Conversion of Industrial Sludge into Activated Biochar for Effective Cationic Dye Removal: Characterization and Adsorption Properties Assessment

Abstract

This paper presents an in-depth characterization of a raw industrial sludge (IS-R) and its KOH-activated biochar pyrolyzed at 750 °C (IS-KOH-B) followed by their application to remove a cationic dye from aqueous solution. Materials characterization shows that compared to the IS-R, the IS-KOH-B has improved structural, textural, and surface chemical properties. In particular, the IS-KOH-B’s BET surface area and total pore volume are about 78 and 6 times higher than those found for the IS-R, respectively. The activated biochar efficiently retained the cationic dye under wide experimental conditions. Indeed, for an initial dye concentration of 50 mg L⁻¹, removal yields were assessed to be more than 92.5%, 93.5%, and 97.8% for a large pH range (4–10), in the presence of high contents of competing cations (3000 mg L⁻¹ of Ca²⁺, Mg²⁺, Na⁺, and K⁺), and a low used adsorbent dose (1 g L⁻¹), respectively. The Langmuir’s adsorption capacities were 48.5 and 65.9 mg g⁻¹ for of IS-R and IS-KOH-B, respectively, which are higher than those reported for various adsorbents in the literature. The dye removal was found to be monolayer, spontaneous, and endothermic for both the adsorbents. Moreover, this removal process seems to be controlled by chemical reactions for IS-KOH-B whereas by both physico–chemical reactions for IS-R. This study demonstrates that the raw industrial sludge and especially its KOH-activated derived biochar could be considered as promising adsorbents for the removal of dyes from aqueous solutions.

1. Introduction

The presence of dissolved dyes in effluents from various industries such as leather, textile, paper, and paint represents a serious threat to the environment [1]. Huge research has been carried out during the recent decades to find suitable solutions/technologies for the removal of dyes from wastewater. These techniques include individual or coupled technologies such as coagulation-flocculation [2], biological degradation [3], membrane filtration [4], and advanced oxidation processes [5]. Dye removal from aqueous solutions through adsorption process has been highly recommended due to its simple design, easy operation, and the abundance of low-cost materials that could transformed into effective adsorbents [6]. Various raw and modified materials have been investigated for the elimination of dyes from industrial effluents. They include lignocellulosic materials [7,8], animal biomasses [9], activated carbons [10], graphene [11,12], magnetic nanomaterials [13,14], and biochars [15]. Biochars, the carbonaceous materials obtained from the thermochemical conversion of biomasses through pyrolysis (in the absence of oxygen), have been pointed out as attractive materials for an efficient removal of various contaminants from industrial effluents [16,17].

Huge amounts of sludge are annually produced in the world. The Europe Union and China are the biggest producers with respective quantities of about 55 and 39 million tons in 2019 [18,19]. Almost half of the sludge produced in China is being incinerated or disposed of in landfills. Therefore, the integrated management of these solid wastes has been considered an important challenge worldwide. During the past decade, the transformation of sludge into biochars through pyrolysis for a subsequent use in the agriculture, energy, and environment sectors has been highlighted as an attractive and eco-friendly strategy [17,20,21]. Biochar derived from raw sludge has been used for the removal of various cationic and anionic dyes from aqueous solutions [22,23]. However, pre-modification of the raw sludge could significantly improve the physico–chemical properties of the generated biochars and consequently their capacities to remove dyes. This process improves various properties including pH of zero-point charge (pHzpc), specific surface area, microporosity volume, and the functional groups present at the biochars’ surface. Depending on the nature of the targeted dye, various pre-treatments have been tested, including the use of acidic solutions such as HNO₃, HCl, CH₃COOH, and H₂SO₄ [24,25], alkali solutions as NaOH, KOH, and K₂CO₃ [26,27], and saline solutions such as ZnCl₂, FeCl₃, MgCl₂, and AlCl₃ [24,28]. These studies have pointed out that basic solutions, particularly KOH, were the most efficient modification reagents due to their positive impact on the physico–chemical characteristics of the generated biochars [17,29]. For instance, Zhang et al. [30] studied the effect of the nature of the sludge-derived biochar (SDB) activation on its lead-removal efficiency from aqueous solution. They reported that compared to CO₂ (physical activation) and CH₃COOH (sludge/CH₃COOH mass ratio = 1:2), KOH activation (at the same ratio) significantly improved the SDB textural properties, especially surface area and microporosity. This led to the adsorption capacity of 57.5 mg g⁻¹, which is 2.6 and 1.2 times more than that of the SDB activated with CO₂ and CH₃COOH. Moreover, Gomez-Pacheco et al. [31] demonstrated that SDB activation with NaOH at a sludge/NaOH mass ratio of 2:1 increased its methylene blue dye adsorption capacity by a factor of 2.5. This was mainly imputed to the presence of basic functional groups at the surface of the activated SDB. These authors observed a similar trend for various types of pollutants, including antibiotic (tetracycline), pesticide (2,4 dichlorophenol), and a heavy metal (cadmium).

The intensive review of the published papers regarding the use of modified SDBs for wastewater treatment shows that only rare studies have used KOH-activated SDBs for dye removal form aqueous solutions [17,32]. Indeed, the majority of the reported works dealt with the use of these chemically modified SDBs for the removal of pharmaceuticals and heavy metals. Furthermore, these studies used high KOH:sludge mass ratios (>2) [30,33,34,35], which will return in a high cost of the activation process. Moreover, some of these studies lack a detailed characterization of the raw sludge or its modified form, which limited the clear appreciation of the involved mechanisms during pollutant removal [36]. Therefore, the current work emphasizes the synthesis of an activated SDB by using relatively low KOH:sludge mass ratio and afterwards its application for the removal of a cationic dye (methylene blue: MB) under wide experimental conditions. With this objective, a deep physico–chemical characterization of these two adsorbent materials (raw and modified) was firstly carried out through the use of different analytical techniques. Then, MB adsorption properties were assessed under various conditions (i.e., contact time, pH, adsorbents dosages, MB concentration, and temperature). Finally, the potential involved mechanisms in MB adsorption were discussed.

2. Materials and Methods

2.1. Biomass Preparation and Biochar Synthesis

Raw sludge was collected from an industrial wastewater treatment plant in Oman. It was firstly washed with deionized water in order to remove impurities. Then, it was dried overnight in an oven (Sanyo, Osaka, Japan) at 85 °C. This dried raw industrial sludge (IS-R) was stored in airtight containers for subsequent use. The synthesis of the KOH-activated biochar (IS-KOH-B) was performed according to the following consecutive steps: (i) the impregnation of 100 g of IS-R within 1 M KOH solution for 24 h by using a magnetic stirrer (Gallenkamp, Cambridge, UK), (ii) the separation of the impregnated solid phase through centrifugation at 3000 rpm by a Beckman centrifuge (Beckman, Indianapolis, IN, USA), (iii) overnight drying at 85 °C, (iv) the pyrolysis of the resulting dried material at 750 °C for 2 h at 5 °C/min in N₂ atmosphere by a using a tubular furnace (TF1-1200, Carbolite, Hope Valley, UK), (v) the neutralization of this KOH-activated biochar with 3 M HCl solution as applied by Ding et al. [37] for a dosage of 50 g L⁻¹, (vi) overnight drying at 85 °C, and finally (vii) the grinding and sieving of the dried solid matrix. Only the solid fraction with diameters lower than 250 µm was e in this work.

2.2. Adsorbent Materials Characterization

The raw sludge as well as the KOH-activated biochar were characterized to assess the following parameters:

2.2.1. Morphology and Structure

The morphology of IS-R and IS-KOH-B was assessed through the use of a scanning electron microscope (SEM) and an energy-dispersive X-Ray spectrometer (EDS) (Jeol, Jsm-7800F, Tokyo, Japan). Moreover, X-ray diffraction (XRD) analysis was used to determine the crystalline structure of these two adsorbent materials. This analysis was performed by a Panalytical X’Pert powder diffractometer equipped with a copper anode (Rigaku, Miniflex 600, Hokuto City, Japan) with diffraction angles (2θ) varying between 10° and 80°. The obtained diffractograms examination and phases identification were determined by using both the Panalytical X’Pert HighScore software and the database established by the international center for diffraction data (ICDD).

2.2.2. Mineral Composition

The contents of the main inorganic elements of IS-R- and IS-KOH-B, including K, Ca, Fe, and P, were determined by using an X-ray fluorescence (XRF) apparatus with a rhodium target X-ray tube (Rigaku, Nexqc, Hokuto City, Japan).

2.2.3. Textural Properties

For both adsorbent materials, the BET surface area as well as the pore-size distribution were determined through the exploitation of N₂ adsorption/desorption isotherm at 77 K. This analysis was performed by a micrometrics, ASAP-2020 apparatus.

2.2.4. Surface Chemical Properties

The surface chemical properties of both adsorbents were assessed using two complementary tests. The first one consisted of the determination of the pH of zero-point charge (pHzpc) by using the pH drift method [38]. It involved the agitation using a mechanical shaker (Edmund Buhler) for 24 h of 0.2 g of the solid materials in 50 mL of 0.1 M NaCl solutions at initial pH values (pHi) varying between 3 and 11. The pHzpc values were deduced from the graph giving the difference between the final pH values (pH_f) and pH_i (∆pH = pH_f – pH_i) versus pH_i. They corresponded to the intersection of this curve with the abscise axis (pH_i). The second test concerned the determination of the existing functional groups on the adsorbent materials surface before and after MB adsorption. It was determined by using a Fourier transform infra-red (FTIR) apparatus (Perkin Elmer, Frontier, MA, USA). For each sample, an adsorbent material to KBr mass ratio of 1/200 was ground and pressed into 1 cm diameter disk with 3.5 tones pressure. This sample was then analyzed by the FTIR device at a spectral resolution of 1 cm⁻¹, for wavenumbers between 400 and 4000 cm⁻¹.

2.3. Batch Adsorption Experiments

2.3.1. Dye Solutions Preparation and Analysis

The dye used in this study was methylene blue. It was purchased from Sigma-Aldrich (Champaign, IL, USA). During the adsorption tests, a stock MB solution of 2000 mg L⁻¹ was prepared and used for the preparation of the required solutions. MB removal efficiency by the two tested adsorbent materials was followed through the measure of the solutions’ absorbance decrease at 664 nm by an UV spectrophotometer (Perkin Elmer, Lambda 12, Crystal City, DC, USA).

2.3.2. Adsorption Assays

Batch MB removal assays were carried out in order to get a precise idea about the enhancement of MB removal when using the KOH-activated biochar instead of the raw sludge. During these assays, a given mass of the tested adsorbent materials was shaken in 100 mL of MB solution at a desired concentration for a fixed time at 600 rpm by a magnetic shaker (IKA, RO15, Staufen im Breisgau, Germany). If not specified, the following default parameters were used during this study: a MB concentration of 50 mg L⁻¹, an initial pH of 6.8 (natural: without adjustment), a contact time of 3 h, an adsorbent dosage of 1 g L⁻¹, and a temperature of 20 ± 2 °C. During this work, the effect of the following parameters was assessed: (i) contact time for durations of 1, 5, 10, 20, 40, 60, 90, 120, 150, and 180 min, (ii) initial pH for values of 4, 5, 6.8 (natural), 8, and 10, (iii) adsorbents dosages for values between 0.2 and 1.0 g L⁻¹ for IS-KOH-B and 0.2 and 3.0 g L⁻¹ for IS-R, (iv) presence of sodium, potassium, calcium, magnesium and all of these components together at respective concentration of 3000 mg L⁻¹ by using NaCl, KCl, CaCl₂, and MgCl₂ analytical-grade salts, (v) initial MB concentrations between 10–50 mg L⁻¹, and 10–100 mg L⁻¹ for IS-R and IS-KOH-B, respectively, and (vi) aqueous solutions temperatures for values of 20, 30, 35, and 40 °C.

The MB adsorbed amount at a precise time ‘t’, (q_t) and the corresponding removal yield (Y_t) were determined by the following Equations (1) and (2):

$$q_t=\frac{\left(C_0-C_t\right)}{D}$$

(1)

$$Y_t(\%)=\frac{\left(C_0-C_t\right)}{C_0}\times 100$$

(2)

where C₀ and C_t (mg L⁻¹) are, the initial and at a time ‘t’, dyes concentrations, respectively, and D is the used adsorbent dosage (g L⁻¹).

On the other hand, the thermodynamic parameters associated with MB adsorption process, namely standard enthalpy ΔH (kJ mol⁻¹), Gibbs free energy ΔG (kJ mol⁻¹), and entropy ΔS (kJ mol⁻¹ K⁻¹), were deduced form the following equations:

$$\mathit{ln}\left(Kc\right)=\frac{\mathsf{\Delta }S}{R}-\frac{\mathsf{\Delta }H}{RT}$$

(3)

$$\mathsf{\Delta }G=\mathsf{\Delta }H-T\mathsf{\Delta }S$$

(4)

$$Kc=\frac{q_e}{C_e}$$

(5)

where R is gas constant (8.314 J mol⁻¹ K⁻¹), T is the temperature (K), and Kc is the standard thermodynamic equilibrium constant (L g⁻¹). C_e and q_e are the pollutant concentration in the aqueous solution and onto the adsorbent material at equilibrium. The values of ΔH and ΔS were calculated from slopes and intercepts value of linear plots of (ln (K)) versus (1/T).

2.3.3. Kinetic and Isotherm Data Modelling

The experimental data regarding the MB removal kinetic were fitted to three famous models, namely pseudo-second order (PSO), pseudo-first order (PFO), and film and intraparticle diffusion models. Details regarding these models were widely cited in the scientific literature [17,39]. Moreover, the measured isothermal data were fitted to three well-known models namely: Freundlich, Langmuir and Dubinin–Radushkevich (D-R) models. Details regarding the original and linearized equations of these models as well as their assumptions are intensively cited in the bibliography [40,41]. The concordance between the experimental and the models’ theoretical data was appreciated on the basis of both determination coefficients of the corresponding curves and the calculated mean absolute percentage errors (MAPE) values:

$$MAP{E}_{kinetic}(\%)=\frac{\sum \left|\left(q_{t,exp}-q_{t,calc}\right)/q_{t,exp}\right|}{N}\times 100$$

(6)

$$MAP{E}_{isotherm}(\%)=\frac{\sum \left|\left(q_{e,exp}-q_{e,calc}\right)/q_{e,exp}\right|}{N}\times 100$$

(7)

where q_t,calc and q_t,exp (mg g⁻¹) are the calculated and measured MB adsorbed amounts at the instant ‘t’, and q_e,calc and q_e,exp (mg g⁻¹) are the estimated and the measured adsorbed MB amounts at equilibrium. N is the number of experimental data.

During this study, all the adsorption assays were carried out in triplicates and the mean values were presented in this work. The standard deviation was lower than 3%.

3. Results and Discussion

3.1. Adsorbent Materials Chracterization

The morphological properties of the two adsorbent materials were assessed through SEM/EDS analysis (Figure 1).

It appears that the IS-R presented a heterogeneous structure with a significant concentration of organic matter conglomerates on its surface (Figure 1a). After KOH impregnation and thermal activation at 750 °C, obtained activated SDB showed a more homogeneous structure with less sharp forms on particle edges. Moreover, it contained fewer conglomerated surface forms with the presence of some porosity (Figure 1b). This result could be imputed to the disintegration of the contained volatile organic matter [42] into soluble short sugar molecules caused by the catalytic effect of basic entities (i.e., KOH) [38]. The mechanism of the porosity development of the activated biochar can be simplified as in the following reactions [43,44,45]:

2KOH → K₂O + H₂O

(8)

C + 4 KOH → K₂O +2 H₂ + K₂CO₃

(9)

K₂CO₃ → K₂O + CO₂

(10)

With the increase in pyrolysis temperature, the dehydration of KOH leads to the formation of K₂O and H₂O (Equation (8)). For temperatures lower than 600 °C, KOH could also react with the biomass carbon to produce K₂O, K₂CO₃ and H₂ (Equation (9)). When the pyrolysis temperature exceeds 700 °C, K₂CO₃ is generally transformed into K₂O and CO₂ (Equation (10)). The formed CO₂ gas could significantly impact the internal structure of the biochar leading to the formation of micropores [43]. The quantitative assessment of K contents as well as porosity increase in the activated biochars (in comparison with the raw biomass) will be developed in the sections below when discussing the XRF, and BET analyses results, respectively.

On the other hand, EDS results (Figure S1) showed that the IS-R presented a typical composition of an industrial sewage sludge with the presence of important peaks of oxygen and carbon. A significant peak of iron was also recorded in this adsorbent material, along with other metals and alkaline and alkaline earth metals (AAEM’s) in trace quantities such as Al, Ca, and Mg. This relatively high Fe content can be mainly linked to the use of iron chloride in coagulation-flocculation process during the effluent treatment in the WWTP from which this sludge has been collected [20]. After the pyrolysis process, carbon peak intensity increased while that of oxygen decreased compared to IS-R (Figure S1). This is mainly attributed to the condensation of organic matter into more stable and fixed carbon during the pyrolysis process [46]. Moreover, the effect of the IS-R activation with KOH can be clearly seen in Figure S1b where a relatively important peak of K appeared at 3.15 Kev.

The mineral composition of the IS-R and IS-KOH-B (

Table 1. Mineral composition of IS-R and IS-KOH-B (mg g⁻¹).

Adsorbent Material	K	Ca	P	Fe	Al	Si	Ni	Cl	S	O
IS-R	1.8	10.4	25.4	84.9	24.3	33.2	0.235	0.929	28.6	787.0
IS-KOH-B	57.2	21.1	7.55	140.0	26.8	85.0	0.279	1.440	11.8	630.0

) showed that the raw sludge contained relatively high contents of Fe, Si, Al, P, and Ca, which is consistent with the literature [47]. The thermo–chemical activation of the IS-R significantly changed its composition. As expected, KOH activation increased the K content in biochar by a factor of about 32 (Table 1). This result, consistent with the EDS analysis, is favored by the small K ion radius (2.8 Å) allowing its easy incorporation in the structure of the industrial sludge derived biochar. Likewise, industrial sludge activation resulted in a significant increase of Ca, Fe, Al, Si, Ni, and Cl contents by about 102%, 65%, 10%, 156%, 19%, and 55%, respectively (Table 1). It has been reported that thermal conversion of biomass into carbonaceous materials allows the elimination of volatile matter and, therefore, the fixation of some minerals into the aromatic structure through electrostatic bonds [46]. On the other hand, oxygen content decreased by about 20% after pyrolysis. This is expected since the increase in pyrolysis temperature reduces the oxygenic content through the release of carbon monoxide, carbon dioxide, and water vapor [48,49]. Similar trend was observed for sulfur and phosphorus contents, which dropped by 59% and 70%, respectively. This is mainly due to the escape of Sulphur in the form of CS₂ and phosphorus in elementary form [44].

To better understand the effect of the activation process on the structural properties of the biochars, XRD analyses were performed on the IS-R and IS-KOH-B (Figure S2). For the IS-R, the initial structure was rather amorphous with the presence of few peaks attributed to cellulose I at 21.69° (2θ) and Silica (SiO₂; COD 4115457) at 26.82° (011) and 32.62° (110) (2θ), respectively. These two peaks are commonly present in raw sludge. Amorphous cellulose is generally derived from vegetal biomass debris, while silica origin could be linked to either the wastewater treatment steps or the surrounding dust. This observation suggests that the minerals detected using the EDS analysis might be mainly soluble in water and were not sufficiently crystallized to be detected in a well-defined crystalline structure. After the pyrolysis, cellulose structure completely disappeared, indicating the degradation of this organic structure [46]. On the other hand, few sharp peaks related to the rearrangement of iron in the structure of the biochar appeared. For instance, peaks detected at 44.62° (121) and 64.94° (212) (2θ) can be attributed to Kamacite (Fe, Ni; ICDD: 00-037-0474). This result confirms the XRF analysis in which Ni was detected at a content of 0.279 mg/g (Table 1). Other peaks observed at 43.00° (110), 50.69° (200) and 74.96° (211) (2θ) were identified as iron carbide (Fe₃C; ICDD: 00-044-1290) [50] (Figure S2). Since chlorine is highly volatile during the pyrolysis step, iron ions were incorporated into the structure of the biochar, which is further favored by the development of the microporosity.

The BET curves (Figure S3) emphasize the significant effect of the potassium-based impregnation and the pyrolysis process on the porosity of these materials (Figure S3a). The IS-R presented a Type III isotherm curve with an exponential increase of adsorbed N₂ by increasing the relative pressure (Figure S3b). This typical curve indicates the presence of a low macroporous structure with a significant interaction of N₂/N₂ atoms on the surface of the raw sludge [51]. The BET specific surface area was about 2 m²/g, which is in agreement with the values of raw sludge in the literature [52,53]. This relatively low surface area might be due to the nature of this adsorbent material, which is composed of aggregates that do not present an arranged aromatic structure or developed porosity [54].

After the activation step, the isotherm of IS-B-KOH presented a Type IV adsorption curve indicating monolayer formation for P/P0 between 0.05 and 0.4. Afterwards, the N₂ adsorption increased exponentially following the multilayer mechanism. The absence of an equilibrium “knee” near the saturation point (P/P0 = 1) indicates that IS-B-KOH structure favors an unlimited multilayer adsorption of gas via a developed mesoporous structure [55]. The hysteresis curve supports this assumption as it could be qualified of Type H3 usually indicating a heterogeneous and non-uniform porous profile with the significant presence of microporosity [56]. The thermo–chemical treatment of IS-R increased its BET surface area by a factor of about 78, reaching a value of 157 m² g⁻¹. Moreover, the chemical/thermal treatment of the IS-R increased its total pore volume (TPV) by a factor of 6 from 0.019 cm³ g⁻¹ to 0.119 cm³ g⁻¹ for the activated biochar. This activation process also significantly reduced the average pore diameter from 388 Å for the IS-R to less than 98 Å for IS-KOH-B. Micropores formation is generally favored by the significant reactivity of potassium with organic structure at carbonization temperatures between 0 to 900 °C, leading to the scouring of carbon skeleton for the formation of CO, CO₂, and H₂ gases [57]. Similar textural parameters improvements were reported in previous studies [43,44,58].

On the other hand, the industrial sludge chemical and thermal treatment significantly affected its chemical surface properties (pH zero-point charge and functional groups). Indeed, the activated biochar had a pHzpc of 10.8, which is about 4.4 units higher than that of the raw material. This increase can be linked to the desorption and/or decomposition of acidic functional groups and the increase in the contents of alkali metal salts during the pyrolysis process [26,40]. A relatively comparable value (10.6) was reported for a biochar derived from the pyrolysis at 550 °C of an anaerobic digested sludge [53]. For cationic dyes (as methylene blue), their adsorption would be favored at aqueous pH values higher than pHzpc, where the surface particles will be negatively charged. Therefore, MB adsorption by IS-R would be favored for a larger interval. The FTIR analyses (Figure 2) shows that the IS-R presented a heterogeneous surface structure with a large number of acid and basic surface functional groups. Peaks detected at 3391, 2923, 1650, 1540, 1338, 1034 cm⁻¹, correspond to –OH, =C–H, C=O, C=C, –CH₂/CH₃, C=C and –C–O bands, respectively [40,49,59].

Significant changes were detected after the thermo–chemical activation process. Initially, a shifting of +23 cm⁻¹ was noticed for the peak –OH. A similar behavior was noted for –C=O and –C–O band that shifted by −60 cm⁻¹ and −34 cm⁻¹, respectively. These findings indicate that the pyrolysis process favored oxygen release and carbon fixation. A similar trend was reported by Wang et al. [44] when producing KHCO₃-activated biochars. Moreover, the peaks attributed to =C–H, –CH₂/CH₃ and C=C peaks disappeared or decreased in intensity after pyrolysis (Figure 2). These functional groups seem to be unstable at pyrolysis temperatures higher than 500 °C [53,59]. Moreover, according to Lillo-Rodenas [57], the activation of carbonaceous materials using alkali reagents, especially KOH, catalyze the release of gases such as CO, CO₂, H₂ and water vapor (see Equations (3)–(7)). In fact, the presence of AAEM, and especially potassium and sodium favors the dehydration and decarboxylation mechanisms [48]. This loss in oxygen-containing functional groups is consistent with the aforementioned EDS and XRF analyses. On the other hand, MB adsorption on IS-R and IS-KOH-B seems to impact their respective functional groups distribution (Figure 2). This aspect will be discussed later in Section 3.3, when exploring the main involved mechanisms.

3.2. Adsorption Study

3.2.1. Kinetic Behavior

The kinetic experimental results (Figure 3) showed that MB adsorption by the tested adsorbent materials is highly time-dependent. In fact, adsorption was relatively faster at the beginning for the KOH-modified biochar in which about 59.7% of MB was removed after only 20 min. This high rate adsorption step is mainly imputed to the fast diffusion of dyes molecules through the boundary layer. After this step, the removal of MB continued but at a slower rate (Figure 3). This second phase is attributed to intraparticle diffusion inside the pores of the IS-KOH-B [60]. For the IS-R, MB adsorption rate stayed almost similar (almost linear variation until a duration of 2.5 h), which suggests that MB adsorption occurs mainly at the surface of the particles for this adsorbent. The equilibrium time (saturation state) was observed after 3 h for both adsorbents. This duration is lower than the ones found by Chen et al. [49]: 8h; Yin et al. [24]: 12 h, Zhao et al. [28]: 24 h, and Fan et al. [61]: 24 h when studying MB removal by non-modified SDB, ZnCl₂ activated biochars derived from an urban and industrial sludge, and a non-activated biochar from a mixture of urban sludge and tea wastes, respectively. However, comparable or lower contact times were reported for biochars derived from the pyrolysis of a sludge mixed with rice husk powder: 3 h [62]; a Fe₂O₃-biochar nanocomposite derived from pulp and paper sludge: 40 min [63]; and a ZnCl₂ activated biochar from urban sludge: 30 min [64]. Having a low equilibrium contact time is an important asset when upscaling this process to real site conditions due to the associated benefits, including shorter treatment duration and lower energy consumption due to the stirring phase.

It is important to underline that at equilibrium, the adsorbed amount by IS-KOH-B was higher than the one of IS-R (Figure 3). This is due to the improvement of the physico–chemical characteristics of the IS-KOH-B after the KOH activation step. Indeed, as cited above, this operation significantly affected the adsorbent texture and surface chemistry (see Section 3.1). This adsorption capacity increase is further discussed in Section 3.2.5 when evaluating the adsorption isotherm results.

The MB kinetic adsorption parameters of the three used models are depicted in

Table 2. Kinetic models parameters for the adsorption of MB onto the industrial sludge (IS-R) and its KOH-activated biochar C0 = 50 mg L⁻¹; adsorbent dose = 1 g L⁻¹; natural pH; T = 20 ± 2 °C).

Model	Parameter	IS-R	IS-KOH-B
	qe,exp (mg g−1)	37.0	45.8
Pseudo first order model (PFO)	k1 (min−1)	0.011	0.018
	Correlation coefficient (R2)	0.988	0.892
	MAPE (%)	21.5	37.1
Pseudo second order model (PSO)	k2 (g.mg−1 min−1)	0.00038	0.00248
	qe,calc (mg g −1)	45.7	47.4
	Correlation coefficient (R2)	0.962	0.947
	MAPE (%)	20.3	14.3
Diffusion model (DM)	Df (×10−13 m2 s−1)	1.79	1.31
	Regression coefficient	0.968	0.881
	Dip (×10−13 m2 s−1)	5.94	1.60
	Regression coefficient	0.987	0.983

. It can be seen that the PFO model fits well to the experimental data of IS-R. Indeed, for this case, the correlation coefficient (R²) was relatively high (0.988) and the mean absolute percentage errors between the experimental and predicted data were close to those observed for the PSO model. Moreover, the theoretical adsorbed amount at equilibrium by the PSO (45.7 mg g⁻¹) was much higher than the experimental value (37.0 mg g⁻¹) (Table 2). This finding suggests that MB adsorption by IS-R is mainly physical [65]. This aspect will be discussed later in Section 3.2.6 regarding the thermodynamic study.

The experimental data related to the adsorption of MB by IS-KOH-B were better fitted with the PSO. Indeed, compared to the PFO, this model has higher R² (0.947) and lower MAPE (14.3%) (Table 2 and Figure 3). Furthermore, the calculated adsorbed amount of MB at equilibrium was close to the experimental value (about 3.4% higher, Table 2). The validity of the PSO model indicates that the MB adsorption rate limiting step might be chemical adsorption including valency forces through sharing or exchange of electrons. Similar findings were reported by Fan et al. [66] and Chen et al. [49] when investigating MB adsorption by non-modified biochar derived from the pyrolysis of urban sludge at 550 °C and 200 °C, respectively. Furthermore, the estimated MB kinetic adsorption coefficients by the activated biochar (k₁, k₂) by both PFO and PSO models are much higher than those determined for the pristine feedstock (Table 2). This confirms that the MB adsorption by the activated biochar is a more rapid process.

The test of the diffusion model showed that the MB adsorption occurs through surface interactions for times lower than 10 min and 120–150 min for IS-KOH-B and IS-R, respectively, and by intraparticle diffusion later (Figure 3). In addition, the calculated film and intraparticle diffusion coefficients values show that film diffusion is the limiting process during MB removal by the raw sludge and its activated biochar. An opposite finding was reported by Fan et al. [66] who showed that intraparticle and chemisorption processes are the main controlling steps of MB adsorption by a biochar derived from an urban sludge.

3.2.2. Effect of Initial pH

The effect of initial aqueous pH on MB adsorption by IS-R and its KOH-activated biochars is given in Figure 4. It can be clearly seen that MB adsorption capacity for IS-R significantly increased with an increase in the initial pH value. Indeed, the MB adsorbed amount rose from 35.8 mg g⁻¹ at an initial pH of 4 to more than 41.0 at a pH value of 10.

This behavior could be linked to the fact that at pH values lower than the pH of zero-point charge of the IS-R (6.4), this adsorbent material is positively charged. Therefore, the adsorption of the MB cationic dye should be limited because of the electrostatic repulsion. However, the MB adsorption through electrostatic attraction was favored at high pH values as that adsorbents particles become negatively charged due to the dissociation of the surface functional groups [62,67]. A similar trend was reported for MB removal by raw biochar generated from the pyrolysis at 300 °C of urban sludge mixed with tea wastes at a mass ratio of 1:1 [61]. These authors reported that MB adsorption rate increased from 80% to 90% when the initial pH was increased from 3 to 10. Similar results were also reported for MB removal by a Fe₂O₃-biochar nanocomposite derived from pulp and paper sludge [63], a sludge-derived mesoporous biochar treated by a cationic polyacrylamide [59], and also another cationic dye (crystal violet) by an urban sludge derived hydrochar [68].

For IS-KOH-B, there was no significant difference in the retained MB amounts versus the pH variation. This behavior could be linked to its relatively high pHzpc (10.8). For all the initial pH values, the final measured pH values were almost constant: 10.4–10.5. This result indicates that the use of this material represents an important asset since high MB removal can be anticipated for a large pH range including both acidic and alkaline media.

3.2.3. Effect of Adsorbents Dosage

The effect of adsorbents dosage on MB removal from aqueous solutions is given in Figure 5. It appears that for the studied range, higher adsorbent doses resulted in higher MB adsorption (Figure 5). For instance, for a small used dose of 0.2 g L⁻¹, MB adsorption efficiency was assessed at 36.2% and 56.1% for IS-R and IS-KOH-B, respectively. The removal yields reached more than 97% and 79% for a dose of only 1 g L⁻¹ for the IS-KOH-B and IS-R, respectively. The high MB removal efficiencies by such a low dose of IS-KOH-B is of great economic importance when this solution would be applied at a field scale. From Figure 5, it can be clearly seen that compared to IS-KOH-B, a higher IS-R dose (2 g L⁻¹) is required to get approximately constant removal efficiency (88.5%). The increase of MB removal efficiency with the increase of the adsorbents doses is mainly due to the presence of more available sorption sites that could react with MB molecules. A similar trend was observed for MB adsorption by a Fe₂O₃-biochar nanocomposite derived from pulp and paper sludge [63] and a biochar from the pyrolysis of an urban sludge mixed with tea wastes [61].

It is important to underline that as expected (see Equation (1)), the removed MB amounts (q_e, mg g⁻¹) decreased with the increase of the adsorbents doses. For instance, for the case of IS-R, q_e values were assessed to be 38.8 and only 6.3 mg g⁻¹ for dose of 0.2 and 3 g L⁻¹, respectively. Similar behavior was observed by Alalwan et al. [69] when studying methyl green removal by eggshell wastes.

3.2.4. Effect of Salts

The effect of competition on MB removal yield is illustrated by Figure 6. It shows that for the raw sludge, all the present cations have significantly reduced the MB adsorption capacity. The highest effect was observed for the divalent cation Ca²⁺, in which compared to the blank test, the MB adsorbed amount decreased by about 67.7%. This decrease percentage reaches more than 75% when Na⁺, K⁺, Mg²⁺, and Ca²⁺ co-exist in the same solution. This is due to the adsorption competition for the same available IS-R’ active sites between the positively charged MB molecules and these cations. This finding could be attributed to the physico–chemical characteristics of both the biochars and the cation such as the average pores of the biochars particles; and size, hydratation energy and electronegativity of the competing chemicals [70].

In the case of the activated biochar (IS-KOH-B), the competitive effect was much lower compared to the IS-R (Figure 6). Indeed, the decrease in MB adsorption varies from about 0.9% to 6.5% due to competition with Na⁺, and with Na⁺, K⁺, Mg²⁺, and Ca²⁺ altogether. This lower competition effect suggests that MB adsorption by IS-KOH-B is a chemical process including high retention energy and an important selectivity for MB.

3.2.5. Effect of Initial Concentration-Isotherm Investigation

Figure 7 gives the fitting of the experimental data to the Freundlich, Langmuir, and D-R models. The calculated constants of these three models are given in

Table 3. Isotherms parameters regarding MB adsorption by raw industrial sludge and its KOH activated biochar.

Isotherm	Parameter	IS-R	IS-KOH-B
Langmuir	KL (L mg−1)	0.26	0.52
	qm (mg g−1)	48.5	65.9
	R2	0.929	0.951
	MAPE (%)	29.4	13.1
Freundlich	n	1.25	3.48
	KF	5.9	26.4
	R2	0.913	0.901
	MAPE (%)	18.6	13.1
D-R	qm,D-R (mg g−1)	1911.6	220.1
	E (kJ mol−1)	9.0	14.9
	R2	0.920	0.909
	MAPE (%)	14.6	15.0

. It can be clearly seen that the experimental data of MB adsorption by IS-KOH-B was best fitted with the Langmuir model. Indeed, compared to other used models, the Langmuir model presented the highest determination coefficient (0.951) and the lowest mean absolute percentage errors (13.1%) between experimental and theoretical data. Moreover, the highest Langmuir parameter value $“R_L=\frac{1}{1+K_{L*C_0}}”$ was assessed to be equal to 0.09, which is much lower than 1, suggesting therefore that MB adsorption by IS-KOH-B was a favorable process. Therefore, the MB adsorption onto IS-KOH-B seems to occur on uniform monolayer coverage at the outer surface of the adsorbent. Comparable findings were indicated by Chen et al. [49] when investigating MB removal by an SDB from the pyrolysis of an urban sludge at 200 °C. Regarding the IS-R, the highest R² (0.92) and lowest MAPE (14.6%) were observed for the D-R model (Figure 7). However, the corresponding theoretical adsorption capacity (q_m,D-R) was considerably high (more than 1900 mg g⁻¹) compared to what should be (see the experimental data pattern (Figure 7)). This unlikely high theoretical value could be attributed to the non-validity of the model assumptions. Indeed, this model assumes that the adsorbent contains a homogenous and uniform micropores structure [38]. The same behavior was found for the IS-KOH-B, where the q_m-D-R was evaluated to more than 220 mg g⁻¹.

The Freundlich parameter “n” was estimated to be 1.25 and 3.48 for MB adsorption by IS-R and IS-KOH-B, respectively (Table 3). Both values belong to the range 1–10, signifying that the adsorption of MB by the two used adsorbents was a favorable process. On the other hand, the free energy values (E = 1/√2β) for MB adsorption by IS-R and IS-KOH-B were evaluated to 9.0, and 14.9 kJ mol⁻¹, respectively (Table 3). These values are higher than 8 kJ mol⁻¹, indicating that MB removal mainly occurs through cation exchange. Similar behavior was reported by Chen et al. [62] when investigating MB removal by a biochar derived from an urban sludge mixed with rice husk powder.

The Langmuir’s adsorption capacity of IS-KOH-B was found to be 65.9 mg g⁻¹, which is about 36% higher than that of IS-R (48.5 mg g⁻¹). This result could be attributed to the improvement of the structural and textural properties of the KOH-activated biochar. For instance, the BET surface area and total volume have increased by factors of about 78 and 6, respectively, after the activation process.

In order to evaluate the efficiency of our adsorbents in removing MB from aqueous solutions, among raw and treated sludge cited in the scientific literature, a comparison based on Langmuir’s q_m was carried out (

Table 4. Comparison of MB removal efficiency by IS-R and IS-KOH-B with other raw and modified sludge (-: no pre-treatment).

Feedstock, Provenance	Pre-Treatment	Pyrolysis Conditions	Adsorption Experimental Conditions	Langmuir’s Adsorption Capacity, qm, (mg g−1)	Reference
Sludge form an urban WWTP, China mixed with tea wastes at a mass ratio of 1:1	-	T = 300 °C; G = -; t = 2 h	C0 = 0–250 mg/L; pH = not adjusted; D = 10 g/L; t = 24 h; T = 25 °C	12.6	[61]
Sludge form an urban WWTP, China mixed with pine sawdust at a mass ratio of 1:1	-	T = 800 °C; G = 40 °C/min; t = 0.25 h	C0 = 20–50 mg/L; pH = -; D = 1 g/L; t = 4 h; T = 25 °C	13.0	[71]
Sludge form an urban WWTP, China mixed with rice husk powder at a mass ratio of 1:1	-	T = 500 °C; G = 7 °C/min; t = 2 h	C0 = 50–300 mg/L; pH = -; D = 1 g/L; t = 24 h; T = 25 °C	22.6	[62]
Sludge form an urban WWTP, China	-	T = 550 °C; G = 10 °C/min; t = 2 h	C0 = 50–500 mg/L; pH = not adjusted; D = 6 g/L; t = 24 h; T = 25 °C	24.1	[66]
Sludge from an urban WWTP, China	-	T = 200 °C; G = -; t = 2 h. Then impregnation for 4 h in 4 M HCl	C0 = 0–400 mg/L; pH = 7.0; D = 0.2 g/L; t = 24 h; T = 25 °C	177.6	[49]
	Demineralization			184.9
Textile industry sludge, Malaysia	Impregnation in H2SO4 solution for a mass ratio of 1:1 for 48 h,	T = 650 °C; G = 5 °C/min; t = 0.5 h	C0 = 10–200 mg/L; pH = -; D = 2 g/L; t = 1 h; T = 30 °C	10.3	[25]
Sludge from an urban WWTP, Spain	Physical treatment with CO2, then impregnation in 3 M K2CO3 for 48 h.	T = 800 °C; G = 15° C/min; t = 0.5 h	C0 = 0–400 mg/L; pH = -; D = 1 g/L; t = 72 h; T = 20 °C	31.2	[26]
Sludge form a pulp and paper mill, Zimbabwe	-	T = 750 °C; G = -; t = 2 h	C0 = 0–250 mg/L; pH = 12; D = 5 g/L; t = 45 min; T = -	33.0	[63]
	Impregnation with FeCl,6H2O (80%: w/v) for 2 h			50.0
Sludge form an urban WWTP, China	Impregnation with 5 M ZnCl2 solution for 2 h	T = 650 °C; G = 15° C/min; t = 2 h	C0 = 0–100 mg/L; pH = 6; D = 0.4 g/L; t = 0.5 h; T = 25 °C	90.9	[64]
Sewage and drainage Plant, China	Impregnation with 2 M ZnCl2 solution for 12 h,	T = 750 °C; G = 9 °C/min; t = 2 h	C0 = 0–500 mg/L; pH = 7.6; D = 2 g/L; t = 36 h; T = RT	91.9	[24]
Pulp and paper sludge, China	Impregnation in ZnCl2 solution for a mass ratio of 1:2 for 24 h. Then preheating at 350 °C for 30 min followed by a heating at 700 °C for 90 min.	T = 700 °C; G = -; t = 2 h	C0 = 50–1000 mg/L; pH = 8; D = 2 g/L; t = 24 h; T = RT	590.2	[28]
Industrial sludge, Oman	-	-	C0 = 20–120 mg/L; pH = 6.8; D = 1 g/L; t = 3 h; T = RT	48.5	This study
	Impregnation with 1 M KOH for	T = 700 °C; G = 5; t = 2 h		65.9

). It can be concluded that both IS-R and IS-KOH-B could be considered as promising materials for MB and perhaps other cationic dyes removal from industrial effluents. For instance, the MB adsorption capacity of IS-KOH-B was about 2.7 to 5.1 times more important than non-modified SDBs [61,62,71]. It is 6.4; 2.1; and 1.3 times higher than activated SDB with H₂SO₄ [25], with a combination of CO₂ and K₂CO₃ [26], and with FeCl₃ [63]. This adsorption capacity is however, lower than SDB treated with concentrated ZnCl₂ solutions [64] and pre-heated at 350 and 700 °C before activation with ZnCl₂ [24].

3.2.6. Effect of Temperature

The study of the temperature effect on MB adsorption by IS-R and IS-KOH-B indicated that for both adsorbent materials, MB adsorbed amounts rose with the increase of the temperature, suggesting the endothermic nature of this reaction (

Table 5. Thermodynamic parameters of MB adsorption onto IS-R and IS-KOH-B.

Adsorbent Material	T (K)	qe (mg g−1)	Regression Coefficient	ΔH (kJ mol−1)	ΔS (J K−1 mol−1)	ΔG (kJ mol−1)
Raw industrial sludge	293	37.0	0.531	30.8	116.0	−5.9
	303	43.8				−5.3
	308	42.1				−5.0
	313	43.9				−4.7
Sludge derived biochar activated with KOH	293	45.8	0.998	78.9	293.0	−6.2
	303	43.8				−9.1
	308	47.8				−10.6
	313	50.1				−12.0

). This may be attributed to an enlargement of the pore size of the biochar [64]. Similar behavior was reported when studying MB removal by biochars derived from the pyrolysis of a textile sludge [25], an urban sludge [66], and an urban sludge mixed with pine sawdust [71]. For both adsorbents, the thermodynamic parameters were calculated by using Equations (3)–(5) given in Section 2.3.2. The values of ΔH and ΔS are positive (Table 5), which reveal that this reaction is endothermic, and an increase in the degree of randomness (disorder) at the solid particles/solution interface during the adsorption process. In addition, negative ΔG values were obtained for all studied temperature ranges, suggesting that MB removal by both adsorbents is a spontaneous and feasible process [12]. The ΔH and ΔS values of the IS-R are comparable to the ones reported by Fan et al. [66] when investigating MB removal by non-modified sludge derived biochar.

It is important to underline that when ΔH is lower than 40 kJ mol⁻¹, the adsorption process is mainly physical and occurs through Van der Walls forces. However, when the value of this parameter is in the range of 50–200 kJ mol⁻¹, the adsorption process is chemical through various mechanisms including ion exchange, complexation, etc. [61]. Therefore, in the present work, MB adsorption by IS-R and IS-KOH-B seems to be controlled by physical and chemical mechanisms, respectively. This result suggests that the MB desorption would be much favored for IS-R compared to IS-KOH-B.

3.3. Involved Mechanisms Exploration

Dye removal by SDBs occurs through various mechanisms including electrostatic interactions, hydrogen bonding, hydrophobic effect, cation exchange, and pore filling [17]. In the present case, according to the kinetic modeling and thermodynamic studies, the electrostatic reactions could represent an important process, especially in the case of IS-R. Therefore, the maximum of MB adsorption was observed for alkaline pH values when the adsorbent active sites became negatively charged. MB adsorption could also include π-π interactions between the aromatic rings of MB dye and the localized π electrons in the conjugated aromatic rings of the SDBs [72]. The FTIR spectra (see Figure 2 in Section 3.1) of the two tested adsorbent materials before and after MB adsorption showed that for the IS-R, there was no significant impact onto the hydroxyl and the carboxylic functions. On the other hand, small shifting in a few peaks at 1600 cm⁻¹ and 1388 cm⁻¹ was observed, which suggests the involvement of aliphatic and alkyne groups in the MB adsorption. At this stage, it is possible that the uptake of MB involved low-energy interactions such as hydrogen and Van Der Waals bonds [73]. When using IS-KOH-B, the only change was noted for the significant decrease in the intensity of –OH peak, which almost disappeared after the MB adsorption (see Figure 2). This observation could be attributed to the combination of effects of the chemical treatment and carbonization process. Indeed, the slow pyrolysis allowed the fixation of carbon, the release of volatile matter, and the enhancement of porosity. K⁺ could catalyze the release of oxygen and hydrogen and further activate the surface functional groups, which allowed their accessibility to the macro-molecules of dye [74]. Therefore, the adsorption of MB onto the KOH-activated sludge-derived biochar is associated with (i) physical mechanisms: pores filling and hydrogen/Van Der Waals bonds and (ii) chemical mechanisms: Electrostatic interactions, hydroxyl bonds and covalent π-π stacking. It is worth mentioning that since the MB adsorption onto the SDBs experimental data were well-fitted with Langmuir’s model, each active adsorption site on the surface of the adsorbent materials might interact with only one MB molecule with no possible interactions between these adjacent adsorbed molecules [75].

4. Conclusions

This study proved that raw industrial sludge and its KOH-activated derived biochar could be used as attractive adsorbents for the removal of methylene blue from industrial effluents. Indeed, IS-KOH-B exhibited improved physico–chemical properties that allowed significant MB adsorption efficiency under wide experimental conditions. The MB adsorption capacity of the activated biochar was evaluated to 65.9 mg g⁻¹, which is relatively important compared to other various raw and modified materials including sludge wastes. An in-depth characterization of the activated biochar combined with kinetic, isotherm, and thermodynamic studies showed that MB adsorption is an endothermic (ΔH = 78.9 kJ mol⁻¹), spontaneous, and favorable process (ΔS varying between −12 and −6.2 kJ mol⁻¹). This process is mainly controlled by chemical reactions involving essentially electrostatic attraction, π-π interactions, and complexation with hydroxyl functional groups.

In practical application, besides the adsorbent effectiveness in removing dye, the economic viability of the overall process should also be precisely assessed. For instance, costs related to: (i) the chemical impregnation with KOH, (ii) energy consumption during the pyrolysis process, and (iii) the management/treatment of the loaded adsorbent should be taken into account. Moreover, given the complex composition of real industrial wastewaters, the effectiveness of the activated biochar in removing mixture of dyes from solution with high range of salinity will be assessed in the future. On the other hand, in order to ensure water and solid closing loops as well as the promotion of the circular economy in this kind of industry, further investigations are being undertaken to optimize the recovery of the dye from the spent adsorbents and the treatment of the desorbing solutions for local reuse of water in the same industry.