The objective of this review paper is to present the findings from the literature and highlight the advancements of the newly prepared materials for more efficient arsenic removal by activated carbons and their modified alternatives. Although several reviews on arsenic removal have been published in the past, only a few of them have focused on the removal of arsenic by activated carbons and their modified alternatives. To the best of our knowledge, only one review paper [ 22 ] was found in our literature search that focused on arsenic removal by activated carbons. Therefore, this review is novel in the sense that it summarizes the findings from the last 5 years, during which a great advancement in the preparations, modifications, characterizations and applications has been achieved.
In the present review paper, several studies using activated carbon for the removal of arsenic ions are reviewed and discussed while providing an insight into the synthesis of these materials and their characterization. Moreover, the adsorption evaluation of these synthesized activated carbons is also reviewed with a special emphasis on the kinetics of the process, isotherms and the effect of pH and temperature. Finally, a section concerning regeneration and the use of some synthesized activated carbons for many cycles is provided and discussed.
Vitela-Rodriguez and Rangel-Mendez [ 19 ] tested the use of several activated carbons that were modified with hydroxide nanoparticles for the removal of arsenic content from natural waters that were taken from a well in Mexico. Their study aimed to determine the effect of temperature and pH on the adsorption process; in addition, the surface morphology of the modified activated carbon was evaluated alongside the kinetics and thermodynamics of the experiment [ 19 ]. Muñiz et al. [ 20 ] focused their investigations on using commercial activated carbon for the purpose of removing arsenic from natural well water from the state of Chihuahua; they used both raw and modified activated carbons in order to determine which materials exhibited the highest arsenic uptake [ 20 ]. Another study by Chen et al. focused on increasing the impregnation of iron into the activated carbon [ 21 ].
Activated carbon is one of the most common materials used in adsorption; it is used generally in water purification and can be provided either naturally or synthetically. In addition, activated carbon is implemented for adsorption purposes as a natural material, or it can go through treatment for surface modification. The reason activated carbon is mainly used in adsorption and for cleanup purposes in general is due to the many advantages that the activated carbon provides that could enhance the removal efficiency of heavy metals or other ions, and these advantages include a surface that is highly reactive and highly irregular alongside a high porosity and a large surface area. All these mentioned qualities are sought-after when dealing with an adsorption process, which suggests activated carbon to be an exceptional material candidate for the removal of ions from water sources by adsorption. The use of activated carbon has been widely studied in the literature for the removal of several ions over the years, but recently, the potential of activated carbon to provide efficient removal for arsenic has attracted great interest, and many studies in the recent years have been conducted to investigate the behavior of activated carbon for arsenic removal.
A major process that has been used for more than a century for water purification is adsorption. Therefore, the development and application of new adsorbents has been studied intensively over the past years. The performance of any adsorbent depends strongly on the chemical form of the adsorbate that is under investigation for removal. Therefore, it is essential to understand the behavior of arsenic compounds in aqueous solutions and their structure in order to provide the best adsorbent for the efficient removal of arsenic from waters. There are plenty of materials from several sources that have been studied as adsorbents for the removal of different heavy metals from water sources or wastewaters such as natural materials, wastes and synthetic adsorbents, and all of those can be used in both treated and untreated forms. The main focus on adsorption for cleanup procedures is related to the advantages that are provided by the adsorption itself, and this includes the simplicity of implementation and operation, the low cost associated with the production process, minimal waste and sludge production and most importantly the possibility of regeneration or reuse for many cycles [ 18 ].
It has been well demonstrated that, in Europe and other developed countries, almost all arsenic violations are observed in small communities with a population of fewer than 1000 people. Large drinking water plants in Northern and Central Europe either use alternative arsenic-free water resources or they apply conventional high-tech removal methods such as coagulation/filtration, activated alumina and ion exchange, Granular Ferric Hydroxide (GFH) or Bayoxide (E33) [ 3 ]. Smaller towns, communities and individual users in rural areas often rely on local water resources, and removal methods developed for large plants are not applicable because of the high operational and capital costs. In addition, these removal units are often too complicated to operate, or their use is limited by the local water composition. In Hungary, Romania and Northern Greece, the population in the affected regions uses groundwater usually without any treatment, or treatment is based only on simple aeration and filtration, which is usually not sufficient for the efficient removal of these hazardous substances. Consequently, small drinking water systems face the difficult challenge of providing a safe and sufficient supply of water at a reasonable cost. In Bangladesh and West Bengal, a number of simple treatments has been developed for use (e.g., oxidation of As (III) with chemical oxidants or with sunlight followed by precipitation with naturally present or added iron and aluminum salts, removal with zerovalent iron and sorption in prefabricated filters with iron and aluminum oxide sorbents). However, there are still open issues regarding the applicability of these technologies as they present several limitations, especially when iron and manganese are not present, or they are present at low concentrations.
Arsenic is found in waters mainly in its inorganic forms: trivalent, As (III) and pentavalent arsenic, As (V), known as arsenite and arsenate, respectively [ 14 ]. Arsenite is 10 times more toxic than arsenate [ 15 16 ]. Figure 1 shows the speciation of As (III) and As (V) as function of pH. Arsenic is considered to be highly toxic and can have major health impacts if humans are exposed for long periods above certain concentration limits. For instance, exposure to arsenic by the consumption of drinking water for a long time is related to a chronic illness that is linked to the development of cancer. Moreover, the exposure of pregnant women to arsenic has caused severe problems regarding fetal development through malfunctions and has led eventually to abortion [ 17 ]. Therefore, arsenic removal from drinking water sources is a matter of great concern for the world population.
Arsenic concentrations found in groundwaters range from less than 0.5 μg/L up to 50 mg/L [ 12 ], with some findings stating that numerous water sources contain arsenic concentrations more than 50 μg/L [ 12 13 ]. There are many factors that are responsible for the presence of arsenic in groundwater, and these factors include the characteristics of the aquifer itself, organic content and the grain size of the sediments in the aquifer alongside other processes such as ion exchange and adsorption–desorption cycles. Between all the water sources previously mentioned, arsenic can naturally occur in high concentrations and in a wide range in groundwater, and this is due to the interactions between the water and the rocks, which favor the mobilization of arsenic, leading to elevated arsenic concentrations [ 12 ]. In addition, human activities such as mining can release to the environment much higher arsenic quantities and cause heavier pollution than naturally occurring arsenic, thus creating a larger issue that needs to be resolved [ 3 11 ].
Most of the arsenic-related problems for humans and the environment are due to geogenic processes, such as reductive dissolution. However, anthropogenic activities such as mining, the combustion of fossil fuels or even the use of arsenic in agriculture (found within pesticides, for example) lead to the additional input of arsenic compounds in the environment, mainly in water bodies [ 11 ].
Taking into consideration the health effects of arsenic for humans, the risk of its presence in drinking water and the way in which it is being treated, the WHO established a guideline value of arsenic in drinking water of 10 μg/L in 1993, which is the limit of arsenic in drinking water in the European Union and also in the United States of America [ 6 7 ]. However, many countries still accept a maximum arsenic contaminant level in their drinking water at the value of 50 μg/L, such as Bangladesh [ 6 8 ], India [ 9 ] and Cambodia [ 10 ].
Worldwide, more than 750 million people have no access to safe drinking water [ 1 ], and more than 250 million inhabitants are regularly exposed to water contaminated with arsenic [ 2 3 ]. Moreover, according to recent estimations, more arsenic-contaminated areas are expected to be discovered, such as recently in the case of Pakistan [ 4 ]. Furthermore, in Europe, there are many areas affected by arsenic, impacting a population of about 1.5 million people [ 5 ].
To conduct this review, the following methodology was followed. First of all, we identified the subject, which was the use of activated carbons and their modifications for arsenic removal by adsorption. This was based on the fact that, during recent years, only one review paper was published focusing on arsenic removal by activated carbons, despite the numerous papers that have since been published. Then, we performed a literature search in databases such as Scopus, Pubmed and Sciencedirect and found the most representative articles, which in our view provided new advancements in the field. For this, we used among others the following keywords: arsenic, activated carbons, waste, wastewaters, modification, adsorption, oxidation, arsenic speciation and metal impregnation. The next step was to read carefully the selected papers and note down the most important information that we wanted to highlight in the review, comprising the methods of synthesis, characterization, application, models of adsorption, adsorption efficiencies and regeneration possibilities and efficiencies.
There are many research works that have focused on the synthesis of adsorbents through the modification of activated carbon for arsenic removal purposes in the literature [ 14 22 ], and this solidifies the belief in the role of adsorption in general, and specifically activated carbons as potential and suitable candidates for the removal of arsenic content from wastewaters and water sources. The next section focuses on the different characterization techniques that have been adapted for some of the reviewed activated carbons in order to get a better understanding of the main structure of activated carbons that leads towards arsenic removal.
Fe-loaded activated carbon was synthesized for arsenic removal by Liu et al. by using a waste biomass of sawdust. The first step was to dry the sawdust for 24 h at 105 °C, after which the sawdust was subjected to air cooling followed by grinding in order to reach dimensions less than or equal to 80 meshes. In total, 20 mL of 1M FeClwas mixed with 20 g of the sample sawdust followed by adding 10 mL of 50% sulfuric acid. The obtained mixture was then ultrasonicated for a period of 2 h and then left to age for 12 h at a temperature of 60 °C. Centrifugal separation was then used in order to obtain the residue, which was then pyrolyzed by heating at 600 °C for 115 min at a rate of 30 mL/min of nitrogen. After the mixture was left to be cooled inside the furnace, the recovered product, which was now in a solid form, was washed with de-ionized water until it reached neutrality, and then it was dried for 24 h at 60 °C in order to obtain the final synthesized modified activated carbon [ 26 ].
An activated carbon impregnated by zirconium was synthesized by Velazquez-Jimenez et al. for the removal of As (V), where 10 mL of ZrOClO.8HO was added to 0.1 g of commercial activated carbon at a 7% weight per volume concentration of zirconium ions, and the mixture was stirred for a duration of two days in order to achieve the impregnation of the activated carbon by zirconium. After being washed by deionized water, the activated carbon was then added to 10 mL of oxalic acid at a concentration of weight per volume of 7%, and the mixture was stirred for 24 h. The obtained compound was then subjected to filtration, after which it was rinsed and finally dried for 12 h at a temperature of 60 °C to finalize the synthesis of the commercial activated carbon impregnated with zirconium [ 25 ].
Kalaruban et al. studied the adsorption of arsenic for water purification by using granular activated carbon functionalized by iron oxides. The activated carbon was sieved into small particles ranging between 300 and 600 μm in order to reduce the variety of the experiment as believed by the authors. In total, 20 g of the granular activated carbon was mixed in an Erlenmeyer flask with 500 mL of FeClof a concentration of 0.1M while adjusting the pH of the solution between 4.2 and 4.5 [ 18 ]. A flat shaker was then used in order to agitate the flask for about 24 h, at a speed of 150 rpm and a temperature nearing 25 °C. The next step was to filter the mixture and then wash it with deionized water several times in order to remove any remaining iron salts or other precipitates residing on the external surface of the iron granular activated carbon (Fe-GAC), after which it was dried for one full day at room temperature [ 24 ].
In order to create doped straw-activated carbon with iron hydroxide and manganese dioxide, 100 mL of 68% nitric acid was mixed with 5 g of the straw-activated carbon for 1 h as an oxidation step at room temperature. Then, the mixture was thoroughly washed by using distilled water to remove acid that remained in the mix. In total, 40 mL of hydrochloric acid at a concentration of 3 mol/L and 150 mL at a concentration of 0.05 mol/L of FeClwere then mixed with the previous solution at room temperature and stirred for about 22 h; then, the whole mixture was heated for 6 h and 46 min at 100 °C. After filtering and washing the prepared iron hydroxide-doped straw-activated carbon, drying was implemented at a temperature of 80 °C for a full day, and then 5 g was taken from the synthesized compound and mixed with 5 g of MnSO.HO, where they were stirred at a temperature of 80 °C for 15 min by using 150 mL of distilled water with a water cooler condenser. On a separate matter, 5 g of MnSO.HO and 10 g of KOH were mixed with 150 mL of deionized water and then added to the synthesized mixture, where reflux was processed for 1 h and 30 min. As a final step, the product that was obtained was filtered and then washed until it became neutral and finally dried at a temperature of 50 °C in order to finalize the product and obtain a 90% yield of iron hydroxide/manganese dioxide-doped straw-activated carbon [ 23 ].
Activated carbons applied for arsenic removal are in all cases modified by being functionalized with active groups, which enhance the removal of arsenic either by direct sorption or firstly by oxidation of As (III) and then the sorption of the resulting As(V). The major groups that have been used to modify AC are iron and manganese oxides and combinations of them, zirconium and other oxides, as shown in Table 1 . A study by Xiong et al. [ 23 ] investigated the use of modified activated carbon by manganese dioxide and iron hydroxide for the adsorption of As (III) ions, where an appropriate amount of straw was ground into powder and then heated in a furnace at a rate of 10 °C/min until the temperature reached 350 °C and for a duration of 2 h. After that, the powder was left to cool at room temperature, and then 50 mL of deionized water and 18 g of potassium hydroxide were added and mixed for half an hour, after which the mixture was dried for 2.5 h at a temperature of 80 °C. The mixture containing the straw was then placed into a tube furnace for heating for one hour at a rate of 15 °C/min until it reached 800 °C, after which the powder was cooled again at room temperature. The straw-activated carbon was finally obtained after the powder was mixed with 0.5 L of 5% hydrochloric acid for half an hour and then washed and dried. The obtained yield of straw-activated carbon that was recorded by Xiong et al. reached 93% [ 23 ].
Xiong et al. [ 23 ] have extensively studied the BET surface area of the different compounds that they have synthesized with straw-activated carbon as their base. These compounds were raw straw, straw-activated carbon both with and without potassium hydroxide treatment, straw-activated carbon with iron doping and finally straw-activated carbon with iron hydroxide and manganese dioxide doping. The authors found that the specific BET surface area of straw was equal to 3.98 m/g, and the largest surface area calculated was 1413.45 m/g for Fe-SAc (iron-doped straw-activated carbon). The other observed surface areas were 130.07 m/g, 1360.00 m/g and 507.5 m/g for straw-activated carbon without KOH treatment, straw-activated carbon and activated carbon doped with iron and manganese, respectively. The synthesis of an activated carbon greatly increased the surface area of straw, and further doping with iron led to an additional increase of the surface area, which is a very important parameter in adsorption, since increasing the adsorbent’s surface area typically enhances the adsorption efficiency of the materials regarding arsenic removal.
3/g, in comparison to the pore volume of 0.2498 cm3/g for straw-activated carbon without KOH treatment. It was also noticed that when adding manganese dioxide MnO2 for doping into the iron-doped straw-activated carbon, the surface area experienced a sharp decrease, and this was attributed to MnO2 residing within the pore canal of the activated carbon, which adversely affected the surface area [The authors observed also that activated carbon with KOH treatment had a larger surface area than straw-activated carbon without KOH treatment, and this was attributed to the fact that the treatment of activated carbon with potassium hydroxide led to the creation of new pore channels within the activated carbons, which led to the increase in the total surface area. This observation is consistent with the obtained results since the activated carbon that was subjected to KOH treatment attained a pore volume of 0.8081 cm/g, in comparison to the pore volume of 0.2498 cm/g for straw-activated carbon without KOH treatment. It was also noticed that when adding manganese dioxide MnOfor doping into the iron-doped straw-activated carbon, the surface area experienced a sharp decrease, and this was attributed to MnOresiding within the pore canal of the activated carbon, which adversely affected the surface area [ 23 ].
SEM analysis was conducted in order to investigate the structure of the different synthesized compounds, as shown in Figure 2 . The SEM imaging of straw-activated carbon showed a large porous structure, and the SEM images that were conducted for iron-doped straw-activated carbon resulted in a similar and slightly better organized and porous structure according to Figure 2 , which agrees with the calculated surface areas of both compounds. This shows that iron resided on the surface of the activated carbon and not within the pore channels. However, that is not the case for manganese and iron-doped activated carbon, as SEM imaging presents an irregular shape while taking into consideration that the surface area is extremely reduced compared to that of iron-doped activated carbon; this is again attributed to the fact that manganese dioxide landed within the pore structure of the activated carbon [ 23 ].
The authors also conducted an XRD analysis of the different synthesized activated carbons, as shown in Figure 3 . The XRD analysis of the straw-activated carbon showed a peak at 22°, which is typical of carbon, and another peak at 36°, which is attributable to the Fe-O bond present in the iron-doped straw-activated carbon In contrast, a peak for Mn-O in manganese/iron-doped straw-activated carbon was not observed in XRD analysis, suggesting that manganese dioxide had a structural change on the surface of iron-doped activated carbon [ 30 31 ]. Moreover, the typical sharp peak in manganese/iron-doped straw-activated carbon is much smaller than the other peaks of the aforementioned compounds, which shows that the presence of manganese dioxide decreased the peaks of the elements that were presented previously on the surface of the activated carbon by loading on this surface [ 23 ].
−1, 1556 cm−1, 1415 cm−1 and 1086 cm−1 respectively. The observed peaks were nearly the same for iron-doped straw-activated carbon with the addition of a new peak at 704 cm−1 which was ascribed to Fe-O stretching [−1, 520 cm−1 and 450 cm−1 did not exist in the graph. This observation is in agreement with the XRD analysis, proving that a change in the structure of manganese dioxide occurred on the surface of the activated carbon [The FTIR analysis of the compounds confirmed the results from the previous characterizations. First, for straw-activated carbon, four peaks were recorded corresponding to O-H stretching, C=O stretching, C-H bending and O-H asymmetric stretching at peaks of 3438 cm, 1556 cm, 1415 cmand 1086 cmrespectively. The observed peaks were nearly the same for iron-doped straw-activated carbon with the addition of a new peak at 704 cmwhich was ascribed to Fe-O stretching [ 32 33 ]. It is noteworthy to point out that the Mn-O peaks that should be present at 720 cm, 520 cmand 450 cmdid not exist in the graph. This observation is in agreement with the XRD analysis, proving that a change in the structure of manganese dioxide occurred on the surface of the activated carbon [ 23 ].
2/g for granular activated carbon in comparison to 876 m2/g for iron-impregnated granular AC. Moreover, the pore volume was roughly equivalent for both compounds, with a slight increment in granular activated carbon (0.62 to 0.6 cm3/g). The suppression of the surface area after the impregnation of activated carbon by iron could be attributed to the iron oxide coating, which blocked some pores of the granular activated carbon, decreasing the surface area. This has been observed also in other studies, which have also reported a decrease in the surface area of granular activated carbon after iron impregnation [Kalaruban et al. [ 24 ] measured the surface area and the pore volume of both granular activated carbon and iron-impregnated granular activated carbon. The obtained results revealed a surface area of 1124 m/g for granular activated carbon in comparison to 876 m/g for iron-impregnated granular AC. Moreover, the pore volume was roughly equivalent for both compounds, with a slight increment in granular activated carbon (0.62 to 0.6 cm/g). The suppression of the surface area after the impregnation of activated carbon by iron could be attributed to the iron oxide coating, which blocked some pores of the granular activated carbon, decreasing the surface area. This has been observed also in other studies, which have also reported a decrease in the surface area of granular activated carbon after iron impregnation [ 34 ] or iron and manganese impregnation [ 27 ] and the explanation provided was based on the hypothesis that iron oxide blocked some pores of the granular activated carbon during the impregnation process.
SEM imaging was also conducted and clearly showed the porous structure of the granular activated carbon. In addition, the images show that iron was bound on the surface of the activated carbon, most likely blocking a portion of the pores [ 24 ].
Velazquez-Jimenez et al. [ 25 ] conducted microscopy studies on zirconium functionalized activated carbon for arsenic removal in order to determine the surface morphology of the compound that was synthesized. The studies showed that adding oxalic acid led to the smallest particles sizes being reached, which ranged between 2 and 10 nm. Moreover, SEM showed an irregular shape of the activated carbon due to complexations of zirconium-oxalate that covered the largest part of the activated carbon. Peaks of zirconium while adding oxalic acid have been also observed in XRD analysis at different shapes, which supports the successful addition of zirconium into the surface of the activated carbon [ 25 ].
3O4 loaded activated carbon and activated carbon impregnated with sulfuric acid. The observed SEM images showed that both materials have a wide variety of pores with different shapes and sizes, and this could most likely be attributed to the treatment with sulfuric acid. Nevertheless, there was a difference observed on the surface of both compounds as the Fe3O4 loaded activated carbon had a more irregular and coarser surface in comparison to the smoother surface of activated carbon treated only with sulfuric acid, and this is due to the deposition of Fe3O4 particles on the surface of the activated carbon.Morphological studies were conducted by Liu and co-authors [ 26 ] to characterize Feloaded activated carbon and activated carbon impregnated with sulfuric acid. The observed SEM images showed that both materials have a wide variety of pores with different shapes and sizes, and this could most likely be attributed to the treatment with sulfuric acid. Nevertheless, there was a difference observed on the surface of both compounds as the Feloaded activated carbon had a more irregular and coarser surface in comparison to the smoother surface of activated carbon treated only with sulfuric acid, and this is due to the deposition of Feparticles on the surface of the activated carbon.
−1 to 1500 cm−1 was observed for both compounds, which could be attributed to carbon double bonding [−1 was observed for Fe3O4 loaded activated carbon and attributed to the torsional vibration of Fe3O4 in octahedral sites and Fe-O bonds in tetrahedral sites [3O4 loaded activated carbon were 349 m2/g and 0.20 cm3/g, respectively, which are less than the values obtained for the activated carbon treated only with sulfuric acid (572 m2/g and 0.28 cm3/g, respectively) [3O4 particles into the activated carbon led to a decrease in its surface area, possibly due to the blockage of some pores during the binding process. Despite the decrease in surface area due to the addition of Fe3O4 particles, the authors still synthesized an activated carbon that has a comparable surface area to other studies that have focused also on synthesizing carbons from waste biomass [The FTIR analysis ( Figure 4 ) showed the similar shapes of both compounds with some logical differences. For instance, a peak ranging from 1900 cmto 1500 cmwas observed for both compounds, which could be attributed to carbon double bonding [ 35 ]. However, an adsorption band at around 586 cmwas observed for Feloaded activated carbon and attributed to the torsional vibration of Fein octahedral sites and Fe-O bonds in tetrahedral sites [ 36 ]. The surface area and pore volume of the Feloaded activated carbon were 349 m/g and 0.20 cm/g, respectively, which are less than the values obtained for the activated carbon treated only with sulfuric acid (572 m/g and 0.28 cm/g, respectively) [ 26 ], showing that the loading of Feparticles into the activated carbon led to a decrease in its surface area, possibly due to the blockage of some pores during the binding process. Despite the decrease in surface area due to the addition of Feparticles, the authors still synthesized an activated carbon that has a comparable surface area to other studies that have focused also on synthesizing carbons from waste biomass [ 37 38 ].
2/g for the activated carbon. The results shown by the obtained isotherms showed that the pore types fitting the used activated carbon can be classified as micropores and mesopores. Those results were also proven by the pore size distribution, which also showed that the majority of the structure of the activated carbon was either mesoporous or microporous with an average pore size of 22 Å. SEM images also supported the obtained results, as the pictures obtained in the process showed that the activated carbon had a rough and porous surface.Hashim et al. [ 39 ] investigated the adsorption of As(V) by activated carbon using a rotating packed bed. The authors implemented FTIR analysis, SEM and BET in order to conduct the characterization of their acquired activated carbon. By using the adsorption/desorption isotherms and the BET equation, the investigators obtained a BET surface area of 583 m/g for the activated carbon. The results shown by the obtained isotherms showed that the pore types fitting the used activated carbon can be classified as micropores and mesopores. Those results were also proven by the pore size distribution, which also showed that the majority of the structure of the activated carbon was either mesoporous or microporous with an average pore size of 22 Å. SEM images also supported the obtained results, as the pictures obtained in the process showed that the activated carbon had a rough and porous surface.
−1 and 3200 cm−1. The results suggest also that alkanes, which are represented by C-H stretching, are not present in the activated carbon since the peak observed between 3000 cm−1 and 2850 cm−1 was of low intensity. The authors observed multiple peaks ranging between 1580 cm−1 and 1650 cm−1, indicating the presence of C-C stretching for aromatic rings and carboxyl groups, which are usually indicated by C=O and C-O stretching [−1 [The data provided by the FTIR analysis showed the existence of O-H stretching, which could be attributed to alcohol or phenol groups due to the peak observed between 3500 cmand 3200 cm. The results suggest also that alkanes, which are represented by C-H stretching, are not present in the activated carbon since the peak observed between 3000 cmand 2850 cmwas of low intensity. The authors observed multiple peaks ranging between 1580 cmand 1650 cm, indicating the presence of C-C stretching for aromatic rings and carboxyl groups, which are usually indicated by C=O and C-O stretching [ 40 ]. In addition, another peak corresponding to C-O stretching and indicating ester, alcohol and carboxylic acid groups was observed at a peak in the range of 1300–1000 cm 39 ].
0 was slightly above 0.4, a typical type IV isotherm with an H3 hysteresis loop was observed for all of the investigated activated carbons, which is in fact connected to capillary condensation [3/g) and GL200 (0.5255 cm3/g) were higher than that of GL100 (0.438 cm3/g) at an average pore diameter ranging between 50 and 500 Å. However, the opposite was observed when an average pore diameter ranging between 20 and 50 Å was taken into consideration since the mesopores volumes of both GL200 (0.2433 cm3/g) and XHIT (0.2405 cm3/g) were much smaller than GL 100 (0.5668 cm3/g), indicating that GL100 has a smaller distribution of mesopores in comparison to GL200 and XHIT [Three different activated carbons—GL100, GL 200 and XHIT—were evaluated and characterized for the removal of arsenic by Gong et al. [ 28 ]. According to nitrogen adsorption/desorption isotherms and at a relative pressure less than 0.2, the nitrogen uptake for XHIT and GL200 was larger than that of GL100, which could suggest that the micropores developed better in these two types of activated carbons. When P/Pwas slightly above 0.4, a typical type IV isotherm with an Hhysteresis loop was observed for all of the investigated activated carbons, which is in fact connected to capillary condensation [ 41 ]. The presence of a mesoporous structure was verified when the nitrogen uptake greatly increased when the relative pressure was larger than 0.8 [ 42 ]. The average pore sizes obtained for XHIT, GL200 and GL100 were 37.904, 34.95 and 38.232 Å, respectively, proving the mesoporous structure of the investigated activated carbons. The mesopore volumes of XHIT (0.5669 cm/g) and GL200 (0.5255 cm/g) were higher than that of GL100 (0.438 cm/g) at an average pore diameter ranging between 50 and 500 Å. However, the opposite was observed when an average pore diameter ranging between 20 and 50 Å was taken into consideration since the mesopores volumes of both GL200 (0.2433 cm/g) and XHIT (0.2405 cm/g) were much smaller than GL 100 (0.5668 cm/g), indicating that GL100 has a smaller distribution of mesopores in comparison to GL200 and XHIT [ 28 ].
The surface chemistry of the activated carbons was also determined after the impregnation with iron/calcium. Boehm’s titration indicated that both the carboxyl group and the basic group were better developed in the modified GL100 and GL200 than XHIT. The carboxyl groups on the surfaces of GL200 and GL100 were 8.21 mmol/g and 8.32 mmol/g, while the basic groups were 8.12 and 8.23 mmol/g, respectively. In addition, GL100 and GL200 had a higher ash content because of the modification of the surface via iron and calcium in-situ impregnation [ 43 ]. In total, 6.0555% and 6.562% of the ash contents were attributed to the in-situ impregnation by calcium and iron for GL200 and GL100, respectively [ 28 ].
−1 and 2500 cm−1, which included hydroxyl groups at 3778 cm−1 and alcohol or phenol groups (O-H stretching) at 3701 cm−1. C-H stretching corresponding to either alkanes or aliphatic groups was observed at 2921 cm−1. Moreover, carboxyl groups represented by C-O or C=O stretching were present at 1591 cm−1, while -NH2 vibrations were observed at 1640 cm−1. The presence of ester, carboxylic acid or alcohol represented by C-O and C-OH stretching was observed in the fingerprint area at 1275 cm−1; furthermore, Si-O and Fe-O stretching were observed at 1082 and 626 cm−1, respectively [−1 of O-H stretching for either alcohol or phenol groups or carboxyl groups represented by C=O or C-O stretching at 1591 cm−1. Furthermore, GL200 contained an aromatic ring (Ca-O stretching) at 466 cm−1. The FTIR results for XHIT indicated the presence of hydroxyl groups via O-H stretching at 3472 cm cm−1, partially N-H stretching, O-H groups and H bonds at 3415 cm−1 alongside C=O stretching at 1616 and 1591 cm−1 [The authors also conducted an FTIR analysis, which showed that O-H stretching for GL100 was present in the area ranging between 4000 cmand 2500 cm, which included hydroxyl groups at 3778 cmand alcohol or phenol groups (O-H stretching) at 3701 cm. C-H stretching corresponding to either alkanes or aliphatic groups was observed at 2921 cm. Moreover, carboxyl groups represented by C-O or C=O stretching were present at 1591 cm, while -NHvibrations were observed at 1640 cm. The presence of ester, carboxylic acid or alcohol represented by C-O and C-OH stretching was observed in the fingerprint area at 1275 cm; furthermore, Si-O and Fe-O stretching were observed at 1082 and 626 cm, respectively [ 44 ]. GL200 showed 3179 cmof O-H stretching for either alcohol or phenol groups or carboxyl groups represented by C=O or C-O stretching at 1591 cm. Furthermore, GL200 contained an aromatic ring (Ca-O stretching) at 466 cm. The FTIR results for XHIT indicated the presence of hydroxyl groups via O-H stretching at 3472 cm cm, partially N-H stretching, O-H groups and H bonds at 3415 cmalongside C=O stretching at 1616 and 1591 cm 28 ].
2/g, probably due to oxidation of labile carbon fractions and some inorganics that dissolved [2/g even though a high amount of iron was deposited into the surface. In terms of the pore fraction, the ultra-micropores experienced the majority of this decrease (15% of the total pore volume), which indicates that some smaller iron particles were able to penetrate the pores of the activated carbon.Nieto-Delgado et al. [ 29 ] investigated the modification of commercial activated carbon with iron and manganese for the removal of arsenic from water. Generally, pristine F400 is characterized by having a wide pore size distribution ranging normally from ultra-micropores to small mesopores. By applying hydrothermal treatment and without the presence of manganese or iron ions, the authors noticed that the surface area of the activated carbon increased from 776 to 891 m/g, probably due to oxidation of labile carbon fractions and some inorganics that dissolved [ 37 ]. On the other hand, the surface area of the modified activated carbon with iron/manganese decreased between 3 and 40 m/g even though a high amount of iron was deposited into the surface. In terms of the pore fraction, the ultra-micropores experienced the majority of this decrease (15% of the total pore volume), which indicates that some smaller iron particles were able to penetrate the pores of the activated carbon.
The crystalline structure of the modified activated carbon was evaluated by XRD. The results showed a poor crystalline structure as a wide band with the values of 44° and 26° at 2θ for all carbons. For all the tested samples, sharp peaks representing hematite, goethite and graphite were observed. Goethite is an element present usually in pristine F400 activated carbon, and after the activated carbon was modified in the presence of iron and manganese, the intensity of the goethite peak sharply increased and the hematite peak appeared [ 37 ].
Even though the XRD for both the iron modified activated carbon (Fe-F400) and the modified activated carbon with both iron and manganese was similar, the morphological structure according to SEM showed some differences. For instance, the average particle size was 40 nm for Fe-F400 widely distributed on the surface of the activated carbon, while the structure of Fe:Mn-activated carbon showed the presence of a quasi-sphere, which normally has an average diameter of 500 nm and is formed by nanorods that have an average length of 20 nm. Generally, the authors observed that the largest surface area of the activated carbon was covered by iron, and they relate this high deposition of iron onto the surface area of the activated carbon with the high adsorption capacity that was obtained during the experiment (5 mg As/g carbon) [ 29 ].
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