Introduction

As dyes are employed in several industries such as printing, pharmaceutical, plastic, and fabrics, their effluents are discharged into water bodies and may harm human and aquatic life beings due to their carcinogenic, toxic, and mutagenic effects (Mostafa et al. 2023; Abd-Elhamid et al. 2020; Ibrahim et al. 2019; Behera et al. 2017; Youssef et al. 2016; Nandi et al. 2009).

Brilliant green (BG) is a derivative of triarylmethane dye. It is employed in various medical applications as it can be used as a veterinary drug, a biological stain, a dermatological agent, and an antimold. It is also used in different industries like printing and paper and fabric dying. BG dye is dangerous in case of skin contact, eye contact, and swallowing. It is toxic to the lungs, through inhalation. During its degradation, it produces CO2, NO, and sulfur oxides. Hence, it is necessary to remove this dye from wastewater (Behera et al. 2017; Ghaedi et al. 2014; Kumar et al. 2014; Rehman et al. 2013; Kumar et al. 2012; Zhang et al. 2011; Nandi et al. 2009).

There are numerous techniques for dye removal, including membrane filtration, coagulation, enhanced oxidation, integrated treatment processes, and adsorption (Akl et al. 2023a, 2023b, 2021; Al-Awadhi et al. 2023; Akl et al. 2022a, b; Serage et al. 2022; Mostafa et al. 2021; Qian et al. 2013; Aziz et al. 2012; Kurt et al. 2012; Lotito et al. 2012; Javaid et al. 2011). For various applications, including wastewater treatment, purification and liquid mixture separation, or polar organic solute recovery from biotechnology operations, the adsorption process is one of the most straightforward and efficient procedures with simple operating conditions (Akl et al. 2023c, 2022b; Abdel-Fadeel et al. 2022; Rehman et al. 2012).

Using activated carbons as an adsorbent is very common as they have a lot of advantages like they have high capability and efficiency to remove many pollutants, low cost, and simplicity of design. However, the large-scale usage of AC is controlled when it is produced from expensive and nonrenewable precursors despite it being commercially available (Islam et al. 2017; Cherifi et al. 2009; Tan et al. 2008). This has prompted researchers to seek and evaluate other sources for producing AC in feasible ways. Agricultural wastes such as sawdust, banana core, fruit stones, and coconut husks are feasible and promising precursors for activated carbon due to their low cost, availability, and eco-friendly. (Murmu et al. 2019) It is very essential to produce highly efficient activated carbon from low-cost, available, and renewable precursors like biomass-based agroindustrial byproducts (Húmpola et al. 2016; Ioannidou and Zabaniotou 2007). Impressively, different agricultural biomasses have been utilized such as sugar cane bagasse, rice husk and straw, fruit peel, animal waste, date and rape seed, and corncob in order to produce a large amount of biochar (Hai et al. 2019; Idrees et al. 2018; Lam et al. 2018; Meng et al. 2018; Alvarez et al. 2014).

Date palm is one of the oldest plants grown by humans and has been used as food for about six thousand years. It is widely planted in dehydrated and semi-dry areas around the world. Huge numbers of date palm trees are present in Yemen and present in great quantities in Iraq, Saudi Arabia, the United Arab Emirates, Egypt, and the rest of the Arab world. Its scientific name is Phoenix dactylifera L. (Mahdi et al. 2018, 2017; Shi et al. 2014). Date fruit output has grown globally, from 4.6 million tonnes in 1994 to 7.68 million tons in 2010, and it is anticipated that this trend will continue (Al-Farsi et al. 2007; Sahari et al. 2007; Besbes et al. 2005). In addition to their nutritional significance, date fruits are believed to have many medicinal activities such as free radical scavenging activities, antioxi­dant, anti-inflammatory, gastroprotective, hepatoprotective, and nephroprotective activities.

The byproducts of date processing industries include date seeds (date pits). Every year, a lot of dates pits might be produced. Relying upon the date cultivar, the weight (%) of pits varies from 4.6 to 15% of the entire fruit weight (Tang et al. 2013; Ashraf and Hamidi-Esfahani 2011; Al-Bowait. 2006). In the future, the Arab World will have access to over 100,000 tons of date pits each year, which are currently being wasted. It is a difficulty when it comes to collection, no doubt. The modern food processing sector, which exports dates with the pits removed, would be the best source for their collection and successful use.

Even though activated carbon is a significant commercial product on the chemical market, production of it in our nation has not yet begun. This stimulated the authors to prepare date pit activated carbons, describe them and investigate their adsorption capabilities for dyes and heavy metals.

The present study was carried out with the following objectives:

  1. (i)

    Design and characterization of from zinc chloride activated carbons from date pits (ZCPs) using various instrumental performances as Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).

  2. (ii)

    Batch sorption experiments utilizing cationic dye (BG) as pollutant.

  3. (iii)

    Studying the optimum conditions of the BG adsorption such as the BG initial concentration, ZCPs weights, pH, temperature, isotherms, kinetics, thermodynamics, and shaking time.

  4. (iv)

    Comparative evaluation of removal efficiency of BG dye, feasibility, and reusability of with other adsorbents.

  5. (v)

    Elucidation of the mechanisms involved in the processes of activation of date pits derived carbons using zinc chloride.

  6. (vi)

    Elucidation of the mechanisms involved in the processes of adsorption of BG onto zinc chloride activated carbons from date pits (ZCPs) composite.

Experimental

Materials

Date pits were bought from market, in Marib City, Yemen. Brilliant Green Dye (C27H34N2O4S) that present in Scheme 1, was used as it is without any purifications and was purchased from Merck Company (Germany) (Almana and Mahmoud 1994). In order to prepare BG 1000 ppm, 1 g of the BG was dissolved in a liter of distilled water. The concentration of the other solutions tested in the current study can be changed by the diluting method. The pH adjustment for the BG solutions occurred by utilizing NaOH and HCl (diluted solutions) (Almana and Mahmoud 1994).

Scheme 1
scheme 1

The structure of Brilliant Green dye (C27H34N2O4S)

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Preparation of date pits derived -Zinc Chloride-activated carbon (ZCPs)

Date pits were powdered, pretreated by washing with 0.5% HCl to remove all dirt, dried in an oven at 378K overnight, ground by a mill, sieved to mesh size of 1–4 mm, and carbonized in a tabular furnace at 873K for 2h to yield the carbonized samples (CPs). CPs was rinsed for one day in Zinc chloride (ZnCl2) solution with an impregnation ratio of ZnCl2: char (w/w) equal to 1:1(ZCP11), 2:1(ZCP21) and 3:1(ZCP31). The mixture was then dried in an oven all-night at 378K to remove moisture and followed by the activation step at 1023 K for 2h. The resulting ZCPs were washed with dist.H2O many times after cooling until the pH of the filtrate reached 7. This was followed by drying the ZCPs overnight in an oven at 378K. The activated carbons were then repeatedly rinsed in hot dist.H2O to remove any excess chloride ions (Tang et al. 2019; Almana and Mahmoud 1994).

Characterization

Through using a surface Area and Size Analyzer (QUANTACHROME–NOVA 2000 Series) and by applying Brunauer–Emmett–and–Teller (BET) equation and N2 adsorption–desorption measurements, the ZPCs biosorbents surface area was obtained at liquid nitrogen temperature, 77 K. The surface micrographs of the biosorbents (ZPCs) were captured utilizing a JSE–T20 (JEOL, Japan) scanning electron microscopy (SEM) apparatus at 40 kW. The samples were totally dried at 383 K prior to analysis, and a thin coating of Au particles was applied to them in order to dissipate sample metallization and charges. The FTIR spectra of the prepared activated carbons before and after activation with ZnCl2 and after the BG adsorption using a Nicolet FTIR spectrophotometer instrument in the range of 4000 to 400 cm−1. The samples were prepared by mixing 1 mg of each sample with KBr (500 mg). The scanning occurred in the 4000 to 400 cm−1 range. The type of carbon surface and the amount of the functional groups that exist on it were estimated via neutralization with bases of differing strengths by applying the acidity of the surface and Boehm titration approaches.

The moisture content is estimated by heating 50 mg of the biosorbent in an oven at 110 °C overnight, then it was cooled in desiccators to 25 °C and finally weighed. To calculate how much ash is in the produced biosorbent by putting 500 mg of the sample in a muffle furnace at 700 °C for six hours. The residual minerals that are present as impurities in the adsorbent are left behind after the carbonaceous component of the adsorbent is burnt off during pyrolysis. According to reported work (Litefti et al. 2019; Nemr 2009), the point of zero charge (pHPZC) for the prepared material was assessed. A pH meter (Hi 931,401, HANNA, Portugal) device was utilized to measure the solution’s pH. The batch sorption experiments were carried out at 240 rpm on a Shaking Water Bath (Nickel Electro Ltd., NE5, UK).

Batch adsorption studies

The current investigation utilized the batch approach, 50 ml of known concentration of the BG solution was put in a 250ml glass stoppered bottle with a known dose of ZCPs. At room temperature, the bottles were kept in a thermostatic shaker with a constant speed of 240 rpm. After the equilibrium was reached, the supernatant solution that includes the remains of the investigated BG was measured at its specific λmax. Various parameters were investigated like contact time, temperature (25–45 °C), CP and ZCPs dose (0.025–0.2 g), pH (from 2 to 10), ionic strength, and initial BG concentration of (50–500 mg/l). The BG pollutant removal percentage (R, %) and adsorption capacity (qe, (mg/g)) were calculated in Eq. (1) and Eq. (2), respectively (Bakr et al. 2018; Qi and Xu 2004). The BG concentration was measured spectrophotometrically at lambda maximum (λmax) 624 nm. The BG pollutant removal percentage (R, (%)) and adsorption capacity (qe, mg/g) were calculated in Eq. (1) and Eq. (2), respectively (Bakr et al. 2018; Qi and Xu 2004).

$$q_{{\text{e }}} = { }\frac{{\left( {C_{{\text{i}}} - C_{{\text{e}}} } \right)V}}{m}$$
(1)
$$R\% = { }\frac{{\left( {C_{{\text{i}}} - C_{e} } \right)}}{{C_{{\text{i}}} }}{ } \times 100$$
(2)

where Ci (ppm) and Ce (ppm) are the initial BG concentration and equilibrium BG concentration after adsorption, respectively. V (L), is the volume of BG solution, and m is the mass of the ZCPs adsorbent (g).

Adsorption isotherms investigations

The equilibrium adsorption isotherm investigation is critical in representing the solute and adsorbent interactive behavior and is also important for the design of the adsorption system of BG pollutant adsorption.

Langmuir isotherm

The Langmuir model, Eq. 3, illustrates the monolayer adsorbate adsorption onto a homogeneous adsorbent surface. Also, the maximum capacity can be estimated by it (Mittal et al. 2015; Goscianska et al. 2014).

$$\frac{{C_{{\text{e}}} }}{{q_{{\text{e}}} }} = \frac{1}{{bq_{{\text{m}}} }} + \frac{{C_{{\text{e}}} }}{{q_{{\text{m}}} }}$$
(3)

As qe and qm, which are expressed by mg/g, are the quantity of the BG at equilibrium and the maximum adsorption capacity, respectively, while b (L/mg) is the Langmuir constant.

A separation factor, RL, is performed to investigate the Langmuir isotherm and is calculated as in Eq. 4.

$$R_{{\text{l }}} = \frac{1}{{1 + K_{{\text{l}}} C_{{\text{o}}} }}$$
(4)

The highest initial concentration of the solute is Co. The separation factor, RL, reveals the isotherm type and the adsorption process characteristics. If the RL equals 0, equals 1, between 0and1, higher than 1 the adsorption can be irreversible, linear, favorable, or unfavorable, respectively (Tang et al. 2019).

Freundlich isotherm

It is often employed for heterogeneous surface energy systems and is described by Eq. 5 (Bakr et al. 2018; Qi and Xu 2004; Almana and Mahmoud 1994).

$${\text{ln }}q_{{\text{e}}} = \ln K_{{\text{f}}} + \frac{1}{n}\ln C_{{\text{e}}}$$
(5)

where Ce, kf, and qe are the equilibrium concentration in the liquid phase (mg/l), Freundlich constant, and the adsorption capacity (mg/g), respectively. n is the empirical parameter depicting the energetic the adsorption sites heterogeneity.

Kinetic studies

The BG adsorption onto the ZCPs surface was described by the following four kinetic models:

Pseudo-first-order

The pseudo-first-order kinetic model is represented by Eq. 6 (Bakr et al. 2018; Qi and Xu 2004; Almana and Mahmoud 1994).

$${\text{log}}\left( {q_{{\text{e}}} - q_{{\text{t}}} } \right) = {\text{log}}q_{{\text{e}}} - \frac{{k_{1} }}{2.303}{\text{t}}{.}$$
(6)

whereas qe and qt are the amount of BG loaded per mass of the ZCPs biosorbent at equilibrium and at a specific time (t, min), respectively; while k1 is the rate constant, 1/min.

Pseudo-2nd-Order

This model is represented by Eq. 7 (Bakr et al. 2018; Qi and Xu 2004; Almana and Mahmoud 1994).

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{e}^{2} }} + \frac{t}{{q_{e} }}$$
(7)

where k2 is the rate constant.

Weber—Morris intraparticle diffusion

The intraparticle diffusion mechanism was investigated to understand the BG adsorption process’s diffusion mechanism by utilizing Weber and Morris equation, Eq. 8 (Bakr et al. 2018; Qi and Xu 2004; Almana and Mahmoud 1994).

$${\text{q}}_{{\text{t}}} = {\text{k}}_{{{\text{int}}}} {\text{t}}^{{{1}/{2}}} + {\text{ C}}$$
(8)

where the intercept, C, is found by plotting qt against t1/2, and kint (g/mg min1/2) is the adsorption constant (Mittal et al. 2014).

Boyd’s equation

The kinetic data were then assessed utilizing Boyd’s delivered kinetic expression to determine whether adsorption happens through an external diffusion or an intraparticle diffusion mechanism which is stated in Eq. 9 (Bakr et al. 2018; Qi and Xu 2004; Almana and Mahmoud 1994).

$$F\left( t \right) = 1 - \frac{6}{{\pi^{2} }}\mathop \sum \limits_{n = 1}^{\infty } \frac{1}{{n^{2} }}\exp \left( { - n^{2} Bt} \right)$$
(9)

where B (t) is a mathematical function of F. While F is the fraction of equilibrium at various times (t) and n is an integer that determines the infinite series solution and is obtained by the expression, Eq. 10.

$${\text{F }} = \frac{{q_{t} }}{{q_{e} }}$$
(10)

Equation (9) can be rewritten to:

$$B_{t} = { } - 0.4977 - {\text{ln }}\left( {1 - F} \right)$$
(11)

Thermodynamic studies

Thermodynamic parameters for the adsorption of BG on the ZCPs biosorbents are evaluated by the equilibrium experimental data observed at different temperatures from 298.15k to 318.18 K.

The difference in the thermodynamic distribution coefficient, kc, with a temperature change was utilized to determine thermodynamic parameters. The following equations were applied to calculate the standard free energy change, ΔG°, Eqs. 12 and 13.

$$\Delta G^\circ = - RT\, ln\, k_{C}$$
(12)
$$\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$$
(13)

As Kc, T, and R are the Langmuir constant, the absolute temperature in K, and the gas constant (8.314 J/(mol. K)), respectively. The entropy (ΔS°) and standard enthalpy (ΔH°) of adsorption may be computed utilizing the Van’t Hoff equation, Eq. 14:

$$\ln {\text{k}}_{{\text{C}}} = { }\frac{{\Delta {\text{S}}}}{{\text{R}}} - { }\frac{{\Delta {\text{H}}}}{{{\text{RT}}}}$$
(14)

where K is the equilibrium constant (g−1). By plotting the graph between 1/T and ln(k), the adsorption process ΔH° and ΔS° were estimated from the slope and the intercept (Bakr et al. 2018; Qi and Xu 2004; Almana and Mahmoud 1994).

Desorption experiments

Desorption investigations were carried out after the adsorption process by utilizing various eluent agents: absolute EtOH, HNO3, HCl, NaOH, and NaHCO3 with concentrations (0.01–0.5 M). Briefly, 0.2 g of BG-loaded ZCPs was immersed in 10 mL of eluent solution for 3 days with shaking at 150 rpm. Then the desorbed BG concentration was analyzed spectrophotometrically after the suspension was stirred for a fixed time. The BG desorption efficiency can be estimated using equation, Eq. 15 (Bakr et al. 2018; Qi and Xu 2004).

$${\text{Desorption}}\% \, = \frac{{\text{Desorbed amount of BG }}}{{\text{Total Adsorbed amount of BG }}} \times 100{\text{\% }}$$
(15)

Results and discussion

Materials’ design and physicochemical studies

Analysis of the carbons’ nitrogen adsorption–desorption data

Both carbons porosity and surface area play effective roles in adsorption capacities (qe) improvement (Brunauer et al. 1938). Conventionally, the adsorption of nitrogen at 77 k is used to measure the textural proprieties of solids, and adsorption data are commonly analyzed utilizing the BET equation (Akl et al. 2013).

The N2 adsorption/desorption isotherms of ZCP11 and ZCP31 carbon samples are represented in (Fig. 1a). It displays shapes that are similar to one another. The Types IV and V isotherms, which are typical of mesoporous materials, are indicated by these shapes, according to the IUPAC classification. (Fig. 1b) displays the linear BET plots for all sorbents of nitrogen adsorption at 77K.

Fig. 1
figure 1

a Nitrogen adsorption- desorption isotherms at 77K for ZCP11, and ZCP31; b nitrogen adsorption linear BET plots at 77K for CP, ZCP11, ZCP21 and ZCP31

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BET

The surface area, SBET, values of the ZCPs materials can be determined by utilizing the linear BET plots of N2 adsorption at a temperature of 77 K, which show varied surface area values relying on modification and activation processes. The specific surface areas of CP, ZCP11, ZCP21, and ZCP31 were found to be 165.91, 289.51, 333.84 and 667.89 m2/g, respectively, as shown in Table 1. The ZCPs’ SBET values can offer various active sites that result in an increase the BG adsorption. ZnCl2 activation process is characterized by the formation of both wide microspores and small mesopores where ZnCl2 acts as dehydrating agent. The degradation and dehydration of cellulosic material caused by the impregnation of the precursor with ZnCl2 lead to carbonization, aromatization of carbon skeleton and pore structure formation. Pore formation can be attributed to releasing of hydrogen gas during the decomposition of phenolic-aromatic structure of the precursor resulting in formation of porous structure and active sites (Youssef et al. 1994). The volume of pores developed is similar to the volume of ZnCl2 introduced into the particles; so, increasing the ZnCl2 impregnation ratio results in increasing in SBET values which increase from 289.51 m2/g for ZCP11to to 667.89 m2/g for ZCP31.

Table 1 The textural properties of ZCPs
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Activation mechanism

Chemical activation agents such as ZnCl2 are frequently employed (Chang et al. 2013; Sun et al. 2013; Yue and Mangun 2002; Hu et al. 2001). Even though a distinct process of chemical activation is employed to produce carbons with numerous porous, the approach is not completely understood. In the beginning, a temperature over 500 °C is suitable for the activation step because the ZnCl2 melting point is 283–293 °C. Meanwhile, the Zn2+ radius of 74 Pico meters is less than that of common metal salt ions such as Na+ (102 Pico meter), K+ (138 Pico meter), and Ca2+ (100 Pico meter), which can contribute to the ZCPs to obtain micropores from 0.54 to 1.311 nm. ZnCl2 is additionally typically thought of as a dehydrator (Sun et al. 2013; Hu et al. 2001). On the one hand, at high concentrations of zinc chloride, ZnCl2 combines with H2O to create hydroxy dichloro zinc acid (ZnCl2 + H2O → H [ZnCl2(OH)]). To clean the CPs impurities and also to make a channel structure, acid that has an etching influence is used. On the other hand, if ZnCl2 is combined with CPs, the amount of ZnCl2 or ZnO (ZnCl2 + OH−  → ZnO + HCl) that remains in the pore structure may prevent ZnCl2 from volatilizing as the temperature rises, altering the carbon’s pyrolysis behavior and resulting in reduced char (Mittal et al. 2015). At the same time, the carbon yield (%) is found to be higher as a ZnCl2 consequence inducing plenty of H and O atoms in the carbon precursor to be removed as H2O rather than developing in hydrocarbons or oxygenated organic matter (Jia et al. 2002).

Moisture% and ash content

The produced activated carbon’s effectiveness is gauged by its moisture and ash contents. As can be shown in Table 2, the CP (precursor of ZCPs) has a very low moisture and ash content, demonstrating its great activation efficiency.

Table 2 Physicochemical characters of the biosorbents
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Surface acidity, pHPZC and Boehm titration

The pH of the carbon substance aqueous solution supplies a suitable indicator of the chemical parameters type and concentration for the investigated ACs.

As presented in Table 2, it can be revealed that: -

  1. (i)

    The pH of the non-active carbon (CPs) surface is neutral, 6.23.

  2. (ii)

    The pH of surface of ZCPs activated carbons are 4.98, 4.86, and 4.59 for ZCP11, ZCP21, and ZCP31, respectively. This suggests that the surfaces have a modest predominance of acidic functional groups.

The pHpzc, the point of zero charges, is the value of pH at which the totality of positives and negatives charges equals zero on the investigated substance’s surface (Jia et al. 2002). At a pH < the carbon pHPZC, its surface is positively charged while it is negative at a pH > the carbon pHPZC. The pHPZC of ZCP11, ZCP21, and ZCP31 is 5.47, 5.59, and 5.73, respectively, as presented in Table 2.

Boehm technique depends on the selective neutralization of the functional groups on the surface in accordance with their acidity because it is known that a functional group with a particular pKa may only be neutralized by a base with a greater value of pKa (Boehm 1994). Bases like sodium hydroxide, NaOH (pKa = 15.74), sodium carbonate, Na2CO3 (pKa = 10.25), sodium bicarbonate, NaHCO3 (pKa = 6.37), and sodium ethoxide, NaOC2H5 (pKa = 20.58) are utilized. NaHCO3 neutralizes carboxylic groups, whereas lactones were neutralized by Na2CO3 but not by NaHCO3. Phenols were neutralized by NaOH but not by Na2CO3. The neutralization of the surface with NaOH revealed all of the acidic functional groups, including carbonyl groups. The basic groups on the surface of the activated carbon, on the other hand, were detected using HCl.

Table 2 depicts the CP and ZCPs biosorbents’ surface acidity and the various acidic groups’ existence. An investigation of the data in Table 2 reveals:

  1. (i)

    The overall surface area of acidic sites is greater than that of basic sites, confirming the acidic character of their surfaces. This observation was completely agreeing with the findings of pHSUP, a pHPZC that is also acidic.

  2. (ii)

    For ZnCl2 activated carbon samples, the total acidity increases with increasing the impregnation ratios of ZnCl2;

  3. (iii)

    Lactones >  > carboxylic > phenolic is the order of the acidic groups on the surface of all samples.

Characterization

Scanning electron microscopy (SEM)

The SEM images of ZCP11, ZCP21, and ZCP31 are depicted in Fig. 2(a–c), respectively. According to the SEM images, preliminary treatments that included impregnation, crushing, carbonization, and activation at varied temperatures may be to blame for the activated carbons’ uneven form, size, and lack of a clearly defined morphology. Due to the effect of various ZnCl2 activating agent ratios, pore development is observed in activated carbon samples. The SEM micrographs, which were taken at magnifications of 1500 × and 3500 x, are quite obvious in revealing the porous structure of the ZCPs.

Fig. 2
figure 2

SEM photographs of a ZCP11, b ZCP21, and c ZCP31

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FTIR spectra

FTIR is an analytical technique used to identify the surface functionalities in activated carbons. Figure 3 shows FTIR spectra of CP, ZCP31and BG-ZCP31 samples.

Fig. 3
figure 3

FTIR spectra of: a CP, b ZCP31and c BG-ZPC31

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In the FTIR of CP and ZCP31, Figure A and B, before BG adsorption, the carbon samples exhibit a wide band at around (3265–3445 cm−1) wavelength due to the hexagonal group’s O–H stretching mode and adsorbed H2O. Additionally, the carbon samples exhibit the shoulders at (2850–2950 cm−1), which are caused by aliphatic (C-H) molecules (Sain and Panthapulakkal 2006; Kennedy et al. 2005). The stretching vibration of the C-H in the methyl group is responsible for a definite band at 1595–1465 cm−1 (Sain and Panthapulakkal 2006). The bands that appear in the region of 1253.414–1015 cm−1 may be returned to the stretching vibration of ROH and C-O in acids, phenols, ethers, and esters (Southichak et al. 2006). Due to the superposition of several large overlapping bands, it is challenging to assign the bands in the range of 1000–1200 cm−1. This overlapping band has been attributed by some writers to the ether (symmetrical stretching vibrations), phenolic structures and epi-oxide that exist in various structural settings (Kennedy et al. 2004; Gómez-Serrano et al. 1994; Faix 1992). As present in (Fig. 3C), the ZCP31-BG FTIR spectrum was compared with that of CP and ZCP31 showed that there are bands at 3442, 2920, 1583, 1440 cm−1, assigned to O–H, –CH aliphatic, –C=C, –CH2 bending, respectively. Also, showed shifts in bands of the ZCP31-BG FTIR spectrum to higher and/or lower with the appearance or disappearance of other bands, which indicates the complete adsorption of the BG dye.

Adsorption studies

Effect of pH

pH can enormously influence the BG pollutant adsorption in aqueous solutions (Mansour et al. 2020; Jamshidi et al. 2016). The BG pollutant adsorption experiment was obtained over the pH range of 2–10 as shown in Fig. 4. The BG removal [R (%)] gradually increased as the pH increased from 2 to 6, and then it almost remained constant at pH from 7 to 10. The BG-loaded percentage on ZCPs is low in a very acidic pH medium (Fig. 4). The explanation is that, at low pH, protons (H+) dominate the competitive adsorption process on ZCPs. As illustrated in Table 2, the ZCPs pHPZC is 5.73. The ZCPs surface is negatively charged at a pH higher than its pHPZC which leads to promoting the adsorption of the BG. The pH at which material surface net charges equals 0 is called the pHPZC (Omrani and Nezamzadeh-Ejhieh 2020). The ZCP surface becomes positively charged and prohibits BG pollutant adsorption when the pH is lower than the ZCP pHPZC. As at a pH higher than the pHpzc, the net positive charges decreased which result in a decrease in the repulsion force between the BG pollutant and the ZCP surface. Subsequent experiments were performed at pH 7.0.

Fig. 4
figure 4

Effect of The pH on adsorption capacity of BG by CP, ZCP11, ZCP21, and ZCP31. (Co = 200 mg/l; dose = 0.05 g/50 ml, T = 25 °C)

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Effect of adsorbent dose

The ZCPs’ weight effect on the removal of the BG pollutant is represented in Fig. 5. It was noticed that the BG uptake increased as ZCPs’ weights were increased. When the ZCPs doses increased from 0.025 to 0.2 g, the removal efficiency of the BG increased significantly from 9.49 to 76.13%, from 18.61 to 97.82%, 19.42 to 98.14% and from 25.03 to 99.97% for CP, ZCP11, ZCP21, and ZCP31, respectively. With the ZCPs’ doses increasing from 0.025g to 0.05g, a rapid increase in BG elimination was observed. While increasing the ZCPs doses over 0.1g, the BG removal efficiency not significantly changed. Thus, a 0.05g dose of the ZCPs can be chosen as the optimum dose. The observed BG removal efficiency increase with the ZCPs doses increases may be returned to the surface area and available active sites increase.

Fig. 5
figure 5

Effect of the CP, ZCP11, ZCP21, and ZCP31 dosage on the adsorption capacity of BG. (Co = 200 mg/l; T = 25 °C)

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Effect of initial BG concentration

The initial BG concentration’s influence on the CP and ZCPs material adsorbing capacity to the studied pollutant (BG) was examined, as displayed in Fig. 6. Once the dye concentration was raised, it was determined that the CP and ZCPs’ capacity to adsorb the BG increased as well. As the concentration increased from 50 to 500 mg/l, the sorption capacity, qe, increased from 42.092 to 75.28 mg/g for CP, from 46.067 to 200.185 mg/g for ZCP11, from 47.57 to 210.11 mg/g for ZCP21, and from 49.719 to 247.75 mg/g for ZCP31. This may be caused by the rise in the adsorbate’s concentration as an impetus to overcome the BG’s mass transfer barrier between the solid and liquid phases. However, it should be noticed that as the initial BG concentration (Co) increased from 50 to 500mg/l, the R (%) was lowered, in spite of the carbons uptake capability (qe) increased. The active sites’ saturation with an increase in the initial concentration of BG may be the reason for the dye removal (%) decrease at Co = 500 mg/l.

Fig. 6
figure 6

Effect of The the initial dye concentration on adsorption capacity of BG by ZCPs. (dose = 0.05 g/50 ml, T = 25 °C)

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Adsorption isotherms

The adsorption isothermal study is applied to relate the adsorbed BG amount at the interface and the adsorbent (ZCPs) concentration in the bulk solution. These investigations are very important for the design of BG adsorption onto the ZCPs mechanism. The R2, correlation coefficient, is utilized to detect the most fitting isotherm model to aid in the description of the BG adsorption process.

Langmuir isotherm

Experimental data may be linearized using the linear form as seen in Fig. 7 by plotting Ce/qe against Ce. For CP, ZCP11, ZCP21, and ZCP31, respectively, the qm (mg/g), which measures the highest adsorption capacity of ZCPs, is found to be 74.98, 192.02, 201.02, and 247.75. Furthermore, all values of the separation factor (RL) in the present study ranged between 0.003 and 0.012 confirming that all adsorbents exhibit favorable BG adsorption.

Fig. 7
figure 7

Langmuir model for brilliant green dye onto ZCPs

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Freundlich isotherm

Figure 8 illustrates the linearized Freundlich model for the BG adsorption onto the ZCPs materials.

Fig. 8
figure 8

Freundlich model for brilliant green dye onto ZCPs

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Table 3 represents the experimental results of the isotherm parameters. R2 of both Langmuir and Freundlich models for ZCPs are 0.999 and 0.969, respectively. Langmuir is well fitted to BG adsorption data which suggests that the BG adsorption into ZCPs is a monolayer process.

Table 3 Parameters of Freundlich and Langmuir isotherm for BG adsorption by CP, ZCP11, ZCP21and ZCP31
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Adsorption kinetics

Kinetic investigations provide essential information that assists in the explanation of the BG dye and the ZCPs interaction mechanism. In order to determine the equilibrium time (qt) for BG adsorption by the ZCPs, the shaking time effect was investigated. As shown in Fig. 9 all kinetic curves for BG adsorption by the ZCPs biosorbents are very similar and formed of rapid initial adsorption followed by very slow diffusion. This may be attributed to the availability of a higher surface area at the beginning. Thus 80% of the adsorption was obtained in the first hour while the adsorption reaches its equilibrium after 2 h.

Fig. 9
figure 9

Effect of contact time on adsorption capacity of BG by ZCPs. (Co = 200 mg/l; dose = 0.05 g/50 ml, T = 25 °C)

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Various models were utilized to investigate the behavior of the BG adsorption on the ZCPs as presented in Table 4 and Fig. 10(a–b) like Boyed, intraparticle diffusion, the pseudo-first-order, and pseudo-second-order models. According to the values of the R2, the pseudo-second-order model matches better than the pseudo-first-order model which indicates the BG adsorption onto the ZCPs is chemisorption. By applying the intraparticle diffusion kinetic model it was proved that there are three main steps implicated in the BG adsorption onto ZCPs biosorbents. The first step is the most rapid and explained as follows BG particles diffused to the ZCPs surface. The second linear stage is an intraparticle BG diffusion inside the ZCPs pores. The third, slow, and final step is a diffusion of the BG particles into the inner micropores of the biosorbents. Furthermore, as it can be noticed from Fig. 10(c–d) none of the straight lines in the intraparticle diffusion and Boyed models passed through the origin indicating that there are other limiting factors that participate in the adsorption process in addition to intraparticle diffusion.

Table 4 Kinetic parameters for the BG adsorption adsorption
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Fig. 10
figure 10

Kinetic models a Pseudo-first-order; b Pseudo-second-order; c Intraparticle diffusion, and d Boyd plot

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Effect of temperature

The effects of temperature on adsorption were investigated at 298, 308 and 318 k. The results are displayed in Table 5. As it can be noticed, the maximum amounts of BG adsorbed by CP, ZCP11, ZCP21, and ZCP31 carbons are found to have raised when the temperature was changed from 298 to 318 k, from 75.280 to 89.292 mg/g, from 192.022 to 208.103 mg/g, from 210.11 to 218.878 mg/g, and from 247.75 to 254.161 mg/g for CP, ZCP11, ZCP21 and ZCP31, respectively. The increase in adsorption capacity may be brought about by chemical reactions between adsorbent sites and adsorbate or by the raised rate of BG molecule intraparticle diffusion into the carbon pores at higher temperatures (Tang et al. 2019).

Table 5 Temperature effect on maximum adsorption capacities of BG by CP, ZCP11, ZCP21and ZCP31
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Thermodynamic studies

The thermodynamic parameters (ΔG0, ΔH0, and ΔS0) study reveals a lot of information on internal energetic changes and they are calculated as was reported in Sect. "Thermodynamic Studies". As presented in Table 6, it can be detected that with the temperature increasing from 25 to 45°C, the maximum adsorption capacity increased, which specifies an endothermic reaction of the adsorption process for BG on ZCPs. As shown in Fig. 11, the plot of qe/Ce vs T−1 was linear over the utilized temperature range.

Table 6 Thermodynamic factors for the BG adsorption by CP, ZCP11, ZCP21, and ZCP31 at various temperatures
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Fig. 11
figure 11

Thermodynamic parameters (lnKc vs 1/T) for the adsorption of BG dye onto ZCPs

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In Table 6, the positive ΔH0 and negative ΔG0 values affirm the BG adsorption on the ZCPs to be an endothermic process and spontaneous, respectively. While the positive ΔS0 value is derived from the enhancive randomness on the solid–liquid interphase. In addition, the increase of the ΔG0 absolute value with temperature increase illustrates that the rising temperature can increase the adsorption spontaneity.

Effect of ionic strength

The influence of ionic strength on the adsorption of BG is shown in Fig. 12. It was shown that when ionic strength increased, the adsorption capacity decreased. The adsorption capabilities for CP, ZCP11, ZCP21, and ZCP31 fell from 77.628 to 52.799 mg/g, from 162.015 to 141.20 mg/g, from 168.021 to 144.131 mg/g, and from 183.419 to 162.417 mg/g, respectively, when the concentration of NaCl increased from 0.05 to 0.5 M. The BG and cations may have been competing with one another for the active sites on the adsorbent surface, which could account for this.

Fig. 12
figure 12

Effect of Ionic strength effect on adsorption capacity of BG by CP, ZCP11, ZCP21, and ZCP31. (Co = 200 mg/l; dose = 0.05 g/50 ml, T = 25 °C)

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Desorption and reusability studies

Desorption of BG from the BG loaded ZCPs was carried out using 0.2 and 0.4 M HCl. The adsorbent was treated with 50 ml of 0.2 or 0.4 M HCl and stirred until equilibrium to remove the adsorbed BG was collected after being loaded with 100 mg/l of BG at the ideal pH. Table 7 shows that the percent recovery of BG for the adsorbents ZCP11, ZCP31, and CP increased with an increase in HCl from 0.2 to 0.4 M and thereafter stayed constant. The percentage of desorption is shown to have increased from 42.11 to 76.835%, from 47.099 to 81.466% and from 54.475 to 93.847% for ZCP11, ZCP31 and CP, respectively.

Table 7 Desorption of BG from ZCPs using HCl (0.2–0.4M)
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As presented in Table 8, the reusability study further demonstrated that the sorbent may be utilized again for 5 times with only a little reduction in adsorption capacity.

Table 8 Repeated adsorption cycles of BG by ZCP31 dose (0.025 g), time of shaking 400 min
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Plausible mechanism of adsorption of BG onto ZCPs

The BG adsorption on the ZCPs biosorbents might be returned to numerous interactions like pore-filling, electrostatic attraction, π–π, and H-bonding interactions (Cheng et al. 2021a, b). A schematic representation of the probable BG adsorption process onto ZCPs is presented in Figs. 13 and 14. The BG adsorption occurred at pH greater than the pHpzc; hence, the ZCPs surface will gain negative charges that interact with the positive charges on the BG this interaction is called electrostatic forces (Fig. 14a) (Cheng et al. 2021a, b). Also, the molecules of the BG can be entered into the ZCPs structure through their micropores and mesopores, and thus, they will subsequently adsorb onto the active sites this process is called pore-filling. The nitrogen atoms of the brilliant green interacted with the hydrogen of the ZCPs’ OH groups in order to form the hydrogen bond (Fig. 14b) (Cheng et al. 2021a, 2021b). π–π interactions (Fig. 14c) occurred through BG’s aromatic backbone interacting with the ZCPs hexagonal skeleton (Cheng et al. 2021a, b).

Fig. 13
figure 13

Schematic illustration of the potential BG adsorption process onto ZCPs

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Fig. 14
figure 14

Plausible mechanism of adsorption of BG onto ZCPs biosorbents

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Analytical applications

Analysis of real water samples

Bidistilled water and tap water samples were collected from our lab (Saba Region University, Marib, Yemen), Wastewater was collected from sewage drainage station- Ibb, Yemen and underground water was collected from Ibb city, Yemen. A sintered glass G4 filter was used to filter all of the samples. All of the selected samples were then preserved in plastic containers for later use after being acidified with strong nitric acid to pH 2. The organic matter was digested using K2S2O8 (0.5–1.0g) which was added to 1L of the chosen water sample and then the mixture was heated for 30 min at 95 °C. After cooling to 25 °C, 80 mg of ZCPs biosorbents were added to a series of transparent stoppered bottles containing different concentrations of BG (0.0, 5, and 10 ppm) at optimum pH and They were shaken at 150 rpm on a shaker for 4 h and then filtered. Another dose of the ZCPs biosorbents (80 mg) was added to the previous filtrate and the pH was adapted to the optimum then the mixture was stirred for 15 min and filtered. The results that are present in Table 9 showed that the prepared ZCPs were successfully applied for the removal of known amounts of BG spiked to different natural water samples. The recovery % ranged between 70.00 and 82.0% for CP and between 90.7 and 98.0 % for the ZCP31 with a relative standard deviation lower than 3 (RSD, %, <3). The noticeable increase in the recovery % using ZCP31 can be returned to its modification by ZnCl2.

Table 9 Recovery of BG from different water samples using unmodified carbon (CP) and zinc chloride activated carbon (ZCP31) (n = 5)
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Performance of the prepared ZCPs

In order to enhance our adsorbents value, we carried out a comparative study of the maximum adsorption capacity obtained for the same pollutant to other adsorbents and activated carbons reported in the literature. Table 10 represents the different qmax values for BG adsorption by different adsorbents. It can be observed that the BG dye adsorption with ZCPs was obtained with acceptable values. The differences in the BG uptake values return to many factors such as the adsorbent structure, surface area, and the functional groups. ZCPs could be an attractive adsorbent for basic dyes owing to their isoelectric point pHpzc.

Table 10 Comparison of maximum sorption capacity of BG by proposed ZCPS with some of the previously published articles
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Conclusion

Here, an investigation of the derived date pits’ activated carbon usage for brilliant green removal was obtained. The following findings were obtained based on the experimental results from the current study:

  1. (1)

    The prepared activated Carbons (CPs and ZCPs) were characterized by the BET, SEM, and FTIR techniques.

  2. (2)

    The activated carbon SBET value increased with increasing the ZnCl2 impregnation ratio, it raised from 289.51 m2/g for ZCP11 material to 667.89 m2/g for ZCP31 material.

  3. (3)

    The ZCPs efficiency was examined for the decolonization of BG; the influence of the solution’s initial pH, dye (BG) concentration, contact time, adsorbent dose, and temperature on the adsorption of BG was investigated.

  4. (4)

    As the BG initial concentration increased, the ZCPs adsorption efficiency for BG increased, whereas the optimum shaking time and pH were shown to be 2 h and 7, respectively.

  5. (5)

    The BG adsorption data are well matched with the Langmuir isothermal and the pseudo-2nd-order kinetic models.

  6. (6)

    The BG adsorption onto ZCPs biosorbents is a spontaneous and endothermic process, based on negative ΔGo and positive ΔHo values. Meanwhile the positive ΔSo value describes that the randomization increased at the solid (ZCPs)-liquid (BG solution) interface.

  7. (7)

    The plausible mechanism of BG loading/adsorption onto the prepared ZCPs might be assigned to various interactions such as pore-filling, electrostatic attraction, H-bonding, and π–π. Figure 15 summaries the synthesis of ZCPs and their use for adsorption of BG.

Fig. 15
figure 15

Synthesis of ZCPs and its use for adsorption of BG

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