 111 represents the progressive filling of large micropores. Finally, the adsorption amount increases abruptly at near the saturation pressure of nitrogen because of active capillary condensation. The activated carbon (PMAC3) prepared in the study shows the type I isotherm according to the IUPAC 114 classification in Fig. 1(a). The activated carbon contains mostly micropores that have average pore diameters of 1.97 nm as shown in Table 2 and Fig. 1(b). For the activated carbon sample average DFT pore width was in the 120 range of 0.79 nm [13]. When KOH -activation was used, small micropores seem to be well developed during activation since the chemical agent was more finely distributed inside anthracite particles than the simple 123 physical mixing of KOH powder. PMAC3 sample prepared from semi-anthracite is highly porous with a wide range of micropores. As shown in Fig. 1(b), PMAC3 has properly developed mesopores of 1 -2 nm. 126 The FTIR spectra of the RA and PMAC3 are shown in Fig. 2. The sample PMAC3 shows O-H stretching between 3300 -3500 cm -1 like RA. In addition to this, C-H aliphatic stretching peak intensity is significantly increased at 2850 -3000 cm -1 and 1300 -1450 cm -1 . Similarly, C-O-C 129 (stretching) phenolic deformation is found between 1000 -1100 cm -1 with increased peak intensity.
Besides there are many small peaks of C=O, C-O is resolved to new lowest small peaks of carbonyl and carboxylic groups at 1639 -1750 cm -1 , and C-H aliphatic bending peaks intensity is slightly 132 increased at 1300 -1450 cm -1 . It can be observed that the sample PMAC3 has much more contents of aliphatic C-H, carboxylic acid, and carbonyl groups (-C=O-). The chemical activation is responsible for the reasonably more oxygen functional groups in PMAC3. The surface hydrophilic nature of a material increases with surface oxygen bonded functional groups [16][17][18][19]. It can be suggested here that the sample PMAC3 has a slightly 138 hydrophilic nature compared to that of RA according to this FTIR analysis. This suggestion was also confirmed by XPS analysis as discussed below. The surface chemical properties of activated carbon (PMAC3) and raw anthracite (RA) samples 141 were characterized by XPS analyses (Fig. 3 and Table 3). Fig. 3 shows the C1 and O1 XPS spectral peaks of two samples, RA and PMAC3 for comparison. The surface elemental and high resolution C1s and O1s XPS spectra results of the samples are listed 144 in Table 3, respectively.     The effect of pH on the Cu (II), and Pb (II) ions adsorption were studied at the temperature of 298 K, agitation speed of 150 rpm for the adsorption contact time required to reach the equilibrium was 120 min from metal ion initial concentration of 100 mg/L. In Fig. 4, the highest adsorbed amount 180 of Cu (II), and Pb (II) ions were obtained at pH 8.0 and the lowest adsorbed amount was illustrated at pH 2.0. At high pH values, the activated carbon surface will convert to a more negative charge compared with low pH values. It might provide metal binding sites for ion exchange reaction and 183 electrostatic interaction that are favorable to adsorb cationic species and high adsorption capacity. In this way, carboxylic and carbonyl groups could provide additional ion binding capacity at pH 8.0.

Effect of adsorption temperature:
The effect of adsorption temperature for adsorption of Cu (II), 186 and Pb (II) ions onto PMAC3 were studied at the temperature of 298, 308 and 318 K, agitation speed of 150 rpm for the adsorption contact time required to reach the equilibrium was 120 min from the metal concentration of 300 mg/L. 189 The adsorption isotherms of the copper and lead ions on the activated carbon sample are presented in Fig. 5. The adsorption isotherm data were optimized with the Langmuir, Freundlich and Sips isotherm models [21] due to well explain the adsorption mechanism of heavy metals. 192 The Langmuir isotherm expression is: Where, Ce is the equilibrium concentration of metal ion (mg/L), qe is the adsorption capacity (mg/g), 195 KL (L/mg) is the Langmuir isotherm constant, qm signifies the theoretical monolayer capacity.
The key characteristics of the Langmuir isotherm have been described by the equilibrium constant (KL), which is defined as: Where, C0 is the initial concentration of metal ion, and b is the Langmuir constant. The equilibrium constant (KL) indicates the nature of adsorption [22] as: 201 KL > 1 (unfavorable), 0<KL < 1 (favorable), KL = 0 (irreversible), 204 KL = 1 (linear). The Freundlich isotherm expression is: The Sips isotherms is given the following a general form: where KF (L/g) is the Freundlich isotherm constant, b is the Sips isotherm constant, 1/n 210 (dimensionless) is the heterogeneity factor [23]. All isotherm models parameters and the correlation coefficient (R 2 ) are listed in Table 4. For the copper ions, adsorption data favorable isotherm was the Langmuir isotherm. Because the Langmuir 216 isotherm constant (KL) is to be below 1.0. This result indicated the homogeneous adsorption of the activated carbon surfaces. From the Table 4, comparing the values of qm and R 2 , the Sips isotherm predicted the lead ions adsorption data better than the Freundlich and the Langmuir isotherms. From the Sips isotherm 222 model [20], the values of 1/n attained were <1 for lead ions adsorption data, which suggests favorable adsorption behavior of lead ions on the activated carbon. The lead ions adsorption on the PMAC3 was found to follow the Sips isotherm model since the correlation coefficient, R 2 values 225 obtained to best fit at all temperatures, in comparison to the Langmuir and the Freundlich isotherms where the values of R 2 were 0.99. The result suggested that heterogeneous adsorption for the lead adsorption data. 228 The results showed that the most favorable isotherm was the Langmuir isotherm for Cu (II) ion adsorption onto PMAC3, but for adsorption of Pb (II) ions was the Sips isotherm. The maximum adsorption capacity was 31.9 mg/g for Cu (II) ion from the Langmuir isotherm model and 134.9 231 mg/g for Pb (II) from the Sips isotherm model with 300 mg/L of initial concentration of metal solution at 318K as shown in Table 4.  The adsorption isotherms are illustrated in Fig. 6, which shows that the adsorbed amount of Cu(II) and Pb(II) ions increased with adsorption temperature. The increase in the adsorption amount of metal ions at high temperatures is due to the negative surface charge of the activated carbons and 237 the adsorption process is endothermic. In other words, the results indicate that the ability of carboxyl groups involved in metal cations adsorption amount increased with the proton exchange adsorption mechanism. 240 In Table 5, the results show that the removal efficiencies of PMAC3 for adsorption of Cu(II) and Pb(II) were 20.9% and 90.0% when the initial concentration of the metal solution of 300 mg/L at 318 K. 243 Table 5. The heavy metals adsorption capacity and removal efficiency percentage (at 300 mg/L of initial concentration of metals and 318K.

Adsorbent
Adsorbate Adsorption capacity Efficiency mg/g (%) PMAC3 Cu(II) 31.9 20.9 Pb(II) 134.9 90.0 Kinetic studies: The kinetic data were fitted with the pseudo-first-order and pseudo-second-order 246 models [24][25] for adsorption of Cu(II) and Pb(II) ions onto PMAC3. The correlation coefficient ( 2 ) is a fitting degree between experimental data and the model-predicted values. It can be assessed by which represents that the applied model is more feasible when this coefficient is close to or equal 249 to 1. In this work, the adsorption kinetic data were predicted with the Lagergren pseudo-first-order and pseudo-second-order models, which have been known as reliable models for a long time to describe 252 the adsorption rate in batch adsorbers. The integrated linear form of the pseudo-first-order model can be expressed as follows [1,24]: where, is the equilibrium adsorption capacity [mg/g], is the adsorption capacity at time [mg/g], and K1 is the rate constant of the pseudo-first order adsorption (min -1 ). Straight line plot of ln( − ) versus time confirms that the adsorption is governed by the first-order kinetics. 258 The pseudo-second-order model [25] can be expressed by a linear equation given below: Where, K2 (g/mg min) is the rate constant of the second-order adsorption.
The nonlinear forms of the above kinetic equations, which can be used to predict the adsorption 261 equilibrium, as shown in equations 9 and 10: The pseudo-first-order and pseudo-second-order kinetic models were used to investigate the kinetics as an important parameter for the adsorption mechanism and kinetics of the adsorption processes. 273 The adsorption amounts of Cu(II) and Pb(II) ions on PMAC3 were measured in terms of contact time. Fig.7 shows experimental data and predicted kinetic data from the pseudo-first-order ( Fig.7(a)) and pseudo-second-order ( Fig. 7(b)) models when 25 mL of 300 mg/L of metals was contacted with 276 0.2 g of the PMAC3 at 298 K.   The parameters of kinetic models are listed in Table 6. The correlation coefficients (R 2 ) for the pseudo-second-order model are slightly low, and the calculated qe values (qe, cal) from the pseudo-282 second-order model do not agree with the experimental data (qe, exp), suggesting that the Cu(II) and Pb(II) ions adsorption on the PMAC3 cannot be explained a pseudo-second-order model. However, the values of the correlation coefficient of the pseudo-first-order model (R 2 = 0.99) were 285 much higher than pseudo-second-order model. On the other hand, the adsorption kinetics follows the pseudo-first-order equation, as shown in Fig. 7. Therefore, the adsorption kinetics were not suggesting a chemisorption process.

CONCLUSIONS
The KOH-activated carbon derived from semi-anthracite (PMAC3) for lead ions shows significantly higher adsorption capacities compared with copper ions adsorption. The lead removal efficiency of 291 activated carbons was up to 90% within the 240 min, where the concentration of contact solution was 300 mg/L at 318 K. In fitting adsorption equilibrium data of heavy metal ions on the PMAC3, three isotherm equations, the Langmuir, Freundlich and Sips, were employed. Among them, the 294 Langmuir isotherm was the most favorable isotherm for the copper ions adsorption data. This result indicated the homogeneous adsorption of the activated carbon surfaces. But for lead ions adsorption onto PMAC3 was found to follow the Sips isotherm model, the result suggested heterogeneous 297 adsorption. The copper and lead ions adsorption kinetic data of the PMAC3 were follow the pseudo-first-order equation, it suggesting that was not chemisorption process.