Potential of TiO2 loaded almond shell derived activated carbon for leachate treatment: isotherms, kinetics, and Response Surface Methodology

ABSTRACT In the present study, an agro-waste, almond shell was derived as activated carbon. Based on chemical oxygen demand (COD) removal of leachate, the optimised conditions of activated carbon were obtained and loaded with TiO2 nanoparticles. Brunauer Emmett Teller analysis technique is used to probe the textural properties through the specific surface area. The specific surface area of the TiO2 nanoparticle loaded activated carbon (TiO2@ASAc-7) and efficiently activated carbon (ASAc-7) were 153.3 m2/g and 54.7 m2/g. A band at 536 cm−1 in FTIR of TiO2@ASAc-7 composite demonstrates the spreading of vibrational modes of TiO2 in 4AS material. SEM-EDX shows the elemental peaks of carbon, oxygen and Ti nanoparticles along with atomic percentages of C 70.1, O 22.61, and Ti 7.29. The optimum values of pH 5.5, dose 1.75 g/50 mL, time 160 minutes and temperature 30°C were observed at maximum COD removal of 78.6%. The adsorption isotherm records were appropriately fitted to Freundlich, Hill, Khan, Redlich–Peterson, Toth, and Koble–Corrigan. The correlation coefficient (R2 = 0.9707) and a comparatively low value of the root mean square error (RMSE = 3.97) demonstrates that the adsorption technique adapted a pseudo-second-order model. Negative values of ΔG0 signify that COD removal by TiO2@ASAc-7 was spontaneous and positive values of the enthalpy change (∆H° = 37.816 kJmol−1) indicate that the process was endothermic in nature. To optimise the COD removal efficiency from the leachate sample, a central composite design for a subset of the RSM was also performed.


Introduction
Globally, municipal solid waste are being widely managed by landfill process due to its less investment and operational cost [1,2].The high distress for landfills is to produce more polluted liquor called leachate.It is produced due to water infiltration into the disposed solid waste and mingled with the inherent wastewater, and through biochemical reactions [3][4][5].Solid waste degradation in landfills occurs in four different stages [6]; i.e. aerobic, anaerobic, primary methanogenic and stable methanogenic.Leachate contains extensive hazardous chemical agents signified by the predictable heavy metals and the supplementary compounds deepened with nitrogen and other complex organic matter [7].The pollutants concentration in the leachate varies with the waste composition [8], climate factor, and the age of the leachate [3,9].Generally, the concentration of organic and inorganic contents in the leachate is so high that its small volume may pollute a substantial amount of groundwater reserves [10,11] and also affect the human health due to the consumption of contaminated water/food [8,12].That is why, the treatment of leachate becomes an immense concern for managing waste in various regions.The leachate treatment procedures are classified into three categories: 1) leachate treatment with domestic wastewater after recycling at landfill site; 2) biodegradation methods that include aerobic and anaerobic treatment and 3) physiochemical treatment processes, i.e. flocculation and coagulation, flotation and precipitation, chemical precipitation, chemical oxidation, advanced oxidation process, photocatalysis and adsorption [13][14][15].Biological techniques are more appropriate for fresh leachate treatment and are not suitable for aged/stabilised leachate [16].Hence, physiochemical techniques for biologically treated effluent are required to further remove bio-recalcitrant organic matter.The adsorption technique has been recounted as a convenient method among physiochemical treatments [17].Adsorption technique has been broadly applied to wastewater treatment at a low cost.For the treatment process, commercial adsorbents are being less preferred due to high cost.The complexes (organic and inorganic) present in the leachate entails economical and consistent sorbents owing to the ability to clean up the recalcitrant [18][19][20].The activated carbon obtained from agro-waste has been reported as a key adsorbent, due to its excessive capability for eliminating impurities from contaminated aqueous solutions [21].The activated carbon preparation using almond shells has been recognised earlier in plentiful research [22,23].In addition to that, the incorporation of metal oxide onto activated carbon represents lots of advantages [24,25].This technique of metal loading on adsorbent enhanced the sorption active sites on the adsorbent.An increase in the thermal constancy and yield of the biochar has also been recently reported [26,27].
Titanium dioxide (TiO 2 ) is a widely used nanoparticle for loading on the activated carbon.The hydroxyl groups committed to the surface of TiO 2 nanoparticles may counter with various functional groups that may enhance the action of adsorbent in this technique [28].Yuningrat et al. [29] prepared the TiO 2 immobilised white cement adsorbent for removing COD from landfill leachate.The activated carbon loaded with TiO 2 nanoparticles has also been applied for phenol adsorption from waters [30].However, no study has been reported for the embedding of TiO 2 in activated carbon created from the almond shell and its application in adsorption of impurities from leachate.In view of the above, a low-cost and effective adsorbent, agricultural waste almond shell loaded with TiO 2 nanoparticles was prepared and characterised.The efficacy of TiO 2 @ASAc for the removal of organic impurities from leachate samples was determined under optimised conditions.The equilibrium isotherms, kinetics, thermodynamics study and surface response methodology (RSM) were investigated to find suitable concepts of COD removal from leachate through adsorption techniques.

Materials and characterisation
Analytical grade TiO 2 , ZnCl 2 , dilute nitric acid, ethanol, and hydrochloric acid purchased from Sigma-Aldrich were used to synthesise TiO 2 loaded activated carbon nanoparticles.Raw Almond shell (RAS) was collected from the local vendors in Hisar City, Haryana.The physical and ultimate analyses of precursor RAS in wt% are illustrated in (Table 1).The ash content, volatile content and moisture content of RAS were determined according to the ASTM D2866-94, ASTM E 872 and ASTM D 2016 methods, respectively.The elemental analysis of RAS was done by using Elementar Vario Micro Cube to evaluate the amount of C, H, N and S. The results show that RAS has a high carbon content (49.89%) and volatile content (78.95%) comparatively low ash content, which is why it is more appropriate for the preparation of porous activated carbon.Leachate sample was prepared from MSW in an anaerobic bioreactor having 20 cm inner diameter and 100 cm height at laboratory scale.After stabilisation for 8 months, the leachate was collected for the post-treatment.The physiochemical descriptions of the leachate are depicted in (Table 2).All the experiments were performed in triplicates.

Almond shell activated carbon preparation
Almond shell was washed properly with deionised water to remove impurities and dried at 80°C for 3 h in an oven.The granular almond shell was grounded and sieved through a 0.2 mm sieve.Then, the raw almond shell (RAS) powder was put in a tubular furnace under the N 2 flow of 200 mL/min at 500°C for 30 min with the ramp rate of 20°C/min.Afterwards, the carbonised almond shell (CAS) was impregnated in the zinc chloride (ZnCl 2 ) activation reagent solution, using (wt.%)ratios of 1:1, 1:2, 1:3 and 1:4 for 24 h at  room temperature.The Pyrolysis was implemented at 600, 700, 800 and 900°C for 60, 90 and 120 minutes, respectively, into N 2 atmosphere.All conditions applied for the preparation of almond shell activated carbons (ASAcs) were illustrated in (Table 3).Finally, all the samples were rinsed in deionised water and dried at 110°C for 24 h.

Preparation of TiO 2 loaded ASAc
After the preparation of ASAc, nanoparticles were loaded on the most efficient ASAc in ratio of 2:1 through the impregnation method [31].0.5 g prepared TiO 2 nanoparticles were dissolved in an ethanol-water ratio of 1:1 mixture under the stirring conditions at room temperature.Then, 1.5 mL of dilute nitric acid was added to the mixture for adequate dispersion of the nanoparticles.Afterwards, 1 g of ASAc was mixed in the above solution.The suspension was given constant stirring for 2 h and bath sonicated for 60 min.Then, the final product TiO 2 @ASAc-7 was collected by centrifugation.Finally, before washing with deionised water, the product was calcinated at 200°C so that nanoparticles adhere to the activated carbon more strongly on its porous walls.

Characterisation of titanium dioxide loaded almond shell adsorbent
The yields of ASAcs were deliberated through the following equation: The adsorbents were investigated through Fourier Transform Infrared Spectroscopy (FTIR technique using the Perkin Elmer Spectrum 400 instruments in the range 400-4000 cm −1 with 1 cm −1 resolution to confirm the formation of activated carbon with TiO 2 .The appropriate ratio of KBr salt was mixed with small quantity of powder sample, and prepared homogeneous mixture was filled in the disc by applying pressure.The prepared pellets of different samples were analysed through FTIR spectrophotometer. The surface area of ASAc and TiO 2 @ASAc-7 was determined using BET (Quantachrome Instruments, ASI-CI-11) liquid nitrogen adsorption-desorption method.Briefly, the dried sample of 0.0335 g was placed in a sample tube at 77.3 K for 478.4 min.to eliminate the humidity and volatile contents.The amount of nitrogen adsorption-desorption was measured through adsorption-desorption curves, and the BET surface area and pore volume were obtained.The adsorbent photomicrographs were taken by using FESEM (JSM-7610FPlus) to determine the morphology, while the elemental composition was explored through EDX (JSM-7610FPlus).The prepared powder in the form of TiO 2 @ASAc-7 was deposited on copper grids and after gold plating; SEM-EDX spectrum analysis was performed.The point of zero charge (pH pzc ) of prepared biosorbent was determined by salt addition technique.0.1 g of the biosorbent was added into 40 mL solution of NaNO 3 (0.1 M) and initial pH (pH i ) was maintained from 2 to 12 using nitric acid (0.1 M HCl) and sodium hydroxide (0.1 M NaOH) solutions.After 24 h shaking, the final pH (pH f ) of the solution was determined using pH metre.The pH pzc of adsorbents were calculated by plots of ΔpH (pH f -pH i ) versus pH i .

Batch adsorption
Batch mode experiments were performed in a 250 ml Erlenmeyer flask containing 50 ml of leachate sample.Different dosages of TiO 2 @ASAc-7 were added to each batch followed by stirring at 150 rpm in an incubator shaker (Orbitek, India).The COD removal percentage was observed at pH (2-10), contact time (20-200 min.),adsorbent dosage (5-50 g/L), and temperatures (25-40°C) and; optimum conditions of these were obtained.The pH of the solutions was maintained through (0.1 M) HCl and NaOH and monitored by pH metre (Eutech, pH tutor, Singapore).The adsorbent mixtures were filtered to reduce interference in COD measurements and the leftover concentration of COD was determined using a dichromate closed refluxed calorimetric method.The percent removal of COD (R) and adsorption capacity of COD at equilibrium Q e (mg/g) were calculated with the following equations: Where C i (mg/L) was the initial liquid-phase concentration of COD, C e (mg/L) was the final concentration, V (L) is the volume of leachate and M (g) is adsorbent mass.
The various isotherm models were applied to monolayer and multilayer adsorption techniques implementing nonlinear equations (Freundlich, Hill, Khan, Redlich-Peterson, Toth, and Koble-Corrigan).(Table S1) shows the nonlinear equation as well as the narrative of isotherms.The adsorption kinetic models of pseudo-first-order, pseudosecond-order and intraparticle diffusion were employed for COD removal analysis through the non-linear plots.
Response Surface Methodology (RSM) not only defines the correlation among different process factors and the responses but also determines the implication of these factors on the number of responses [32].The most effective central composite design (CCD) of RSM was employed to investigate the relationship between response (output) parameters and input parameters for the efficient removal of COD onto the surface of TiO 2 @ASAc-7.Four independent variable levels (i) pH (4-8), (ii) adsorbent dose (1.25-2.25 g/50 mL), (iii) contact time (100-200 minutes) and temperature (25-40°C) were considered on the basis of experimental studies.For the removal of COD, CCD was employed to generate a set of 30 experiments using Design-Expert®software 12. Five levels (−1, 0, +1, −α, +α) were used for each independent variable.A pure quadratic polynomial equation was employed to describe association among independent variables and its detected responses [33].The model equation is described as: where Y pred is the predicted value of the response variable, β 0 is constant, β i, β ii, β ij are linear, quadratic and interaction constants of regression coefficients and x i and x j are the independent variables in the form of coded values, respectively.The outcomes of COD removal by TiO 2 @ASAc-7 adsorbent were investigated through analysis of variance (ANOVA) and model graphs such as predicted versus actual value plots, threedimensional graphs and contour plots.

Reusability of nanoparticle loaded activated carbon
The nanoparticle loaded adsorbent was regenerated and reused during the adsorption process.The organic material loaded spend adsorbent (1 g) was washed with the different concentration of HCl (0.05, 0.1 and 0.15 M) and kept in a shaker (150 rpm) for 180 min.The acidic washed adsorbents were filtered and rinsed several times with distiled water until the pH of 7, followed by drying in a hot air oven at 100°C for 24 h.The Spent adsorbents were reused for organic material reduction up to three successive cycles.

Optimisation of ASAc
The most efficient adsorbent was evaluated through a number of batch adsorption experiments as illustrated in (Table 3).An adsorbent dose of 30 g/L [34] was used to treat 20 mL leachate (pH 6.2 and COD 9,250 mg/L) for determining the optimum conditions of impregnation ratio, contact time and temperature of the adsorbent ASAc.Result shows that (ASAc1-ASAc3) the carbon yield improves with the increasing impregnation ratio and the 1:3 ratio was found to be most suitable.However, at the high temperature, the yield of carbon decreased due to more release of volatiles [35].(Table 3) indicates the COD removal percentages at various adsorbent dosages along with the impregnation ratio, activation time and yield production.It was found that the carbonisation temperature had substantial effects on the COD removal.The COD removal was increased with increase in temperature from 600 to 800°C.Further increase in the temperature does not show a significant effect on the COD removal.This behaviour might be due to a violent gasification reaction occurred at temperature >800°C and micropore structures damaged by combining or collapsing to each other [36].Activation time reinforced the effect of temperature; therefore, 90 minutes at 800°C activation time was selected based on COD removal.Among the activated carbons investigated ASAc-7 performed better with COD removal of 52%; therefore, it was chosen for further experiments.

Characterisation of the ASAc
FTIR spectra of biomass stipulate information about functional groups as well as the possible trace minerals in the materials.The biochar has an amorphous behaviour, but it shows vibrational characteristics of diamond-like carbon materials with nitrogen, oxygen and hydrogen impurities [37].The FTIR spectrum finds the bands attributed to lignocellulosic substances related to the functional groups such as hydrogen-carbon bonds aromatic rings, ethers, esters, aldehydes, hydroxyl and amine bonds [38].Yet, as a description activated carbon be obligated to any absorption bands in the spectral section [37].(Figure 1(a)) represents the FTIR spectra of almond shell materials obtained at different conditions i.e. 1AS (raw almond shell), 2AS (physical activated almond shell), 3AS (physical and chemical activated almond shell) and 4AS (TiO 2 nanoparticle loaded activated almond shell).It was observed that less adulterated AC (3AS, 4AS) reveals bands associated only with O-H and C = C groups on 2946 cm −1 and 1566 cm −1 in 3AS and on 2937 cm −1 and 1566 cm −1 in 4AS, respectively.In addition to these bands, the band at 536 cm −1 corresponds to the spreading of vibrational modes of TiO 2 in 4AS material [32].While in the case of 1AS and 2AS, a wider band was observed at 3380 cm −1 and 3708 cm −1 corresponds to the presence of O-H stretch vibrations.This was mainly due to the presence of water molecules in the sample [39].Furthermore, the band near 2900 cm −1 and 2939 cm −1 demonstrated the C-H vibration in 1AS and N-H vibration in 2AS, respectively.The band centred at 1586 cm −1 for 1AS and 1577 cm −1 for 2AS specifies the enhancement in the aromatic The porous structure of the activated almond shell loaded with TiO 2 was determined by N 2 adsorption-desorption isotherm characteristics as depicted in (Figure 1(b)).The prepared samples show the classical IV type isotherm with the irreversible domain of adsorption-desorption isotherm.The curves illustrate the type H3 hysteresis in the adsorption isotherm [40].This indicates that the material has non-rigid aggregates forming slit-shaped pores.The hysteresis loop is caused by the pressure difference between capillary condensation and evaporation [37,41].The BJH specific surface area of TiO 2 @ASAc-7 (153.3 m 2 /g) was observed to be nearly three times larger than that of efficient ASAc (54.7 m 2 /g).The total pore volume also improved from 0.039 cm 3 /g to 0.136 cm 3 /g.The average pore size values for activated almond shell and TiO 2 loaded activated almond shell were 2.91 nm and 3.47 (Figure 1(b)).
The morphological and elemental changes of ASAc and TiO 2 @ASAc-7 are illustrated in the FESEM-EDX spectrum (Figure 2(a)) depicts the aggregates of the activated carbons with porous morphology that might be due to activation of the almond shell with zinc chloride (Figure 2  ΔpH (pH f -pH i ) versus pH i (Figure 3).The pH pzc of TiO 2 @ASAc was found to be 6.5 at 28°C.It indicates that the surface charges of materials was positive at pH below pH pzc and was negative at pH greater than pH pzc .

COD removal experiment
The percentage COD removal of leachate was investigated at different conditions of pH, dose, contact time and temperature.During optimisation process, the value of one parameter was changed while keeping the other factors constant.

Effect of pH on COD removal
The influence of pH on COD reduction from leachate is depicted in (Figure 4(a)).pH is an important factor since it has effects on surface charge and also the degree of ionisation of the adsorbent [42].To assess the pH influence, experiments were performed with varying pH range from 2 to 10, while adsorbent dose, temperature and COD concentration were kept constant at 30 g/L, 30 ± 1°C and 9,250 mg/L.An increase in the percentage of COD removals was observed from pH 2 to 5.5 and the maximum reduction was observed to be 73.2% at pH 5.5.Further increase in pH resulted in decreased COD removal.The results are in agreement with the literature [34,43].The pH pzc of prepared TiO 2 @ASAc-7 was 6.5.However, in a strong acidic medium, lower adsorption can be attributed due to electrostatic repulsion between positive charges of adsorbent and organic matter.With an increase in pH up to 5.5, maximum COD reduction occurred.This might be due to more generation of hydroxide ions at pH 5.5 to 6.5, which is counter to the valence band holes and creating hydroxyl radicals that stimulate the TiO 2 degradation efficiency [44,45].Incomparable at a high pH value, the ionic repulsion among negatively charged surface and solution (containing OH − ) increases, leading to a reduction in COD removal [46,47].Hence, pH 5.5 was estimated as the optimal pH for doing subsequent experiments.

Effect of dose on COD removal
The substantial effect of dosages on the COD reduction was examined by adjusting the adsorbent dose from 0.25 g/50 mL to 2.5 g/50 mL at pH 5.5 and temperature 30 ± 1°C.The percentage removal of COD was increased from 21.1% to 88.9% with an increase in TiO 2 @ASAc-7 dose from 0.25 g/50 mL to 2.5 g/50 mL (Figure 4(b)).This may be recognised by more accessibility of binding sites or surface area at the rising dose of adsorbent [23].However, no expressive effect was found on the COD removal beyond 1.75 g/50 mL.Thamilselvi and Radha [48] has also observed similar observations for COD removal from the effluents.A dose of 1.75 g/50 mL of TiO 2 @ASAc-7 was elected as the optimum dose with uptake capacity of 207.73 mg/g and COD reduction of 78.6%.

Effect of contact time on COD removal
In the adsorption process, the contact time is one of the key parameters and the effect of contact time on COD reduction is illustrated in (Figure 4(c)).The experiments were performed by changing the contact time from 20 to 200 min at pH 5.5, dose 1.75 g/ 50 mL, temperature 30 ± 1°C and at a constant COD concentration of leachate.TiO 2 @ASAc-7 adsorbent has removed 43.9% of COD (along with the adsorption capacity 116.02 mg/g) in the first 20 minutes.Further increase in the contact time up to 160 minutes shows an increase in the COD removal (78.6%) with adsorption capacity of 207.73 mg/g.No major changes in COD removal were obtained thereafter.This might be due to the fact that initially there was more availability of active sites on the adsorbent leading to an increase in the sorption rate.However, with further increase in the contact time, decreased in sorption was observed due to filling of active sites by the pollutants and adsorbent reached at the equilibrium [49,50].

Effect of temperature on COD removal
The temperature influence on the removal of COD was observed by varying the temperature in the range of 25°C to 40°C (Figure 4(d)).The COD removal (70.8% to 83.8%) and adsorption capacity (187.11 to 221.47 mg/g) increased with increase in temperature, which suggested that the process showed endothermic behaviour.This might be due to with increase in the temperature the diffusion of the adsorbate across the external boundary layer increased as well as the intraparticle diffusion inside the pores of the adsorbate also increased [51].A maximum COD removal of 78.6% with adsorption capacity of 207.73 mg/g was observed at 30°C and no significant difference in COD uptake was obtained with a further increase in the temperature.Therefore, the 30°C was selected as the optimum temperature.

Adsorption isotherms
Adsorption isotherms of leachate treatment by TiO 2 @ASAc-7 were performed under different adsorbent dose (0.25 − 2.5 g/50 mL).The adsorption data was examined through non-linear forms of various isotherms and their relations between Q e and C e are represented in (Figure 5).Hill's isotherm model described a cooperative phenomenon of the COD species binding onto TiO 2 @ASAc-7.The value of the Hill cooperative coefficient of the binding interaction n h was calculated to be 0.52568.Khan isotherm was developed for single and multi-component adsorption processes (Table 4) shows that the value of A k (0.4744) is not equal to 1 and along it the values of final concentration C e is also high.This may be due to a model converted towards the Freundlich isotherm.The Redlich-Peterson model follows Henry's law and the Langmuir model at low concentrations and, the Freundlich model at higher concentrations (b R near from zero).In the present study, the value of b R (0.4) was near to zero, due to the fact that it obeys the Freundlich model.Toth isotherm model describes the heterogeneous adsorption process at low and high concentrations throughout the quasi-Gaussian energy asymmetrical distribution.Toth isotherm constant t h was calculated to be 2.10.The Koble-Corrigan isotherm is normally applied for heterogeneous adsorption processes and is also used for the combination of both Langmuir and Freundlich isotherms.The calculated value of parameter D (0.5674) is less than 1, which shows that the Koble-Corrigan model could not described experimental data.Based on the values of nonlinear models, the Freundlich isotherm was found to be appropriate that expresses the adsorption process of COD with R 2 (0.9884) and 1/n (0.5256) as shown in (Table 4).The fitting of these models directed that the reduction of COD by TiO 2 @ASAc-7 could endure multilayer adsorption.

Adsorption kinetic studies
The mechanism or rate-controlling step of adsorption to the COD reduction through TiO 2 @ASAc-7 was investigated by applying pseudo-first-order, pseudo-second-order and intraparticle diffusion.The non-linear plotting between Q t and t is shown in (Figure 6) and its resultant constant, correlation coefficient (R 2 ), root mean square error (RMSE) and the calculated value of Q e are depicted in (Table 5).The results indicate that pseudo-second-order kinetics was best fitted because the calculated value of Q e (210.63 mg/g) is found to be more substantial along with the experimental data (210.37mg/g).On the other hand, the correlation coefficient (R 2 ) value of 0.9707 was significant and the value of RMSE 3.97 was lower than the pseudo-first-order model.A similar finding has been observed by Mahmoud et al. and Tanzifi et al. [32,63].In the case of pseudo-first-order, the reported results of R 2 (0.9081), RMSE (8.55) and the calculated value of Q e (209.57mg/g) specified that the order rate expression does not adapt to the adsorption process.
The plots between Q t vs. t 1/2 for the intra-particle diffusion rate give the value of the intra-particle diffusion rate constant (K id ) and the intercept (C), show the thickness of the boundary layer effect (Figure 6).It is not only the rate controlling step if the plot does not pass into the origin.The 0.8844 values of the correlation coefficient R 2 indicate that the intra-particle diffusion isotherm is not of much significance for adsorption study.

Thermodynamic studies
The temperature has a more significant effect on adsorbent capacity during the adsorption study.To obtain the thermal behaviour of the adsorption process, thermodynamic studies of COD removal were performed at four different temperatures (298 K, 303 K, 0.00332 1.24 0.47443 0.9884 [56,57] Redlich-Peterson  308 K and 313 K).Thermodynamic factors include Gibb's free energy change (ΔG 0 ), entropy change (ΔS 0 ) and enthalpy change (ΔH 0 ) were calculated to reveal the spontaneity of the reaction.The following equations were applied to compute the factors: The evaluated ∆H° and ∆S° were deliberated through the slope and intercept of the linear plots between lnK d and 1/T.Due to the high value of R 2 the data were confirmed to be the best fit (Table 6).The negative values of ΔG 0 signified that COD removal by TiO 2 @ASAc-7 was spontaneous.The ΔG 0 values decrease with increase in temperature replicates that high temperature was favourable for COD adsorption.The positive value of the enthalpy change (∆H° = 37.816 kJmol −1 ) indicated that the interface of COD adsorption with TiO 2 @ASAc-7 was endothermic, and COD adsorption equilibrium grown up with rising temperature.The positive value of the entropy change (ΔS 0 = 134.753Jmol −1 K −1 ) indicated some structural changes and an increase in randomness at the adsorbent solution interface.

RSM-CCD Analysis
The RSM-CCD through six replications in the central point was concerned with the experimental design.Four independent variables, i.e., pH, dose, time and temperature along with the ranges and levels are revealed in (Table 7).
(Table S2) depicts the experiment designed through CCD.The quadratic equation ( 8) as given below defines the correlation between variables and responses, which were found to be fitted for interaction among variables and appropriate to the prediction of the COD adsorption.

COD (%) removal
The Statistical analysis computed using ANOVA, shows the values of p < 0.0001 and F-395.55,indicated model is the best fit (Table 8).The value of adequate precision (signalto-noise ratio) was obtained (66.16) greater than 4, which indicates the authenticity and model suitability.The developed model is desirable for postulating the design space.The CV values of 1.37% validated the infallibility of the experimental data.The Adjusted and Predicted R 0.9948 and 0.9877, respectively, which are in close agreement indicated that the designed experiment was well fitted in the quadratic model (Figure 7(a)).The combined effect of variables in the form of contour plots and 3-D surface response plots are shown in (Figures 7(b-g) and 8).This shows a reduction in COD with an increase in pH and adsorbent dose.The COD reduction was increased as the pH value varied from 4 to 6.After that, the percent removal was decreased with an increase in pH, indicates an inhibitory effect at high pH.The COD removal efficiency was increased with increase in dose from 1 g to 1.75 g and later on, no major change was observed.The effect of pH in combination with contact time and temperature shows the maximum COD removal at the centre point (Figure 8(b,c)).The COD removal percentage was increased with increase in contact time and temperature.The maximum COD removal was found to be 76.68% at optimum conditions of pH 6, dose 1.75 g/50 mL, contact time 150 minutes and temperature 32.5°C that are shown in contour and three-dimensional graphs.These optimum conditions evaluated through the RSM-CCD algorithm model were in good promise with the experimental data for the COD reduction using TiO 2 @ASAc-7 adsorbent.

Regeneration and organic content removal efficiency of TiO 2 @ASAc-7
Regeneration efficiency of adsorbent is essential for cost-effective and sustainable wastewater treatment process.An outstanding adsorbent should not only be better in adsorption efficiency but also be significant in regeneration and reusable practices (Figure 9) represents the COD removal efficiencies of the TiO 2 @ASAc-7 adsorbent regenerated with HCl solution.The COD removal efficiency of adsorbent was increased with increase in HCl concentration from 0.05 to 0.15 M. The COD removals evaluated at 0.15 M HCl concentration were 71.5, 62.2 and 56.3% for cycle 1, cycle 2 and cycle 3, respectively (Figure 9).A more or less equal COD removal capacity has also been observed with silver nano particles loaded corncob [48].More than 50% COD removal in three regeneration cycles of adsorbent showed the economic and leading properties of prepared TiO 2 @ASAc-7.Uptake capacity of TiO 2 @ASAc-7 in the current study is comparable with other similar study presented in (Table 9).

Conclusion
The activated carbons were synthesised using precursor almond shells through the physical and chemical activation processes.The specific surface area exhibited the pre-eminent texture properties of TiO 2 @ASAc-7 and it was found suitable to treat leachate.The pH pzc value of the prepared biosorbent TiO 2 @ASAc-7 was obtained to be 6.5 at 28°C indicating the surface charges of the material.The best removal efficiency of COD 78.6% with adsorption capacity 207.73 mg/g was found at pH 5.5, dose 1.75 g/50 mL, time 160 minutes and temperature 30°C.Nonlinear isotherm studies of Freundlich, Hill, khan, Redlich-Peterson, Toth, and Koble-Corrigan fitted well to the adsorption data with R 2 values of 0.9884, 0.9884, 0.9884, 0.9885, 0.9885 and 0.9823, respectively, confirming the multilayer adsorption behaviour.In the kinetic study the highly significant value of the correlation coefficient, low value of  Pseudo-first order kinetic constant (min −1 ) K 2 Pseudo-second order kinetic constant (g mg −1 min −1 ) K id Intra-particle diffusion rate constant (mg g −1 min −
(b)) expresses the TiO 2 nanoparticle loaded almond shell activated carbon surface and it is observed that the shape of the TiO 2 particles (evidenced included in red circle) was irregular (Figure 2(c)) represents the EDX spectra of activated carbon having atomic percentage of carbon 75.52 and oxygen 24.48 (Figure 2(d)) shows elemental peaks of carbon, oxygen and Ti in TiO 2 @ASAc-7 nanoparticles along with the atomic percentage of C 70.1, O 22.61, and Ti 7.29.The pH pzc values of the prepared biosorbents were determined from plots of

Figure 2 .
Figure 2. SEM micrograph (a) Activated almond shell, (b) TiO 2 nanoparticle loaded almond shell activated carbon, c) EDX spectrum of almond shell activated carbon, d) EDX spectrum of the TiO 2 nanoparticle loaded almond shell activated carbon.

Figure 3 .
Figure 3. Point of zero charge (pH pzc ) of the TiO 2 nanoparticle loaded almond shell activated carbon.

Figure 4 .
Figure 4. Effect of (a) pH, (b) Dose, (c) Contact time and (d) temperature on COD removal percentage.

Figure 7 .
Figure 7. Actual vs predicted and contour plot for COD removal.

Figure 8 .
Figure 8. Three dimensional response plots for COD removal.

Figure 9 .
Figure 9. COD removal efficiency of regenerated TiO 2 nanoparticle loaded almond shell activated carbon.

Table 3 .
Conditions for preparation of different ACs.

Table 4 .
The values of various parameters of the non-linear adsorption isotherms.

Table 8 .
Analysis of Variance (ANOVA) for COD removal percentage.

Table 7 .
Independent variables with level in RSM-CCD and quadratic model statistics.

Table 9 .
Adsorption uptake capacity of various adsorbents for COD removal from leachate.RMSE and equivalent value of calculated adsorption capacity with experimental data revealed that adsorption technique adapted pseudo-second-order model.The spontaneous and endothermic behaviour of adsorption was observed through the thermodynamics process.The high correlation values of Adjusted R 2 (0.9948) and Predicted R 2 (0.9877) indicated that Surface response methodology with CCD model was well fitted.The optimum values of pH 6, dose 1.75 g/50 mL, contact time of 150 minutes and temperature 32.5°C were obtained with a design experiment, which were wellmatched with the experimental data.The regenerated adsorbent efficiency was found to be significant up to three cycles.The overall analysis indicates that TiO 2 @ASAc-7 is a promising low-cost adsorbent for effective and efficient treatment of leachate.However, before industrial application further study is required on TiO 2 release in solution.