Biochar-layered double hydroxide composites for the adsorption of tetracycline from water: synthesis, process modeling, and mechanism

Antibiotic-contaminated water is a crucial issue worldwide. Thus, in this study, the MgFeCa-layered double hydroxides were supported in date palm–derived biochar (B) using co-precipitation, hydrothermal, and co-pyrolysis methods. It closes gaps in composite design for pharmaceutical pollutant removal, advances eco-friendly adsorbents, and advances targeted water cleanup by investigating synthesis methodologies and gaining new insights into adsorption. The prepared B-MgFeCa composites were investigated for tetracycline (TC) adsorption from an aqueous solution. The B-MgFeCa composites synthesized through co-precipitation and hydrothermal methods exhibited better crystallinity, functional groups, and well-developed LDH structure within the biochar matrix. However, the co-pyrolysis method resulted in the LDH structure breakage, leading to the low crystalline composite material. The maximum adsorption of TC onto all B-MgFeCa was obtained at an acidic pH range (4–5). The B-MgFeCa composites produced via hydrothermal and co-pyrolysis methods showed higher and faster TC adsorption than the co-precipitation method. The kinetic results can be better described by Langmuir kinetic and mixed order models at low and high TC concentrations, indicating that the rate-limiting step is mainly associated with active binding sites adsorption. The Sip and Freundlich models showed better fitting with the equilibrium data. The TC removal by B-MgFeCa composites prepared via hydrothermal, the highest estimated uptake which is around 639.76 mg.g−1 according to the Sips model at ambient conditions, and co-pyrolysis was mainly dominated by physical and chemical interactions. The composite obtained via the co-precipitation method adsorbed TC through chemical bonding between surface functional groups with anionic species of TC molecule. The B-MgFeCa composite showed excellent reusability performance for up to five cycles with only a 30% decrease in TC removal efficiency. The results demonstrated that B-MgFeCa composites could be used as promising adsorbent materials for effective wastewater treatment.


Introduction
In the last decade, there has been increasing concern about the quality of water supplies due to various emerging pollutants, including drugs or antibiotics.Disposing different active and processed pharmaceuticals without adequate treatment into water bodies caused severe threats to human and aquatic environments.Widespread overuse of antibiotics and high concentration presence and long persist in the natural and reclaimed wastewater cause severe water pollution affecting food chains and posing new challenges to the environmentalist.Among various pharmaceuticals, tetracycline (TC), an imperative antibiotic, is widely employed to guard against livestock microbial contagions (Granados-Chinchilla and Rodríguez 2017).TC is generally used in reducing the spread of bacterial infections that affect respiratory tract infections.In addition, it is commonly used as a feed additive for aquaculture and poultry operations.The acceptable daily intake and drinking water guideline for TC in Australia are 30 μg. kg −1 .day−1 and 105 μg.L −1 , respectively (Patel et al. 2019).Therefore, the existence of pharmaceuticals in water above the threshold limit is a serious environmental risk.Scientists and researchers are exerting tremendous efforts to develop a cheaper, low-cost treatment technology for efficiently removing antibiotics before discharging them into the environment.
Numerous treatment methods like coagulation and flocculation (Kooijman et al. 2020), bioremediation (Silva et al. 2019), photodegradation (Wang et al. 2020), membrane process (Sahu et al. 2023), and adsorption have been explored to remediate noxious contaminants from wastewater before discharge to the environment (Sahu et al. 2022).Adsorption technology is one of the most simple and facile treatment strategies for successfully removing pollutants and pharmaceuticals, even at low concentrations, from an aqueous medium (Erattemparambil et al. 2023) (Hmoudah et al. 2023).The major challenge in the adsorption process is selecting a low-cost, sustainable, and promising adsorbent material to remove pollutants costeffectively.In recent years, utilizing biomass waste and its derivatives to remove various hazardous contaminants from wastewater has become increasingly prevalent.Biochar is a lightweight carbon-rich material produced after the thermochemical treatment of biomass like agricultural and industrial biowaste and municipal biowaste in an oxygenrestricted atmosphere (Weber and Quicker 2018).Biochar exhibited a porous surface structure, higher specific surface area, and abundant surface functionalities making it a promising low-cost adsorbent material for the remediation of numerous pollutants from wastewater (Qiu et al. 2021).Biochar has been widely investigated for environmental applications, including pharmaceutical wastewater purification (Fidel et al. 2019) (Tang et al. 2013) (Lee et al. 2017).
For instance, Zhao et al., explored bovine-based biochar to remove tetracycline (TC) from wastewater and reported outstanding TC adsorption with a maximum adsorption capacity of 5.82 mg.g −1 (Zhao et al. 2021).In another work, biochar is derived from agro-waste and showed a high affinity towards antibiotics (Hoslett et al. 2021).The Middle East is the largest producer of date palm fruits and discharge huge amount of waste annually.It is estimated that around 200 thousand tons of date palm waste are generated in Saudi Arabia which is directly burned or disposed to landfills.Converting date palm waste to value added product like biochar significantly reduces the environmental concern.Numerous studies investigated the environmental application of biochar derived from date palm wastes.For example, Rambabu et al. produced biochar from date seed waste and reported excellent sequestration of herbicides from agriculture water (Rambabu et al. 2023).In another work, date palm biochar exhibited 80% removal of phenol from secondary wastewater indicating promising adsorbent for remediation of emerging contaminants from wastewater systems (Fseha et al. 2023).Besides, some studies reported the application of biochar from date palm wastes as filler or cement replacement in construction industry (Al-Kutti et al. 2018;Khan et al. 2022).
Layered double hydroxides (LDHs) are layered-type materials and are identified as nano clays (Zubair et al. 2017).LDHs exhibited excellent anion exchange performance, unique surface area, versatile composition, and easily coupled with other materials resulting in improved physicochemical characteristics.LDHs and its derivative showed outstanding performance in water treatment owing to their excellent and favorable sorption properties (Gu et al. 2015).For instance, Rathee et al. fabricated ternary NiAlTi-layered double hydroxide by hydrothermal technique and observed high sorption of TC, and the adsorption capacity reported to be ~238.1 mg of TC per gram of biochar (Rathee et al. 2020).In another study, Mourid et al. reported outstanding adsorption performance of sulfamethoxazole onto calcined LDH (Mourid et al. 2019).Despite their promising adsorption performance, the application of LDH for treating commercial wastewater is limited due to aggregation in the aqueous phase and dissociation in an acidic environment.Therefore, coupling or intercalation of LDHs with other materials is a promising approach to producing high-performance sorbent materials for wastewater purification systems.
Recent studies demonstrated that biochar can serve as a low-cost, porous, and sustainable support matrix for LDH.The synergistic effect of LDH and biochar facilitated substantial improvement in the physicochemical properties of the resultant biochar-LDH composites, including specific surface area, surface functional groups, structural heterogeneity, and adsorption performance (Hudcová et al. 2022) (Hai et al. 2022).Various studies emphasized the design and fabrication of biochar-LDH composites for the removal of different pharmaceuticals from water.For example, Ridha et al. prepared rice husk-derived biochar-supported MgFe LDH composite.The B-MgFe composite adsorbed 43.3 mg.g −1 meropenem antibiotic from water (M-Ridha et al. 2021).Similarly, Gholami et al. showed 92.7% degradation of gemifloxacin using biochar-Zn-Co-photocatalyst produced through a hydrothermal route (Gholami et al. 2020b).In another study, researchers investigated biochar-FeCu composite for improved degradation of cefazolin sodium (Gholami et al. 2020a).These biochar-LDH composites can be produced by various techniques including co-precipitation, hydrothermal, and co-pyrolysis.The physicochemical properties (crystallinity, surface morphology, functional groups) of biochar-LDH can be tailored by selecting different synthesis routes which greatly affect the adsorption performance of various pollutants (Zubair et al. 2021).Therefore, the selection of appropriate synthesis route(s) to produce biochar-LDH of favorable sorption properties is desirable for enhanced removal of pharmaceuticals from water.
To date, there is no study investigating the effect of various production routes of biochar-LDH composites and their adsorption performance for the removal of antibiotics from water.Therefore, the main objective of this research work is to evaluate the removal efficiency of TC from water using biochar-MgFeCa composites produced via different synthesis methods.The environmental hazard presented by the discharge of tetracycline owing to its extensive usage and persistence in the environment requires remediation by effective, inexpensive, and sustainable adsorbents such as those prepared in this work.Furthermore, the effect of the synthesis method on the physicochemical properties of B-MgFeCa composites was examined by various characterization methods.The research work investigated the impact of various adsorption parameters (pH, time, dosage, co-existing ions, and temperature) on the removal of TC.Finally, the TC adsorption mechanism onto B-MgFeCa is deeply evaluated by employing different isotherm, kinetic, and thermodynamic models.

Materials and methods
The metal precursor salts (magnesium nitrate hexahydrate, iron nitrate hexahydrate, and calcium nitrate hexahydrate) for LDH synthesis and sodium salts (chloride, carbonate, phosphate, and sulfate) for co-existing ions were purchased from Sigma-Aldrich.Date palm fronds for biochar production are collected from Al-Hassa, Saudi Arabia.The TC stock solution of 200 mg.L −1 concentration was prepared by dissolving approximately 200 mg of TC in 1000 mL of deionized water and used to prepare all the studied TC concentrations.

Production of biochar (B)
In this work, biochar is produced from date palm fronds collected from a date farm factory located in Al-Hassa, Saudi Arabia.The date palm fronds were first washed with water and air dried.To produce biochar, the dried date palm fronds were loaded in stainless steel stubes and transferred in tubular furnace.The date palm fronds were pyrolyzed at 500 °C for 2 h under nitrogen environment.The resultant biochar (B) was cooled at room temperature and later used for biochar-LDH production.

Production of biochar-MgCaFe composites (B-MgCaFe)
In this study, four biochar-MgCaFe composites were prepared by using co-precipitation, hydrothermal, and co-pyrolysis methods.The details of each production method are illustrated in the following subsections.

Co-precipitation method
The biochar-MgFeCa-layered double hydroxide (B-MgF-eCa) composites were produced via a facile co-precipitation technique.Firstly, magnesium (7.68 g) and iron (8.06 g), and calcium (2.72 g) salts of a 03:02:01 mole ratio were added to 100 mL of distilled water and transferred to a round bottom glass reactor.In parallel, around 5 g of biochar produced (B) was mixed in 100 mL deionized water, sonicated (60 amplitude) for about 1 h, and transferred to the glass reactor.The mixture was agitated at 800 rpm for 15 min at 80 °C.The pH of the mixture was adjusted to 9.5-10 by dropwise addition of 1 M NaOH solution.Once the pH is maintained, the reaction solution was refluxed to 18 h at 80 °C.Afterward, the produced B-MgFeCa precipitates were washed 3-4 times using double-distilled water, and lastly using pure ethanol to get rid of all unreacted salts and impurities.The final composite B-MgFeCa-1 was dried in the oven at 80 °C for 48 h.

Hydrothermal method
The biochar-MgFeCa-layered double hydroxide (B-MgF-eCa) composites were produced using the hydrothermal method.Firstly, magnesium (7.68 g) and iron (8.06 g), and calcium (2.72 g) salts of a 03:02:01 mole ratio were added to an autoclave vessel containing 50 mL of 1 M NaOH solution (pH about 10).Concurrently, a precisely 5 g of biocharderived date palm fronds (B) was added in 100 mL deionized water, sonicated (60 amplitude) for about 1 h, and transferred to the autoclave.The reaction mixture heated was at 160°C for 18 h for hydrothermal carbonization.Afterward, the produced B-MgFeCa precipitates were washed 3-4 times using double-distilled water, and lastly using pure ethanol to get rid of all unreacted salts and impurities.The final composite B-MgFeCa-2 was dried in the oven at 80 °C for 48 h.

Co-pyrolysis method
In the co-pyrolysis method, first, date palm fronds-MgFeCa LDH composites were prepared by using magnesium (7.68 g) and iron (8.06 g), and calcium (2.72 g) salts of a 03:02:01 mole ratio and 5 g of date palm frond biomass according to coprecipitation and hydrothermal method as demonstrated in previous subsections ("Co-precipitation method" and "Hydrothermal method" sections).The synthesized biomass-MgFeCa LDH composites are further pyrolyzed at a temperature (500 °C) and time (2 h) under a nitrogen environment.The resultant B-MgFeCa-3 and B-MgFeCa-4, were washed 3-4 times with double-distilled water, and finally with ethanol to remove all unreacted salts and impurities.The final product B-MgFeCa-3 and B-MgFeCa-4 composites were kept at 80 °C in the oven for 48 h.

TC adsorption experiments
In this work, the TC adsorption experiments were conducted in batch mode using B-MgFeCa prepared composites according to the previous study (Manzar et al. 2023), to evaluate the influence of adsorption conditions including initial solution pH (3-10), composite dosage (2-20 mg), agitation time (0-240 min), and adsorption temperature (25-45 °C) in a shaker with water bath using 50 mL round bottom plastic vials with 40 mL solution.The adsorption experiments were conducted in duplicate, and the average values are reported herein.After the adsorption experiment, the sample with TC solution is centrifuged for 5 min (4500 rpm), and the concentration of TC in the supernatant is measured by UV-Vis spectrophotometer at a maximum wavelength of 359 nm.
where C o and C e are the initial and the equilibrium concentrations (mg.L −1 ) of TC, Q e is the equilibrium amount of TC adsorbed (mg) per unit mass of adsorbent (mg.g −1 ), V is the volume of the solution (L), and w is the mass of the adsorbent in the solution (g).

Kinetic modeling
Adsorption kinetic experiments were conducted to evaluate the performance of the prepared biochar-LDH composites and to investigate the mass transfer mechanisms of TC adsorption.In this study, TC adsorption data at two different concentrations (20 mg.L −1 and 100 mg.L −1 ) and contact time (0-240 min) were fitted to the three models, namely, the mixed-order (MO) model, the phenomenological internal mass transfer (IMT) model, and the Langmuir kinetics model (LKM).These model selections were based on the adsorption system's complexity, the existence of intra-particle diffusion effects, and the necessity to comprehend the kinetic elements of the adsorption process.The empirical MO model is developed in 2019 by Guo and Wang (Guo and Wang 2019) as per Eq. ( 3): where Q t and Q e (mg.L −1 ) are the adsorbed amount of the adsorbate (TC) at time t (min) and the experimental adsorption capacity, respectively, k ′ 1 (min −1 ) and k ′ 2 (g.mg −1 .min−1 ) are the first-order and pseudo-second-order rate constants, respectively.The first term of Eq. ( 3) describes the diffusion step while the second one represents the adsorption step on the active site (Wang and Guo 2020a).Wang and Guo (2020a) defined the diffusional IMT model as shown in Eq. 4: where k int is the internal mass transfer rate constant (min −1 ), and Q et is the equilibrium adsorption capacity in the pores of the adsorbent in mg.g −1 , herein, and is defined in terms of the Langmuir isotherm model.This IMT model suggests that (i) the internal diffusion is the slowest step and (ii) that equilibrium is obtained in the liquid-solid interface.On the other hand, the LKM model (Eq.5) assumes that the adsorption onto the active site is the slowest step.
where k a is the adsorption rate constant (L.mg −1 .min−1 ) and k d is the desorption rate constant (min −1 ).

Isotherm modeling
Adsorption isotherm models provide mechanistic insights into the adsorption processes that are important for the design of the adsorption system.Herein, three adsorption isotherm models, Langmuir, Freundlich, and Sips, Eqs.
(6-8), were used to fit the experimental data (Freundlich 1907;Sips 1948;Wang and Guo 2020b).These isotherm models are commonly used to characterize adsorption equilibrium data and calculate the adsorption capacity, intensity of adsorption, and heterogeneity of the adsorption sites.
Each model has benefits and can give useful insights into the adsorption mechanism and the nature of adsorbent-adsorbate interactions.
where Q e is the amount of TC adsorbed per mass of the biochar composites (mg.g −1 ), C e is the equilibrium concentration of TC in the supernatant (mg.L −1 ), K L is the Langmuir equilibrium adsorption constant related to the affinity of binding sites (L.mg −1 ), Q max is the Langmuir maximum adsorption capacity for complete monolayer coverage (mg.g −1 ), K F is the Freundlich constant related to the adsorption capacity ((mg.g −1 )(L.mg −1 ) 1/n ), 1/n is the adsorption intensity factor (unitless), Q ms is the Sips maximum TC adsorbed amount (mg.g −1 ), K s is the Sips equilibrium constant of adsorption related to the adsorption affinity (L ns .mg −ns ), and n s is the Sips constant related to the surface heterogeneity ().All these estimated parameters are listed in the results section along with the coefficient of determination (R 2 ), the statistical error functions of the nonlinear chi-square (χ 2 ), and residual sum of squares error (SSE) as per Eqs.(9,10,11), to evaluate the fitness of the adsorption isotherm models (Freundlich 1907;Langmuir 1916;Sips 1948). (5) where Q mean is the average value of the experimental adsorption capacity (mg.L −1 ), Q cal is the calculated adsorption capacity (mg.L −1 ), and Q exp is the experimental adsorption capacity (mg.L −1 ).The higher value of R 2 and lower value of χ 2 and SSE, the better the fitting.All fittings and error function estimations were performed using Excel Microsoft 365 (Version 2211).

Reusability experiment
The recyclability of B-MgFeCa composites was performed by agitating 100 mL of TC concentration of 20 mg.L −1 consisting of 50 mg of each B-MgFeCa composite for 2 h.As indicated in previous studies, alkaline solution is effective to desorb the TC pollutant by reducing the electrostatic attractions between the anions and the adsorbents surface (Jang and Kan 2019a) (Wang et al. 2021b).Therefore, the regeneration of the spent adsorbent was conducted using 1 M NaOH solution.After adsorption, the spent adsorbent was placed in a conical flask containing a 50 mL solution of the regenerating agent.The entire mixture was then agitated for 2 h; the adsorbent was separated by centrifugation and dried for further adsorption cycle.The regenerated adsorbent was used for the adsorption of TC solutions of 20 mg.L −1 .The adsorption-desorption process was repeated for five consecutive cycles.

FTIR analysis
The FTIR spectra of biochar-MgFeCa composites are shown in Figure 1a.It is depicted that all similar associated characteristic peaks of biochar and LDH are present with few changes in their peak intensities.The weak broad peak observed at 3634 cm −1 is associated with the hydroxyl group (OH) stretching of physically adsorbed water molecules and within the interlayers of B-MgFeCa composites (He et al. 2019).Similarly, the peak around 2370 cm −1 is attributed

XRD analysis
The XRD patterns of B-MgFeCa composites are displayed in Figure 1b.The broad peak at 21.99° is related to the (002) plane of graphitic carbon found in all B-MgFeCa composites (JCPD card 41-1487) (Chen et al. 2021) (Wang et al. 2021a).The sharp and strong peak of calcite (CaCO 3 ) at 2θ = 29.36° is observed in both B-MgFeCa-2 and B-MgFeCa-4 composites.However, the low intensity of calcite peak in B-MgF-eCa-1 and B-MgFeCa-3 obtained using co-precipitation method.This is mainly due to poor calcite crystal formation due to low process temperature than hydrothermal method (Putri et al. 2016).The peaks observed at 11.08°, 37.93°, 43.25°, and 47.2° are characteristic peaks of hydrotalcite (MgFeCa) (JCPD card 22-0452) (Li et al. 2017).These characteristic peaks are found more dominant in B-MgFeCa-1 and B-MgFeCa-2 which indicates the successful formation of LDH structure onto biochar surface.However, B-MgF-eCa-3 and B-MgFeCa-4 composites showed a weak intensity of LDH peaks indicating poor composite crystallinity mainly associated with the breakage of LDH structure during the co-pyrolysis method.

SEM and EDS analysis
The surface structures of biochar-MgFeCa composites were evaluated by scanning electron microscopy.As shown in Figure 2, all the synthesized B-MgFeCa composites showed heterogeneous structures with randomly distributed small particles.For B-MgFeCa-1, the surface morphology indicates the presence of circular shape LDH particles with varied sizes (80-200 nm) randomly distributed within the flakes of biochar (Figure 3a, b).The B-MgFeCa-2 composite showed flower-like structure LDH particles of size 50-100 nm homogenously intercalated within the biochar surface (Figure 2c, d).The B-MgCaFe-3 revealed an amorphous and porous structure of LDH compared to B-MgFeCa-1 and B-MgFeCa-2, randomly distributed with biochar flakes (Figure 3e, f).Similarly, the B-MgCaFe-4 composite showed the formation of aggregates of amorphous LDH particles in a biochar matrix (Figure 2g, h).
The weight percentage of C, O, Mg, Al, Si, Ca, and Fe in the synthesized B-MgFeCa composites was investigated using the EDX spectra (Figure 3).The percentage of Mg, Fe, and Ca was 3.93%, 2.32%, and 5.95% and 0.48%, 0.62%, and 8.89% found in B-MgFeCa-1 and B-MgFeCa-2, respectively.In addition, B-MgFeCa-3 and B-MgFeCa-4 prepared by copyrolysis also indicated the noticeable presence of Mg, Fe, and Ca.The results confirmed that MgFeCa was successfully embedded into the biochar matrix via co-precipitation, hydrothermal, and co-pyrolysis methods.

TEM analysis
To further evaluate the structure and distribution of MgF-eCa LDH within the biochar matrix, TEM analysis was performed and the results are displayed in Figure 4.As seen, the B-MgFeCa-1, prepared by the co-precipitation method, showed the formation of circular MgFeCa particles uniformly spread within biochar surface.In the case of the B-MgFeCa-2 composite, the flakes of MgFeCa LDH homogenously decorated onto the biochar matrix with some aggregation.The results of B-MgFe-1 and B-MgFeCa-2 agree with XRD and SEM results indicating high crystallinity due  to the successful formation of MgFeCa LDH particles onto the biochar surface.In contrast, in B-MgFeCa-3 and B-MgF-eCa-4 composites, the breakage of LDH structure during co-pyrolysis resulted in a porous and rough surface structure consisting of tiny particles surrounded by biochar structure.Similar behavior has been reported by previous studies on biochar-LDH prepared by the co-pyrolysis method.

Surface area analysis
The textural properties (surface area, pore volume and pore size) of B-MgFeCa composites obtained by N 2 adsorptiondesorption isotherm are tabulated in Table 1.As shown, the specific surface areas of B-MgCaFe-1 and B-MgCaFe-2 are 321.99 m 2 .g−1 and 257.77m 2 .g−1 , while that of B-MgCaFe-3 and B-MgCaFe-4 are 249.33 m 2 .g−1 and 130.42 m 2 .g−1 , respectively.The highest surface area of B-MgCaFe-1, which is prepared by co-precipitation method, is attributed to the formation of less aggregation of LDH and pore blocking on biochar surface as displayed in SEM analysis.However, in hydrothermal method, the MgFeCa LDH flakes effectively intercalated onto biochar surface and pores leading to high crystalline B-MgFeCa-2 composite as confirmed by XRD analysis.Similarly, co-pyrolysis resulted in low surface area; this is mainly associated to the hindrance in formation of porous structure due to the presence of LDH during pyrolysis process.All the prepared B-MgCaFe composites exhibit a microporous characteristic having a pore size distribution in the micro range (1-2 nm).

Effect of solution pH
As solution pH significantly influences the adsorption efficiency owing to its impact on the pollutant speciation, adsorbent surface charge, and the formation of reaction species, it was considered an essential factor for TC adsorption (Luo et al. 2022).The results of the effect of pH (from 3.0 to 10) on TC adsorption onto the synthesized B-MgFeCa composites are displayed in Figure 5.The results showed that the initial solution pH had profoundly affected the B-MgFeCa composites' efficiency for removing TC from the aqueous phase.Firstly, Figure 5a shows the effect of pH when the initial concentration of TC was 20 mg.L −1 .Notably, all the materials demonstrate low efficiency at pH 3. The B-MgF-eCa-2 and B-MgFeCa-3 composites exhibited a similar behavior that depicted the adsorption efficiency decreasing with increasing the initial pH beyond 5. Noticeably, the TC adsorption for the B-MgFeCa-1 and B-MgFeCa-4 composites slightly decreased with increasing the pH of the solution.Thus, the highest adsorption capacity for all four adsorbents was observed at the investigated pH value of 4.Moreover, the best obtained adsorption capacity of 120 mg.g −1 and 110 mg.g −1 was obtained for the B-MgFeCa-2 and B-MgF-eCa-3 composites, respectively.Similarly, when the initial concentration of TC was increased to 100 mg.L −1 , the results depicted in Figure 5b show also that all the composites' uptake capacities drastically decreased with increasing pH solution.Likewise, in this case (100 mg.L −1 TC initial concentration), the B-MgFeCa-2 and B-MgFeCa-3 composites still yielded the best adsorption capacities at the optimum pH value of 4, which significantly improved to 218 mg.g −1 and 182 mg.g −1 , respectively.The results can be demonstrated by the ionization state of TC at different pH conditions.The TC molecules can form three ionization states with different binding sites as per the TC pKa values of 3.3, 7.7, and 9.7 (Jang and Kan 2019b).Accordingly, the species of TC in aqueous solutions could be in cationic (TCH 3 + ) or anionic form (TCH − and TC 2− ) when the value of pH is less than 3.3 or greater than 7.7, respectively (Hoang et al. 2022a).Meanwhile, when the pH value is between 3.3 and 7.7, the TC will be predominantly in neutral molecular form TCH 2 (zwitterionic).Therefore, at a low pH value of 4.0-6.0, the B-MgFeCa composites interacted with TCH 2 .However, as the pH increases, the tendency for the TC molecules uptake decreased as they were gradually becoming deprotonated (negatively charged) as confirmed by Figure 5. Thus, the good adsorption capacity at the acidic pH range could be attributed to ion-exchange, pi-pi, and chemical attraction between the B-MgFeCa composites (as anionic) and the TCH 2 ions.However, at a pH above 7.0, the similarity of the charges reduced the efficiency of the adsorption due to the electrostatic repulsive force (Zaher et al. 2020), while at pH 3.0, the LDH is prone to dissolution, which in turn decreases the abundance of the active sites, resulting in the decline in the adsorption capacity.Similarly, good adsorption of TC within pH 4.0-6.0 which was also found to decrease as pH was increased was reported elsewhere (Zhao et al. 2011).These authors attributed this trend to a higher exchange of nitrate (NO 3 −1 ) ions and chemical complexation with zwitterion species complexation (Zhao et al. 2011).This suggests that besides physical interactions, other mechanisms (ion-exchange and chemical complexation) also played a vital role on TC adsorption onto the B-MgFeCa composites (Gao et al. 2012) (Ji et al. 2009) (Zhao et al. 2011).It is worth noting that at low TC concentrations (20 mg.L −1 ), the high adsorption capacity of B-MgFeCa-1 and B-MgCaFe-3 compared to B-MgFeCa-2 and B-MgCaFe-4 at pH value of 4.0 is mainly attributed presence of abundant NO 3 − and MMO as confirmed from FTIR spectra (Figure 1a) which facilitated TC adsorption via ion-exchange and chemical complexation mechanism.Moreover, B-MgFeCa-2 also showed the highest TC adsorption at high TC level (100 mg.L −1 ), mainly attributed to better crystallinity, surface functional groups, and heterogeneous structure which adsorbed TC through multi-interactions including physical adsorption, ion-exchange, and chemical interaction.

Effect of B-MgFeCa dosage
Figure 5c shows the effect of the adsorbent dosage (2.5-20 mg) on the TC adsorption capacity and percentage removal for the different B-MgFeCa composites at fixed initial concentrations and initial pH.As the B-MgFeCa composites amount was increased, the adsorption capacity for TC uptake decreases for all the investigated adsorbents.Considering that the initial TC concentration was fixed, increasing the B-MgFeCa composites amount led to additional active sorption sites introduced into the solution, more than required for the fixed TC molecules, thereby resulting in the adsorption capacity decreased with increasing dosage.On the other hand, the percentage removal of TC increased with increasing B-MgFeCa composite dosage (5-10 mg).The maximum percentage removal of 50.3, 55.4, 53.9, and 23.3% of TC by B-MgFeCa-1, B-MgFeCa-2, B-MgFeCa-3, and B-MgF-eCa-4, respectively.Further increase in dosage to 20 mg did not significantly improve the removal which is mainly associated to the agglomeration of composites in aqueous phase.The results indicated that uptake of higher TC molecules render, especially, the B-MgFeCa-2 and B-MgFeCa-3 composites' excellent adsorbents for TC aqueous uptake and corroborate their higher affinity towards TC adsorption.

Effect of co-existing anions
Real wastewater is known to usually contain mixed contaminates including both organic and inorganic (Crini 2006;Yaseen and Scholz 2019).During the adsorptive removal of TC from the aqueous phase, the impact of the presence of inorganic ions in water has a subject of interest due to the expected increase in solution ionic strength which has been reported to significantly influence the process for a wide range of different adsorbents (Zhao et al. 2011) (Yu et al. 2020) (Ji et al. 2010) (Duan et al. 2014) (Cheng et al. 2016).Thus, in this study, the effect of the presence of different cohabiting inorganic ions (chloride Cl − , carbonate CO 3 2− , phosphate PO 4 3− , and sulfate SO 4 2− ) on TC adsorption onto the B-MgFeCa composites was investigated.The results shown in Figure 5d show that the TC removal efficiency for all composites declined due to the presence of these anions with the negative impact following the order: CO 3 2− ˃ Cl − ˃ SO 4 2− ˃ PO 4 3− .This suggests that PO 4 3− experienced lesser affinity towards the active sites of the B-MgFeCa composites, thereby resulting in the higher degree of the TC uptake than SO 4 2− and Cl − , then followed by CO 3 2− .Inferably, the reduction in the adsorption capacities of all the adsorbents can be attributed to the competitive environment created in presence of these ions which significantly reduced the interactions between TC and B-MgFeCa composites (Nguyen et al. 2021).

Effect of contact time
The dependency of the B-MgFeCa composite adsorption capacities for TC aqueous removal on contact time (0-240 min) was investigated at two different concentrations of TC (20 mg.L −1 and 100 mg.L −1 ) while other parameters such as pH, temperature, and dosage were kept constant.The results presented in Figure .6a, b show that at both initial concentrations of TC, the adsorption capacities for all the adsorbents for TC uptake gradually increased with contact time reaching equilibrium time of 90 and 120 min for B-MgFeCa-2 and B-MgFeCa-3, respectively.Meanwhile, for both B-MgFeCa-1 and B-MgFeCa-4, a higher contact time of 180 min was required to reach equilibrium.In the early stage, it was obvious that the plenty of active sites on the surface of the adsorbents caused the rapid adsorption of TC.Consequently, the adsorption rate was slow and became stable at equilibrium time due to the gradual reduction of the active adsorption sites.The presence of abundant active sites such as functional groups (NO 3 , C-O-C, C-OH, and MMO) onto B-MgFeCa-2 with excellent crystallinity and surface structure facilitated faster uptake of TC ions from water compared to other B-MgFeCa composites.For instance, the adsorption capacity of B-MgFeCa-2 compared with other adsorbents was the highest and it more rapidly reached equilibrium at 90 min while yielding up to 12 mg.g−1 and 98 mg.g −1 for 20 mg.L −1 and 100 mg.L −1 TC initial concentration, respectively.Lower equilibrium adsorption capacities were obtained for the other three adsorbents for both TC initial concentrations.Meanwhile, the adsorption equilibrium time for B-MgFeCa-2 and both B-MgFeCa-1 and B-MgFeCa-4 were the same (90 and 180 min, respectively) regardless of the initial TC concentration, while that of B-MgFeCa-3 had increased from 120 to 180 min when the TC concentration was increased from 20 to 100 mg.L −1 .

Adsorption kinetics
Figure 7 shows an excellent agreement between the experimental data with kinetics adsorption models, MO, LKM, and IMT evidenced by high R 2 values (Table 2).However, looking into the χ2 and SSE values in Table 2 confirms the better fitness of LKM for all composites at low initial TC concentration (20 mg.L −1 ) while the MO model was the best fitting for all composites at the higher concentration of TC (100 mg.L −1 ).Noticeably, the k a values are higher than k d values, indicating that a low initial concentration of TC the diffusion, as well as the desorption steps, is negligible and the adsorption rate-limiting step is the adsorption onto the active site (Wang and Guo 2020b).As for the MO model, it describes the whole adsorption process; hence, according to Guo and Wang (2019), it represents the conditions: "i) arbitrary stage of the adsorption, ii) the rate controlling step is the diffusion or the adsorption, and iii) arbitrary initial adsorbate concentration."Thus, at a high TC initial concentration (100 mg/L), the estimated k ′ 1 values were higher than k ′ 2 values for all prepared B-MgFeCa composites except for B-MgFeCa-3, indicating that the pseudo-first-order (PFO) rate, i.e., diffusion step, is the overall rate controlling step for the composites of B-MgFeCa-1, B-MgFeCa-2, and B-MgFeCa-4.However, the pseudo-second order (PSO), i.e., adsorption onto the active sites, is the rate-controlling step for B-MgFeCa-3.The latter kinetic behavior (PSO > PFO) was similar to the TC adsorption kinetics obtained by Sun et al. (2021) over biochar and magnetic biochar composites as well as the kinetic results obtained by Hoslett et al. (2021).

Adsorption isotherms
As seen in Figure 8, there are different degrees of experimental uptake values (Q e ) as well as the C e values.For instance, the highest Q e and lowest C e values were for B-MgFeCa-2 followed by B-MgFeCa-3 > B-MgFeCa-1 > B-MgFeCa-4, indicating that the synthesis protocol plays a vital role in adsorption behavior for each prepared biochar-LDH composite.Also, the increase in Q e along with the temperature increase for the B-MgFeCa-3 composite is noticeable while the opposite trend appears for other composites, suggesting the favorability of adsorption nature for B-MgFeCa-3.Furthermore, a good agreement was obtained between the experimental data and the three adsorption isotherm models: Langmuir, Freundlich, and Sips.Based on the statistical parameters (Table 3), biochar-MgFeCa composites (B-MgFeCa-1 and B-MgFeCa-3) prepared via the co-precipitation and co-precipitation-pyrolysis method showed the highest fitting with Sips and Langmuir models.Whereas Sips and Freundlich models were the best-fitted isotherm models for biochar-MgFeCa composites (B-MgF-eCa-2 and B-MgFeCa-4) prepared via the hydrothermal and co-hydrothermal-pyrolysis method.In addition, the values of n s are close to one for both composites B-MgFeCa-1 and B-MgFeCa-3, especially at higher temperatures; thus, the composites' surfaces are more homogenous involving monolayer adsorption.However, the heterogeneous surface of B-MgFeCa-2 at higher temperatures leads to multi-layer adsorption and high adsorption capacity.This is due to effective intercalation and structural properties attained during B-MgFeCa-2 creation which may be credited for its exceptional adsorption performance, which was accomplished via the hydrothermal approach.The creation of distinct LDH structures with more surface functionality and high crystallinity was confirmed by FTIR and XRD results.This improved structure increases the TC molecules' accessibility to the active sites, increasing the adsorption capability.The hydrothermal process produces highly ordered LDH layers with increased interlayer gaps as confirmed previously by the SEM and TEM images ("SEM and EDS analysis" and "TEM analysis" sections) that can hold more TC molecules because of the regulated growth conditions.The interaction between the biochar matrix and LDH layers with TC for B-MgFeCa-2 could be more favorable possibly providing additional adsorption sites due to the homogeneous surface functional groups, thus, having higher experimental and estimated uptakes at elevated temperature as shown in Table 3.
Recent studies of the adsorptive removal of TC over different types of biochar composites were found in the literature (Ji et al. 2009;Liu et al. 2021;Nguyen et al. 2021;Sun et al. 2021).In the study of Sun et al. (2021), TC removal   was enhanced using prepared magnetic biochar composites compared with biochar adsorbent alone.The reported Q ms values were around 153 and 70 mg.g−1 at 45 °C for magnetic biochar and biochar alone, respectively.This could be due to the synergistic effect of such composites.Hoslett et al. investigated the removal of TC from aqueous solutions using prepared biochar adsorbents from agricultural wastes; excess food, and garden materials (Hoslett et al. 2021).Though the Freundlich was the best fit, the reported Q ms value at room temperature was around 88.05 mg.g −1 with n s value of ~0.43 suggesting a heterogenous adsorbent.Enhanced biochar stability and experimental adsorption capacity were noticed by Zhao et al. (2019) by incorporating silica particulates into bamboo biomass via a pyrolysis process at 700 °C in which the adsorption uptake increased from ~2.5 mg.g −1 (biochar alone) to ~20 mg.g −1 (silica composite biochar).Also, Jang et al. achieved a high adsorption capacity (~275 mg.g −1 ) of TC over Pinus taeda-derived activated biochar due to the large surface area (~960 m 2 .g −1 ) of the prepared composites (Jang et al. 2018).Compared to the findings of this study, all prepared B-MgFeCa composites are competent towards TC removal at low adsorbent dose level (~0.67 g.L −1 ), especially B-MgFeCa-2 which exhibits superior adsorption capacity.

Effect of temperature and thermodynamics study
To further understand the extent of adsorption of TC onto biochar composites with respect to temperature and based on thermodynamic parameters, changes in Gibbs free energy (∆G o ), enthalpy (∆H o ), and entropy (∆S o ) were obtained using the following equations (El-Qanni et al. 2017;Hmoudah et al. 2022): where R is the universal ideal gas constant (8.314J/mol.K), T is the temperature (Kelvin), and K is the adsorption equilibrium constant (unitless).The values of K can be determined from Eq. ( 13): K s , n s , and K L were obtained from the isotherms study (Table 3), C s is the solvent concentration based on the density of water at the given temperatures of 25, 35, and 45 °C.Additionally, ∆H o and ∆S o were obtained using the van't Hoff equation shown below and by plotting the estimated values of K from Eq. ( 13) versus 1/T, of which the slope and the intercept equal to −∆H o /R and ∆S o /R, respectively.
The estimated thermodynamic parameters are listed in Table 4.As seen, all ∆G o values were found to be negative, ( 12) confirming the spontaneous nature of the adsorption process.Besides, ∆G o values decreased with temperature, especially for the case of B-MgFeCa-3, meaning the spontaneity of the adsorption is proportional to the temperature.Additionally, the estimated values of ∆H o are positive for B-MgFeCa-3 implying the endothermic adsorption behavior and lining up with isotherms obtained in Figure 8, while negative values were obtained for the other composites implying the exothermic adsorption process, thus, making it thermodynamically favorable.The associated values of ∆H o for B-MgFeCa-1, B-MgFeCa-2, and B-MgFeCa-4 are less than 40 kJ.mol −1 suggesting a physisorption with the pi-pi interaction and physical adsorption (Worch 2012).
As for B-MgFeCa-3, its ∆H o > 40 kJ.mol −1 implying a potential covalent bond, i.e., chemisorption interaction with TC.For B-MgFeCa-1 and B-MgFeCa-2, it is observed that the values of ΔS • ads were negative, whereas they were found to be positive for B-MgFeCa-3 and B-MgFeCa-4.This suggests that while the adsorption process for B-MgFeCa-3 and B-MgFeCa-4 is getting more disordered due to the increase in randomness at the solid-liquid interface, it is becoming more ordered for B-MgFeCa-1 and B-MgFeCa-2.Based on the measured thermodynamic characteristics, the influence of temperature on the adsorption process (Figure 8) may be explained.The negative values ΔS • ads show that the adsorption process becomes more ordered at lower temperatures, which might lead to a reduction in adsorption uptake with rising temperatures.In contrast, the positive ΔS • ads suggests that the adsorption process becomes more disordered at higher temperatures, which can result in an increase in adsorption uptake with increasing temperature, especially in the case of B-MgFeCa-3.Hence, thermodynamic results are in line with the experimental findings from this investigation, which showed that the adsorption uptake decreased with rising temperature, except for B-MgFeCa-3, due to its endothermic chemisorption.

Comparison with other adsorbents
The adsorption capacity of various studied adsorbents to remove TC from water is compared with the B-MgFeCa composite (Table 5).The results showed that the synthesized composite indicated an excellent affinity for the adsorption of TC and exhibited a comparatively high adsorption capacity than various adsorbents such as magnetic graphene oxide, magnetic CuCoFe 2 O 4 -chitosan and Zn-based LDH.
In addition, the adsorption rate of B-MgFeCa is 1.5 h which is faster or comparable to recently investigated adsorbents.
The comparative analysis results suggest that the produced B-MgFeCa from the hydrothermal method has great potential to employ as cost-effective adsorbent material for efficiently removing TC from the water system.

Adsorption mechanism
The adsorption of TC onto biochar-MgFeCa composites prepared by different synthesis routes is governed by physical and chemical interactions (Figure .9a).For instance, B-MgFeCa-1 and B-MgFeCa-2 prepared by co-precipitation and hydrothermal method exhibited better crystallinity presence of NO 3 , metal-OH, C-O, and O-C-O functional groups.Consequently, B-MgFeCa-2 showed a smaller particle size from 50 to 100 nm than B-MgFeCa-1.Furthermore, the favorable physiochemical characteristics of B-MgF-eCa-2 promoted a stronger interface with TC ions via multiinteractions involving surface adsorption, ion exchange, and chemical interactions with oxygen functional groups onto B-MgFeCa-2 composites.Similarly, B-MgFeCa-3 and B-MgFeCa-4, prepared by co-precipitation-pyrolysis and co-hydrothermal-pyrolysis method exhibited homogenous and porous structure due to breakage of LDH structure, and formation of mixed metal oxides, C-O and C-O-C surface functional groups.These functional groups serve as active sorption sites for the uptake of TC ions from water through pi-pi and chemical interactions.The kinetic LKM results further confirm that at low TC concentrations, the chemisorption is the dominant mechanism of TC adsorption onto all prepared B-MgFeCa composites, while at higher TC concentrations may involve physical adsorption involving TC diffusion behavior within the surface of the composite.
In addition, the thermodynamic modeling further supports that the B-MgFeCa composites prepared through co-precipitation and hydrothermal techniques mainly interacted with TC molecules through chemical interactions (∆H o > 80 kJ.mol −1 ) and physical sorption (∆H o < 80 kJ.mol −1 ) such as pi-pi interaction and diffusion behavior dominated in B-MgFeCa composites obtained through the co-pyrolysis method.

Reusability performance
The recyclability results of B-MgFeCa composites after five regeneration cycles are illustrated in Figure .9b.The adsorption-desorption results revealed that all prepared B-MgFeCa composites exhibited excellent potential to efficiently remove TC from water without any significant decrease in removal after two reusability cycles.The TC removal efficiency decreased to 70, 69, 62, and 61% after five consecutive cycles for B-MgFeCa-1, B-MgFeCa-2, B-MgFeCa-3, and B-MgFeCa-4, respectively.Such a slight drop could be attributable to the interaction of several variables.The surface characteristics of the composite might be subtly altered by repeated contact with TC molecules during adsorption-desorption cycles.This pattern may also be influenced by the partial reversibility of the desorption process and any potential buildup of leftover TC molecules on the surface.Other cumulative effects such as physical deterioration, leaching of active ingredients, and others also contribute (Erattemparambil et al. 2023).The high reusability performance of B-MgFeCa-1 and B-MgFeCa-2 is associated with better crystalline and surface structure compared to B-MgFeCa-3 and B-MgFeCa-4.The results demonstrated that B-MgFeCa composites could be used as promising cost-effective sorbent material for the remediation of TC from real wastewater streams.

Conclusions
This research investigated engineered biochar-layered double hydroxides-MgFeCa composites produced through different methods for the adsorptive removal of tetracycline (TC) from the water phase.The co-precipitation and hydrothermal production methods resulted in a crystalline MgFeCa LDH structure with excellent functionalities onto the biochar matrix.Consequently, the low crystalline and amorphous-engineered B-MgFeCa was obtained via copyrolysis.All the produced biochar-MgFeCa composites exhibited high TC at an acidic pH range (4-6).The results indicated that B-MgFeCa composites from hydrothermal and co-pyrolysis methods showed a better affinity for TC leading to high sorption capacity and faster adsorption rate.
The kinetic outcomes at low and high TC concentrations can be better explained by Langmuir kinetic and mixedorder models.A better match with the equilibrium data was demonstrated by the Sip and Freundlich models.High TC adsorption on B-MgFeCa-2 and B-MgFeCa-3 was ascribed to a combination of physical and chemical interactions, as opposed to B-MgFeCa-1, which was produced through coprecipitation, where the chemical reaction was controlled by the physical bonding of the surface functionalities to the TC molecules.The conclusions of this study open the door to a wide range of fascinating future investigations.Additional research may be necessary to fully understand the complex relationships between synthesis techniques, composite architectures, and pollutant interactions.The practical applicability of B-MgFeCa composites will be improved by investigating larger pollutant spectra and realworld application scenarios.Furthermore, using these components for continuous flow systems and improving regeneration procedures might lead to long-term water treatment solutions.Thus, this work brings in a new age of customized composite materials that are ready to transform the field of pollution reduction and environmental stewardship.
Environmental Science and Pollution Research (2023) 30:109162-109180   to the asymmetric stretching vibration of CH 2 .The sharp peak at 1570 cm −1 and 1073 cm −1 can be related to the C=O and C-O-C groups and associated with biochar(Carabineiro et al. 2012).The sharp peak at 1357 cm −1 corresponds to the nitrate (NO 3 − ) group observed in all B-MgFeCa composites except B-MgFeCa-4(Elhaci et al. 2020).This indicates the release of NO 3 −1 in B-MgFeCa-4 due to the co-pyrolysis method.The peak at 562 cm −1 is the characteristic band of mixed metal oxides MMO (Mg, Ca, and Fe)(Su et al. 2021).This peak is observed more dominant and stronger in B-MgFeCa-2 and B-MgCaFe-4 composite.The FTIR spectrum demonstrated that biochar-MgFeCa composites consist of various oxygen-containing functional groups including -OH, CH 2, C=O, NO 3 − , C-O-C, and MMO, which are expected to play a dominant role in the removal of TC anions from water.

Fig. 8
Fig. 8 Macroscopic solution phase adsorption isotherms of TC onto prepared B-MgFeCa composites at three different temperatures.Experimental conditions are composite dose, 0.67 g.L −1 ; shaking rate,

Table 2
Estimated MO, IMT, and LKM kinetic constants for the adsorption of TC over prepared composites at different two initial concentrations

Table 3
Estimated Langmuir, Freundlich, and Sips isotherm constants for the adsorption of TC over prepared composites at different three temperatures

Table 5
Adsorption capacity and parameters of TC on other adsorbents