1 Introduction

The rapid expansion of industry activity has resulted in the contamination of the aquatic environment with a wide array of toxic and hazardous chemicals [1,2,3]. Effective wastewater management and treatment are crucial to mitigate the environmental impact of these pollutant sources and to ensure safe water disposal and conservation. Addressing this issue requires innovative approaches, and recent research has made significant contributions in the field of pollutant detection and removal [4, 5]. Among these pollutants, heavy metals stand out as highly perilous inorganic contaminants, even when present in low concentrations in aqueous solutions [6]. These metals have long half-lives and cannot be degraded [7]. Chromium is found in two forms: trivalent Cr(III) and hexavalent Cr(VI) chromium. Hexavalent Cr(VI) is particularly pernicious due to its elevated oxidation potential and remarkable transmembrane mobility, classifying it as a Group I carcinogen [8]. The World Health Organization (WHO) has established stringent guidelines, specifying limits of 0.05 mg/L for Cr(VI) in drinking water and 0.1 mg/L for surface water [9].

Chromium sources in wastewater are diverse and can originate from various industrial, municipal, and natural origins. Industries such as metal plating, tanneries, and chemical manufacturing contribute significant amounts of chromium through their processes [10]. Municipal sewage also contains chromium from domestic sources, like household products. Natural sources, such as geological deposits and atmospheric deposition, can introduce trace amounts of chromium into wastewater.

Methods including coagulation/flocculation [11], precipitation [12], adsorption [13], ion exchange [14], reverse osmosis [15], and membrane filtration [16] are employed to remove Cr(VI) from wastewater. The adsorption process has been identified as the most efficient and environmentally sustainable method [17]. The ability to regenerate the adsorbents when necessary is another benefit [8]. The use of various adsorbents for wastewater purification, such as bentonite [18], mesoporous silica [19], humic acid-derived resin [20], nanoadsorbent [21] composite materials [22, 23], nanocomposite [24], biosorbents [25, 26], and activated carbons [27, 28], has been the subject of extensive research in recent years.

Activated carbon (AC) emerges as a remarkably significant material with diverse applications across various fields, including energy storage [29,30,31], and wastewater treatment. Its prominence is attributed to factors such as large surface area, cost-effectiveness, high reactivity, and minimal negative environmental impact [32, 33]. have considerable in the removal of hexavalent chromium from water [34, 35]. Moreover, the exploration of enhancing activated carbon (AC) through surface modification with various chemicals, including acids, basic materials, and polymers, adds an intriguing dimension due to its potential to boost AC efficiency [36, 37].

Through a series of procedures, activated carbon can be created from any source material with a high carbon concentration [38]. These procedures transform carbonaceous materials into a highly porous and adsorptive substance. According to Liang et al. [39], wood is the most widely used raw material for the production of activated carbon [40], followed by agricultural wastes. Various forms of agricultural waste, including seeds [41, 42], leaves [43, 44], peels [45, 46], and stems [47], coffee-derived biowaste [48], sawdust [49], and corncob [50, 51], have been harnessed for the production of AC. This study focuses specifically on the utilization of leaves as a biomass source for AC production. The significance of leaves as valuable precursors in material production is underscored by a variety of studies that explore their distinctive properties and applications [52,53,54,55].

Carbonization and activation are the two key stages involved in the manufacture of activated carbon. The first stage entails heating carbonaceous materials beyond 600 °C without oxygen in order to transform them into solid carbon products [56]. The technique of applying physical activation or chemical activation to increase the porosity and surface area of activated carbon is the second phase. Physical activation is a less effective, but more expensive, way to create activated carbon than chemical activation. Typically, chemical activation is employed to produce activated carbon with large pore volumes and surface areas [57, 58].

The impregnation of a raw material with an activating agent and subsequent heating to a high temperature constitute the chemical activation process. By creating holes in the raw material by an activating agent reaction, the surface area and adsorption capacity are increased. Potassium hydroxide (KOH), sodium hydroxide (NaOH), zinc chloride (ZnCl2), and phosphoric acid (H3PO4) are the most often utilized activating agents. The final one has a variety of benefits over other chemical agents, including high efficiency, a non-corrosive acid, relative environmental friendliness, ease of use, and affordability. It creates activated carbon with a variety of pore sizes and high surface areas [59, 60].

The objective of this study focuses on the chemical activation of an agricultural waste, Ficus carica leaves, utilizing H3PO4 as an activating agent for a range of impregnation ratios (30, 60, 100, 150, and 200%) to produce activated carbon. By using XRD, SEM, TGA, FTIR, iodine number, methylene blue (MB) index, pHPZC, and Boehm titration, the prepared ACs were characterized. Using batch adsorption process, the prepared ACs were employed to remove Cr(VI) from an aqueous media. It was investigated how different parameters, including adsorbent dosage, solution pH, Cr(VI) concentration, and contact time, affected the adsorption process. Ethylenediamine (EDA) has been used to modify the prepared ACs surfaces in order to improve their performance. Also, the equilibrium adsorption isotherms of Cr(VI) on unmodified and EDA-modified ACs were investigated.

2 Materials and methods

2.1 Materials

The following chemicals were supplied by Sigma-Aldrich, Algeria: K2Cr2O7 (99.5%), H3PO4 (85%), 1,5-diphenylcarbazide (99%), ethylenediamine (C2H8N2) (≥ 99%), KI (99%), I2 (99%), NaOH (98%), H2SO4 (98%), HCl (36.5–38%), and acetone ( 99%). In order to make the Cr(VI) stock solution (1 g/L), 2.83 g of K2Cr2O7 and 1L of distilled water were used.

2.2 Activated carbon preparation

Ficus carica leaves were collected in the local region in eastern Algeria. The leaves were cleaned with distilled water to get rid of any pollutants before being dried outside for a couple of days. In order to obtain particles with a diameter of less than 900 m, the acquired precursor was crushed and sieved. A chemical activation was carried out by the socking of 60 g of the precursor with a desired amount of H3PO4 (30, 60, 100, 150, 200 wt %) in an ultrasound probe (BRANSON 2510) for 2 h. The mixture was heated to 600 °C for 1.5 h in a turbulent muffle furnace (LINDBERG 54576-S HEVI-DUTY) with a heating rate of 15 °C min−1 after being evaporated at 105 °C in an oven. The resulting sample was rinsed several times with hot distilled water to remove excess phosphoric acid. After drying at 105 °C for 24 h, crushing, and sieving the activated carbon (AC) to a diameter of less than 75 μm, the activated carbon (AC) noted is ready for use. The prepared ACs were given the labels AC-30, AC-60, AC-100, AC-150, and AC-200 in accordance with their impregnation ratios. Ethylenediamine (EDA) has been added to the produced ACs to improve their adsorption abilities. To achieve this, 2 mL of ethylenediamine was gradually added to a mixture of 300 mg of AC and 50 mL of distilled water in an ultrasound probe (BRANSON 2510) at 40 °C for 2 h. The resultant sample was then dried after being rinsed with ethanol and water in a 1:1 ratio and filtered using a 0.45-μm filter. The label for the modified activated carbon read EDA-AC.

2.3 Characterizations

The elemental composition and surface appearance of the AC were characterized using scanning electron microscopy (SEM). A spinning Cu K anode (1.54060) was used in an X-ray diffractometer (XRD; Rigaku MiniFlex600) investigation to ascertain the composition of AC. Shimadzu’s DTG-60 thermogravimeter was used to analyze the thermogravimetry of ACs between 25 and 1000 °C (10 °C/min). Based on FTIR spectra collected with a Perkin Elmer Two FTIR spectrophotometer from Perkin Elmer, USA, the chemical functional groups of the ACs were identified. The ACs were examined as prepared KBr pellets with 20 scans between 4000 and 400 cm−1 and a resolution of 4 cm−1. Additionally, the Boehm titration method (Boehm, 1966) was used to calculate the amount of chemical functional groups present on the surface of AC [61].

The point of zero charge (pHPZC) of AC was determined using a batch technique [62]. In a series of Erlenmeyer flasks containing 50 mL of NaCl solution (0.1 mol/L), a mass of 0.2 g of AC was added to each flask, and the mixture was stirred for 48 h at room temperature. The pH was varied from 2 to 10 by added HCl (0.1 mol/L) and NaOH (0.1 mol/L) to the pH noted pHi. The curve of pH as a function of pHi was used to determine pHPZC. The pH at the point where the surface charge of the activated carbon is zero represents pHpzc [63]

The adsorption performance of the produced ACs was assessed by measuring the methylene blue and iodine numbers. Iodine number was determined using the experimental protocol proposed by ASTM [64]. A mass of 0.1 g of AC is placed in a clean bottle in contact with 10 mL of 5% HCL and allowed to boil for 30 s. Then, 100 mL of iodine solution (0.1 N) was added to the mixture, which was agitated for 30 s, and the mixture was quickly filtered on filter paper. A volume of 10 mL of the filtrate is dosed by a sodium thiosulfate solution of 0.1 N in the presence of starch as an ending indicator. Methylene blue (MB) index was determined according to the Chemviron-Carbon Company (Zoning Industriel C. B-7181 Feluy, Belgium) TM-1 standard test method [28]. In an Erlenmeyer flask containing 0.1 g AC, a standard MB solution (1500 mg/L) is added in small volumes, and the mixture is shaken for a few minutes. Once equilibrium is reached and the AC is saturated with MB, a very light blue color appears with the last amount of MB added just before reaching equilibrium. The total volume (\({{\text{V}}}_{MB})\) adsorbed is then calculated, and the MB index, expressed in terms of milligrams of MB adsorbed per gram of activated carbon, is determined using Eq. 1.

$$\text{MB index}=\frac{\text{1500 }\times {{\text{V}}}_{MB}}{0.1}$$
(1)

2.4 Adsorption study

Adsorption study carried out using a batch process, a predetermined volume of the Cr(VI) solution was combined with a certain mass of AC in a reactor and agitated until equilibrium. The mixture was centrifuged at 4000 rpm for 20 min after equilibrium was reached, and the filtrate was then combined with 1,5-diphenylcarbazide and examined using a UV spectrophotometer at 554 nm. Investigation was done into the effects of various operating parameters, including the AC dose, solution pH, time, and beginning Cr(VI) content. Furthermore, the equilibrium adsorption was investigated under the optimum condition. The adsorption capacity of Cr(VI) at equilibrium, qe, and at time t, qt, were calculated from Eq. 2 and Eq. 3, respectively.

In a batch-processed adsorption investigation, the equilibrium adsorption was examined under optimal operating conditions. Equation 2 and Eq. 3 were used to calculate the adsorption capacity of Cr(VI) at equilibrium, qe, and at time t, qt, respectively [65, 66].

$${q}_{e}({\text{mg}}/{\text{g}})= \frac{{(C}_{0}-{C}_{e})({\text{mg}}/{\text{L}}) }{{m}_{AC} ({\text{g}})}V({\text{L}})$$
(2)
$${q}_{t}({\text{mg}}/{\text{g}})= \frac{{(C}_{0}-{C}_{t})({\text{mg}}/{\text{L}}) }{{m}_{AC} ({\text{g}})}V({\text{L}})$$
(3)

3 Results and discussion

3.1 Characteristics of the prepared ACs

The X-ray diffraction (XRD) patterns of the ACs that were prepared are presented in Fig. 1. As shown in this analysis, the produced ACs exhibit a similar structure across various impregnation ratios. The prominent and wide peaks appearing between 20 and 35° can be attributed to graphite disorder, as reported by [67]. Furthermore, a minor peak observed between 40 and 50° may be associated with the stacking arrangement of aromatic carbon layers. Similar findings for various prepared AC types have been reported in numerous research [68,69,70] as well as for commercial AC [71]. According to [72], the peaks at about 24° and 43° correspond to, respectively, the (0 0 2) planes of disorder aromatic carbon and the (0 0 1) planes of graphite’s hexagonal structure, showing the predominance of an amorphous structure.

Fig. 1
figure 1

The XRD diagrams of the prepared activated carbons

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Scanning electron microscopy (SEM) analysis of the surface morphologies of the produced ACs, shown in Fig. 2, reveals a different pattern. Notably, these ACs have a rough surface structure and uneven porosity. The apparent porosity supports the successful pyrolysis and activation procedure that was accomplished by utilizing phosphoric acid.

Fig. 2
figure 2

SEM analysis of the prepared activated carbons

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The thermogravimetric analysis (TGA) results for all the prepared ACs demonstrate a two-stage weight loss pattern, as illustrated in Fig. 3. The release of adsorbed water is thought to be responsible for the initial stage, which occurs up to 250 °C. The next stage involves the thermal breakdown of oxygen functional groups and occurs between 300 and 800 °C. Other researchers [42, 58] for a variety of prepared activated carbons have reported similar findings.

Fig. 3
figure 3

ATG analysis curves of the prepared ACs

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The FTIR spectra of the prepared ACs (Fig. 4) have consistent structural features with distinct bands. The broad band observed at 3425 cm−1 can be attributed to the presence of free hydroxyl groups (O–H) on the surface of the ACs, as reported by [73, 74]. Furthermore, the absorption peaks at 2923.92 cm−1, 1620 cm−1, and 1097 cm−1 may be due to -CH2 groups (methyl and methylene groups) [75], C = C bonds [76], and C-O stretching vibration in carboxylic acids, alcohols, phenols, and esters, respectively. The peaks falling within the 612–779 cm−1 range can be attributed to the interaction of phosphorous species resulting from the phosphoric acid activation process, as suggested by [77].

Fig. 4
figure 4

FTIR spectra of the prepared ACs

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The results of the Boehm titration analysis of the prepared CAs are illustrated in Fig. 5. It is clear that the prepared ACs contain more acidic than basic groups, with lactonic and phenolic groups making up the majority of the acidic groups. The point of zero charge (pHPZC) values in Fig. 6, which vary from 5.44 to 5.61, support the conclusion that the prepared ACs are mildly acidic. Table 1 lists additional properties for all prepared ACs, including the iodine number, methylene blue (MB) index, yield, apparent density, slurry pH, humidity, porosity, and pHPZC.

Fig. 5
figure 5

Functional groups of the prepared CAs

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Fig. 6
figure 6

The pHPZC of the prepared ACs

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Table 1 Physicochemical characteristics of the prepared ACs
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The iodine number values obtained for the prepared ACs exhibit a notable range, spanning from 1117 to 1346 mg/g. As can be observed in Table 1, the iodine number rises as a result of the phosphoric acid impregnation ratio. These high iodine numbers show that the activation process has produced desirable microporosity and that the activated carbon that has been formed has a sizable microporous surface area [78]. According to the literature, the iodine number values expected for high-quality activated carbons are equal to or higher than 900 mg/g. Moreover, the literature provides a broader context by specifying a typical iodine number range of 500–1200 mg/g, equivalent to a surface area range of 900–1100 m2/g [79]. This suggests that the prepared activated carbons exhibit a surface area that exceeds the typical expectations, further emphasizing their potential for efficient adsorption. In addition, the low-density values validate the porous structure of the ACs and confirm the existence of a large specific surface area [80]. The MB index values ranged from 35.82 to 46.8 mg/g, demonstrating that chemical activation with phosphoric acid had a detrimental effect on the development of mesoporous surface area. This shows that the structure of the prepared ACs is largely microporous. These ACs can also be used to remove substances with tiny diameters, such as hexavalent chromium.

3.2 Adsorption study

3.2.1 Effect of parameters on adsorption of Cr(VI)

The effect of pH (2–12) on the removal of Cr(VI) using the prepared ACs is presented in Fig. 7. The highest Cr(VI) removal efficiency was accomplished at very low pH, with removal yields ranging from 98.18 to 75.83% at pH 2–4. This finding is in line with earlier studies [35, 81]. This is due to the anionic form of chromium, \({{\text{HCrO}}}_{4}^{-}\), which predominates in low pH solutions and is drawn to the positively charged surfaces of the ACs. The prepared ACs all have an acidic point of zero charge (pHPZC), which indicates that at pH levels below their pHPZC, their surfaces are positively charged. For additional research, pH 2 was kept constant.

Fig. 7
figure 7

The effect of pH on the adsorption of Cr(VI) in the prepared ACs (time = 2 h, ambient temperature, an initial Cr(VI) concentration = 100 mg/L)

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The effect of the AC dose on the adsorption of Cr(VI) is illustrated in Fig. 8. It is evident that with an increase in the amount of AC, the adsorption efficiency also increases, which is due to the availability of more adsorption sites on the surface of AC. All activated carbons exhibit comparable behavior, with only slight variation in their removal efficiency. The optimal ACs dose for the effective removal of Cr(VI) was determined to be 2 g/L, achieving a removal efficiency of 98%. This dose was subsequently maintained constant for further study.

Fig. 8
figure 8

The effect of ACs dose on the adsorption of Cr(VI) (pH = 2, time = 2 h, ambient temperature, an initial Cr(VI) concentration = 100 mg/L)

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The effect of contact time on the removal of Cr(VI) was investigated using the prepared activated carbons (ACs) at an initial concentration of 100 mg/L, as depicted in Fig. 9. The results reveal a clear trend, wherein an extension of the contact time results in a corresponding increase in the removal efficiency of Cr(VI). This phenomenon can be attributed to the prolonged contact time facilitating the diffusion of Cr(VI) onto the microporous surface of the ACs, allowing it to effectively bind to available adsorption sites. Equilibrium in the adsorption process is observed to be achieved at approximately 2 h of contact time, and this duration was subsequently adopted as the equilibrium contact time for further study.

Fig. 9
figure 9

The effect of contact time on the equilibrium adsorption of Cr(VI) onto the prepared ACs (AC dose = 2 g/L, pH = 2, ambient temperature, an initial Cr(VI) concentration = 100 mg/L)

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3.2.2 Equilibrium adsorption isotherms

The equilibrium adsorption isotherms of all prepared ACs were investigated in the range of initial Cr(VI) concentration spanning from 50 to 450 mg/L, under optimal operational conditions (ACs dose of 2 g/L, pH of 2, a contact time of 2 h, and ambient temperature). The resulting isotherms are illustrated in Fig. 10. It is evident that the adsorption capacity for Cr(VI) rises with increasing residual Cr(VI) concentrations at equilibrium, reaffirming the favorable adsorption characteristics of Cr(VI) on all the prepared ACs, albeit with slight variations in equilibrium capacity among the different AC samples.

Fig. 10
figure 10

The adsorption isotherms of Cr(VI) on the prepared ACs (ACs dose of 2 g/L, pH of 2, a contact time of 2 h, and ambient temperature)

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Two adsorption isotherm models, namely the Freundlich model (Eq. 4) proposed by [82] and the Langmuir model (Eq. 5) introduced by [83], were employed to assess the Cr(VI) adsorption parameters (Fig. 11).

$${q}_{e} ={\text{K}}_{\text{F}}{{\text{C}}_{\text{e}}}^{ 1/n}$$
(4)

where qe (mg/g): the equilibrium adsorption capacity, Ce (mg/L): the equilibrium concentration of Cr(VI), KF (mg1−(1/n) L1/n g−1): the Freundlich constant, and n: a measure of the adsorption intensity.

$${q}_{e}=\frac{{{q}_{m}{\text{K}}}_{\text{L}}{{\text{C}}}_{\text{e}}}{{1+{\text{K}}}_{\text{L}}{{\text{C}}}_{\text{e}}}$$
(5)

where qm (mg/g): the maximum adsorption capacity for a monolayer coverage of the AC (mg/g), and KL (L/mg): the Langmuir constant.

Fig. 11
figure 11

Linear fit of Langmuir (a) and Freundlich (b) isotherms models for Cr(VI) removal by the prepared ACs

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The parameter values for each adsorption isotherm are provided in Table 2. The correlation coefficient (R2) values reveal that the Langmuir model suitably describes the adsorption data for all ACs. The maximum adsorption increases from 93.28 to 155.27 mg/g as the impregnation ratio increases from 30 to 100%. Subsequently, with a further increment in the impregnation ratio from 100 to 200%, the maximum capacity decreases to 112.86 mg/g. Among the ACs, AC-100 stands out with the highest maximum capacity, signifying the optimal impregnation ratio of the activating agent (H3PO4). This result highlights the crucial role of impregnation ratio in modifying the adsorption capacity of activated carbons for Cr(VI) removal.

Table 2 Adsorption isotherm constants for removal of Cr(VI)) using the prepared ACs
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The maximum adsorption capacity of the selected AC (AC-100) is compared with that of several other prepared ACs used for Cr(VI) removal, as illustrated in Table 3. It can be seen that the prepared AC-100 exhibits a notably greater maximum adsorption capacity in comparison to the majority of previously reported adsorbents, reaffirming its effectiveness for Cr(VI) removal. This AC can be considered a promising adsorbent for the effective removal of Cr(VI) from water.

Table 3 Comparison of the prepared AC (AC-100) with other activated carbons
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The effect of the surface modification by EDA

Ethylenediamine (EDA) was employed as a surface modification agent to enhance the performance of the prepared ACs for the removal of Cr(VI). In Fig. 12, equilibrium adsorption isotherms for all modified ACs are presented. To determine the adsorption parameters and the most suitable isotherm model, the adsorption data were fitted using the Freundlich and Langmuir isotherms, as shown in Fig. 13. The results in Table 4 affirm that the adsorption of Cr(VI) onto all modified ACs (EDA-ACs) is well-described by the Langmuir isotherm, as indicated by the high correlation coefficient (R2) values. A comparison of the maximum adsorption capacities of the unmodified ACs and the EDA-ACs confirms a notable enhancement in the performance of the ACs. Specifically, the maximum adsorption capacity increased from 155.27 to 203.25 mg/g for EDA-modified AC-100 (EDA-AC-100). This improvement in AC performance can be attributed to the introduction of new functional groups on the AC surface and an increase in the surface area, resulting in an elevated Cr(VI) adsorption capacity. The results were further corroborated through iodine number, SEM analysis, and FTIR spectra analysis of EDA-AC-100. The iodine number increased from 1270 for AC-100 to 1422 for EDA-AC-100, signifying an augmentation in the microporous surface area. In Fig. 14, the SEM images confirm the enhancement of surface porosity in the modified AC. The FTIR analysis of EDA-AC-100, as shown in Fig. 15, revealed the emergence of new peaks, including one between 1425 and 1375 cm−1, due to the stretching vibration of C(O)-N in amide groups [90], shows the presence of an amidation reaction between AC and EDA. This amidation resulted in the covalent attachment of EDA to AC, characterized by the vibrations of N–H, C-H, and C-N [91]. Another peak appeared at 868 cm−1, indicating the presence of C-N stretching. Additionally, a peak near 3000 cm−1 was associated with N–H stretching, and the last peak at 1579 cm−1 was possibly related to N–H (amine or amide groups). These peaks provided conclusive evidence of the chemical modification of the prepared AC with EDA. Furthermore, the introduction of new amino groups on the AC surface enhances electrostatic attraction and chelate complex formation with Cr(VI) ions. These results were in agreement with those obtained by [90, 92].

Fig. 12
figure 12

The isotherms adsorption of the Cr(VI) on the EDA-modified ACs (ACs dose of 2 g/L, pH of 2, a contact time of 2 h, and ambient temperature)

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Fig. 13
figure 13

Linear fit of Langmuir (a) and Freundlich (b) isotherms models for Cr(VI) removal by the prepared EDA-ACs

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Table 4 Adsorption isotherm constants for removal of Cr(VI) using the prepared EDA-ACs
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Fig. 14
figure 14

SEM analysis of EDA-AC-100

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Fig. 15
figure 15

FTIR spectra of EDA-AC-100 and AC-100

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4 Conclusion

In this work, preparation, characterization, and adsorption performances of activated carbons derived from Ficus carica leaves for Cr(VI) removal were investigated. The prepared ACs (H3PO4, 600 °C, 1 h) were characterized by DRX, SEM–EDS, ATG, FTIR, iodine number, and pH of zero charge. It was found that the prepared ACs have a high microporous surface area and acidic functional groups. The effect of the impregnation ratio of the activating agent (H3PO4) and surface modification by ethylenediamine (EDA) was also studied to enhance the ACs performance. Results show that an impregnation ratio of 100% gives the best AC performance, with a maximum capacity adsorption equal to 155.27 mg/g evaluated using Langmuir isotherm under optimal operational conditions such as ACs dose of 2 g/L, pH of 2, and a contact time of 2 h. Results of ACs surface modification show a significant improvement in the adsorption capacity of the produced ACs; the maximum adsorption capacity of Cr(VI) increases from 155.27 to 203.25 mg/g for unmodified and EDA-modified AC-100. Moreover, iodine number, SEM images, and FTIR spectra analysis have successfully confirmed the surface modifications. The introduction of amino groups via EDA not only enhances the adsorption capacity but also facilitates electrostatic attraction and chelate complex formation with Cr(VI) ions. This approach holds considerable promise for enhancing the adsorption performance of ACs and their usefulness in water treatment and environmental remediation applications.