Characterisation and adsorption properties of calcinated eggshell, salicylic acid-modified eggshell, and 2, 4-dihydroxy benzoic acid-modified eggshell

ABSTRACT This study synthesised eggshell waste, an agricultural by-product, into a calcinated porous material modified by salicylic acid as a new and effective adsorbent for remediation of Cu(II) from an aqueous solution. The impregnation of salicylic acid onto the surface of calcinated egg-shell improved the adsorption property of the eggshell. Furthermore, it was found that the salicylic acid-modified egg-shell; (SAL-E) has a highly developed functionalised surface and rich pore property compared to that of calcinated egg-shell investigated by FTIR and BET surface analysis, respectively. Moreover, the surface area of the SAL-E composite was found to be 6.65 m2g−1 which is six times more than the calcinated eggshell; (E-500). The salicylic acid procreated calcinated egg-shell showed the superior adsorption property towards copper heavy metal compared to that of calcinated eggshell and 2,4-dihydroxybenzoic acid-modified eggshell; (DHB-E). The results of the study showed that the Langmuir adsorption of salicylic acid procreated egg-shell was up to 177.74 mgg−1 which is four times more than the calcinated egg-shell; (E-500). The maximum adsorption showed by salicylic acid-modified eggshell; (SAL-E) occurred at pH 7. The efficiency of modified egg-shell for the removal of copper from wastewater is directly related to pH, temperature, and contact time. FTIR spectroscopy study affirmed that the vital functional groups engaged in the uptake of Cu(II) onto the surface of modified – eggshell wastes, were hydroxyl group, carboxyl, carbonyl, and C = C of aromatic ring via Cu(II)−π interaction. FTIR, Raman, and UV – DRS evaluations proclaimed the introduction of functional groups that can be rendered more emplacements for binding of Cu (II). The results of this study displayed that the adsorption accept, the pseudo-second-order, Langmuir isotherm, and intraparticle diffusion, indicating that the adsorption mechanism is a chemical adsorption process and physisorption to some extent. The thermodynamic parameters such as ΔG0, ΔH0, and ΔS0 were estimated for all the porous adsorbent materials.


Introduction
The quality of drinking water is a vital concern for mankind since it is directly linked with human welfare.It is a matter of history that fiscal pollution of drinking water caused water-borne diseases which wiped out an entire population of these cities.At present, the menace of water-born diseases and epidemics still looms large on the horizons of developing countries.Polluted water is the culprit in such case.The application of ammonium thioglycolate-modified eggshell membrane for the adsorption of heavy metals from an aqueous solution has been assessed [1].It was confirmed that the adsorption can be described by Langmuir isotherm.Biogenic hydroxyapatite derived from eggshell waste modified with humic acid was used for the removal of methylene blue from an aqueous solution [2].The combined application of hydroxyapatite and humic acid remediated the methylene blue more efficiently as compared to that of un-modified hydroxyapatite.A Chemically modified eggshell membrane was used for the removal of methylene blue from an aqueous solution [3].Humic acidmodified -polycation -montmorillonite clay and activated carbon was employed for the remediation of pyrene [4] from an aqueous solution.The employment of humic acid as an improving agent in pollutant remediation is novel and has considerable priority in wastewater treatments [5].Heavy metals have become of particular interest in recent decades within the frameworks.The diffuse constituents result from traffic activities, roadway maintenance operations, and atmospheric deposition.Tire wear is a source of Zinc and cadmium [6].Brake wear is a source of copper, lead, chromium, and manganese.Engine wear and fluid leakage is a source of aluminium, coppernickel, and chromium.Vehicular component wear and detachment are a source of iron, aluminium, chromium, and zinc.Electroplating, smelting, paint pigments, batteries, fertilisers, and alloy industries is a rich source of cadmium [7].In Philippines millions of fishes has been died due to the high concentration of cadmium in river water [8].High concentration of cadmium in drinking water can damage the vital organs of animals and humans includes (kidney, liver, lungs) immune system and reproductive system [9], so it becomes essential to assess the quality of drinking water in present days, so that good quality of drinking water is available to future.In the past few years, several techniques have been carried out for remediation of heavy metals from wastewater such as electro-coagulation [10], chemical precipitation [11], Ion exchangers [12], and photo catalysis [13], but most of them have so many disadvantages such as incomplete remediation, high energy requirement, high operating cost and production of toxic sludge during the operations of these techniques, to overcome the shortcomings of above techniques, adsorption phenomenon has been introduced for removal of heavy metals from wastewater.Low-cost adsorbents obtained from biowastes such as, activated carbon and eggshell which are readily available, easy to handle, at no cost, and eco-friendly have been used as super adsorbents in recent years.In recent years, adsorption becomes one of the famous technologies which is highly effective, low cost, and eco-friendly.Numerous adsorbents obtained from biowastes to adsorb heavy metals from wastewater and aqueous solution have been reported in the literature and these include; modified-weight brain [14], apricot stone [15], pecan shell [16], seaweed [17], and corn straw [18].The ability of eggshells to adsorb heavy metals more effectively as compared to that of minerals clays and activated carbon.Over the years eggshell has been used as super adsorbents for remediation of heavy metals such as cadmium, lead, and mercury, along with dyes [19].The chief constituent of the eggshell is calcium carbonate it is the only calcium carbonate that makes the eggshell a super adsorbent where remediation of heavy metals mainly occurs through exchange reaction, hence it is very important to study the composition and property of eggshell as previous studies showed it to be a highly efficient adsorbent.Furthermore, it was found that eggshell as an adsorbent for removal of pollutants from wastewater is highly efficient and low cost, therefore it is very important to observe and investigate the structure of eggshells.An eggshell is composed of three components; a semipermeable membrane, calcium carbonate, and cuticle (bloom).The present problem has been undertaken to develop a highly functionalised eggshell surface by impregnating the eggshell with humic substances (salicylic acid,2,4-dihydroxy benzoic acid).Keeping in view the need for cheap and easily available water purification devices in the areas where municipal water is not available.The present project on the synthesis of eggshell complexes of humic substances (salicylic acid and 2,4 dihydroxy benzoic acid) has been undertaken.Humic substances contain a wealth of functional groups, adsorption of humic substances onto the surface of the eggshell brings sufficient improvement in adsorbents performance.Previous studies showed that the humic substances found in water and soil can be adsorbed onto metal oxides that altered their surface property, helps decrease the movement of pollutants in the environment.The results showed that the adsorption of humic substances onto the metal oxides mainly occurs through different interaction that includes; hydrophobic interaction [20], ligand exchange interaction [21] electrostatic interaction [22], hydrogen bonding [23], cat ion bridge [24].Calcite obtained from eggshells has a rhombohedra crystal structure.The Ca 2+ and CO 3 2-on calcite produced a polar hydrophilic surface.Organic compounds which contain hydroxyl and carboxyl groups interact with calcite both by electrostatic interaction between Ca surface and oxygen of hydroxyl group and interaction of hydrogen of the hydroxyl group with the oxygen of carbonate group.More ever it was found that the carboxyl group adsorbs more strongly onto the calcite surface as compared to that of the hydroxyl group.Surface functionalization lapsed proved an intended potent way to build such a favourable adsorbent because functional groups besides can contribute binding sites for the remediation of heavy metals from wastewater but additionally can enhance the adsorption property.Indeed, surface functionalization with carboxylate, hydroxyl, and nitrogen-containing functional groups onto the surface of biosorbent has been published for heavy metal remediation [25].The oxygen-containingfunctional groups are an estimable ligand because of their mighty sympathy for diverse kinds of heavy metals, consequently of lewis -acid-base reaction [26].Calcite display extraordinary chemical affection for organic molecules possess polar functional groups like carboxyl and hydroxyl groups.Furthermore, it was found that salicylic acid molecules chemisorb onto calcium sites employing carboxyl and hydroxyl groups [27].This study aims to explore the practicability of using salicylic acidfunctionalised (SAL-E)calcite derived from eggshell waste via calcination as a biosorbent for the remediation of toxic heavy metals from wastewater.Even so, no report of the exploration of the salicylic acid-functionalised calcinated eggshell powder for the remediation of heavy metals has been published.

Materials and reagents
Eggshell obtained from kitchen dust was used in this study.The chemical composition of eggshell and calcinated eggshells had been reported in the literature.The salicylic acid, 2, 4-dihydroxy benzoic acid, and Gallic acid were purchased from India, the chemical composition of calcinated eggshell was found as, C (Wt%) (9.94), O (wt %) (47.36) and Ca (wt %) (42.7).The salicylic acid, 2, 4-dihydroxybenzoic acid, and Gallic acid were purchased from Indian.The eggshell was crushed and grind into a fine powder and was sieved at 15 microns using mortar and pistil to get uniform-sized starting material.Humic acids are organic compounds, which are not soluble in water but become soluble at higher pH values.Humic acid contains a wealth of functional groups such as aromatic rings, and main oxygencontaining functional groups like carboxyl (COOH) and hydroxyl groups (OH).
Copper nitrate Cu (NO 3 ) sodium hydroxide Na (OH) and hydrochloric acid HCl were attained from the sigma Aldrich.Double distiled water was being used to make all the aqueous solutions.The reserve solution was made using 1000 mg/L of Cu (II).By diluting the reserve solution to proper volumes, operating solutions have been produced.

Preparation of eggshell waste
Eggshell obtained from kitchen dust, rinsed and washed several times with tap water followed by deionised water to remove dirt and impurities and kept in a hot air oven at 45°C for overnight to obtain wasted eggshell powder.

Preparation of calcinated eggshell
The fresh eggshell ground before activation .10g of the sieved material was placed in a muffle furnace and heated at 500°C for 4 h to obtain CaCO 3 .After this, the sample was allowed to cool at room temperature and hived in desiccators for later use.

Preparation of humic acid-modified eggshell
Humic acid solutions were prepared with a deionised water pH value of humic acid solutions were analysed by pH metre and adjusted by using 0.1M NaOH or HCl.Humic acid solutions (SAL, DHB, GA) were prepared by dissolving the proper amount of HAs into deionised water with a pH value of 8. Humic acid-modified eggshells were prepared by adding 150 mg of eggshell powder into 70 mL of HA solutions, subsequently resulted from solutions were dispersed for 20 min in an ultrasonic bath for sonication.The Solution which contains HA and eggshell powder was analysed again and finally kept at pH 8 using 0.1M NaOH.The suspensions were kept in an isothermal oscillator at room temperature to be shaken for 10 h, subsequently, the suspensions were allowed to stand at room temperature for 24 h.The humic acid eggshell complexes were recovered from their solutions and washed thoroughly with NaOH solution with pH value 8, dried in desiccators for later use.

Batch-adsorption experiment for adsorption of humic acids onto the calcite
The adsorption experiments were executed in a batch equilibrium method.The humic acid solutions with various concentrations (20 mg/L,35 mg/L,50 mg/L,80 mg/L,100 mg/L, 150 mg/L) were first prepared by adding an appropriate quantity of humic acid within the noted volume of NaOH solution with a pH value of 10.Exploration suspensions were attained by dissolving 150 mg CaCO 3 mineral derived from eggshell waste via calcination method within 70 mL of humic acid solution and were dispelled for 9 min in an ultrasonic bath.Subsequently, the pH value of the test respites solution was examined repeatedly and was reconciled eventually to carry at pH 10. Entire suspensions were settled within a constant temperature oscillator at 25°C intended stirred for 69 h.Consequently, the suspensions were retained standing for 23 h before the humic acid adsorption were resolved using a model lambda 365 UV spectrophotometer at a wavelength 320 nm the amount of salicylic acid and 2, 4-dihydroxy benzoic acid impregnated onto E-500 was estimated and found to be 2.76 mg/g and 2.60 mg/g, respectively.
Where C i is the initial salicylic acid concentration,Ce is the salicylic acid concentration at adsorption equilibrium (ppm) V is the volume of salicylic acid solution m is the mass of adsorbents (calcite) in (g).

Batch -adsorption experiment for adsorption of Cu (II) on humic acidmodified calcite
To explore the impact of humic acid solution concentration on materials accomplishment, 150 mg of each material was appended into a 100 mL centrifuge tube with 40 ml of 24.0 mgL −1 Cu (II) solution.The pH of the resulted solution was adapted by using 0.1M HCl and NaOH, subsequently, the solutions were agitated at 130 r.min −1 for 2 h at 313 K temperature.The composite SAL-E displays the best performance was selected for use as the modified adsorbent in afterword experiments by observing the initial and final concentration.In the investigation on the pH effect, respites were attained by dissolving 20 mg of all the three porous materials into 50 mL of 25 mgL −1 Cu (II) solution, the pH values of various suspensions were adapted to 2,3,4,5,6 and 7 were agitated at 130 r min −1 at room temperature for 2 h.The final accommodates of Cu(II) prevail, subsequently calculated and the ideal pH value for Cu (II) remediation was decided.The consequence of contact time on the adsorption process was investigated at an excellent pH value of 25mgL −1 of the Cu (II) solution eventually, adsorption isotherm investigations were accomplished at room temperature.In the exploration, the Cu (II) concentration was examined utilising AAS, and Cu (II) adsorption was evaluated via the mass balance.
Where C i is the initial Cu (II) concentration (ppm), Ce is the Cu (II) concentration at adsorption equilibrium (ppm) V is the volume of Cu (II) solution mis the mass of adsorbents in (g).

Surface area of SAL-E and DHB-E composite
In a previous study, it was found that the surface area of eggshell waste and calcinated eggshell powder is 1.05 ± 0.42 and 1.276 ± 0.56s m 2 /g respectively [28] after the adsorption of humic acids onto the surface of calcinated eggshell powder not only crystalline nature increased but the surface area as well.The surface area of the synthesised SAL-E and DHB-E composites was found to be 6 .65m 2 /g and 5.25 m 2 /g respectively.The adsorption isotherm affirmed the virgin eggshell adsorbent may exclusively assimilate the basic dye of less than 1 mgg −1 in aqueous solution, which was accredited to their shabby pore property, after the adsorption of SAL not only surface area increased but pore property as well.The nitrogen adsorption isotherm of SAL-E composite has been presented in Fig S5, the main pore and physical properties of calcinated eggshells and their composites with SAL and DHB were presented in S1 and S2 Table.

Adsorption kinetics
Adsorption kinetic model put in to elucidate the experimental data to decide the mechanism of Cu (II) adsorption from aqueous solution.Pseudo-first order; Pseudo -first-order [29] is comprehensively applied for the adsorption Cu(II) from wastewater.This kinetics depends on the assumption that the rate of change of Cu(II) uptake concerning time increases with increasing the difference in saturation concentration.Equation ( 3) is a differential form of pseudo-first-order whereas Equation (4) presents integrated form qt is the amount of Cu (II) adsorption per unit of adsorbent at a time t.The value of the first-order rate constant(K 1 ) can be estimated from the graph plot of Log qe À qt ð Þv=st.Initial adsorption rate can be explained as; H(mgg −1 min −1 ) at t !0 Intraparticle diffusion; is generally applied for analysing the adsorption mechanism for design determination.The influence of Intraparticle diffusion on adsorption can be defined as: To obey the Intraparticle diffusion a graph plot of qtv=st 0:5 must provide a linear line equation in which slope is (Kid) Intraparticle diffusion rate constant and intercept is I. Values of I provide information concerning the thickness of boundary walls higher is the value of I larger is the thickness of boundary walls.
Pseudo-second-order; Ho and McKay [30] presented the Pseudo-second-order as: Integrating the above equation from t = 0 to q t = 0 the attained equation might be rearranged into a linear form by plotting the graph between t/qt v/s T, the values of K 2 , and qe might be estimated.

Results and discussion
The adsorption of salicylic acid and 2, 4-dihydroxy benzoic acid per unit mass of calcinated eggshell was more compared to that of Gallic acid shown in Figure 1 (d).Furthermore, it was found that when the initial concentration of humic acids (salicylic acid, 2, 4-dihydroxy benzoic acid) was increased, the amount adsorbed onto the calcinated eggshell increased.more ever, it has been found that Gallic acid is hardly adsorbed onto the calcinated eggshell.The amount of salicylic acid adsorbed onto the calcinated eggshell is little more than that of 2,4-dihydroxy benzoic acid adsorbed onto calcinated eggshell.The Mechanism of salicylic acid and Gallic acid adsorption onto calcinated eggshells is very different.Carboxyl acid, water, and alcohols adsorb onto the hydrophilic calcinated eggshell through their polar hydroxyl group end; furthermore, it was found carboxylic acid adsorbs onto the hydrophilic calcinated eggshell more strongly as compared to that of water, alcohols, and aldehydes.Calcite obtained from eggshells are hydrophilic in nature, the adsorption of large-sized organic compounds onto the calcinated eggshell changes their hydrophilic nature into hydrophobic.Gallic acid hardly adsorbed onto the hydrophilic eggshell, it is due to the hydrophilic nature of Gallic acid, that inhibits its adsorption onto the calcinated eggshell.Gallic acid is more hydrophilic than salicylic acid and 2,4-dihydroxybenzoic acid due to this fact its adsorption on calcite is almost negligible shown in Figure 1(d).Salicylic acid and 2,4-dihydroxy benzoic acid had been adsorbed on the calcite.

FTIR study of 2, 4-dihydroxy benzoic acid-coated eggshell
The FTIR spectra of 2, 4-dihydroxy benzoic acid-modified eggshells have been shown in Fig S3, the extra peaks that are present in spectra of a modified eggshell, but absent in the spectra of un-modified belongs to 2,4-dihydroxy benzoic acid but not eggshell.
The FTIR spectra of modified eggshell contain various new adsorption peaks including various wave numbers 1782, 1637 and 1263 cm −1 the peak at 1782 cm −1 which was attributed to the C=O,1637 cm −1 indicates the presence of C=C in aromatics ring, the peak at 1263 cm −1 assigned the C-OH phenol group.The FTIR spectra of calcinated eggshells before and after adsorption of Gallic acid are summarised in supplementary materials.Gallic acid is hardly adsorbed onto the calcinated eggshell the spectra of modified and unmodified eggshells were almost the same, and there was no change in adsorption peaks, which assigned that Gallic is not adsorbed onto the calcinated egg.Furthermore, it was found that the creation of doublet near OH zone in the FTIR spectrum peak of SAL-E composite after the adsorption of copper conveys the coordinated OH group of carboxyl or hydroxyl group.Disappearance of carbonate band near 1400 cm −1 in the FTIR spectrum of SAL-E after Cu (II) accumulation assigned that carbonate was involved in Cu (II) remediation.FTIR study of salicylic acid-modified eggshell FTIR study of the calcinated eggshell before and after adsorption of salicylic acid was displayed in Fig S3 furthermore it was found that the adsorption peaks in salicylic acid-modified eggshell could not be found in FTIR spectra of the unmodified -eggshell.The salicylic acid-modified eggshell contains more functional groups as compared to that of the calcinated eggshell.The extra peaks that are present in salicylic acid-modified eggshell include wave numbers, 1598, 1293, 1224, 1163 and 755 cm −1 , the peak at 1598 cm −1 indicates the presence of C=C in the aromatic ring, the peak at 1298 cm −1 assigned the stretching vibration of C-O in carboxyl, the peak at 1224 cm −1 was attributed to the stretching vibration of C-OH of phenol group, the peak at 1163 cm −1 was attributed to the stretching vibration of C-O in hydroxyl.The extra peaks that are present in salicylic acid-modified eggshell belong to salicylic acid but not eggshell, indicates the adsorption of salicylic acid onto the eggshell, so that surface functionality of an eggshell had been changed, which brings sufficient improvement in adsorbents performance.

UV -visible spectroscopy of E-500, SAL-E, and DHB-E
The Uv-visible absorption spectra of CaCO 3 derived from eggshell waste and their complexes with SAL and DHB were displayed in Fig S6 .All complexes approximately displayed no adsorption of visible light in the wavelength range of 400-800 nm, speculated their peculiarities as white inorganic materials, nevertheless the, four complexes are unlike in the absorption of Uv light in the wavelength range of 200-400 nm.CaCO 3 derived from eggshell waste displayed one adsorption band at a wavelength of 205 nm.SAL-E composite particle showed three absorption bands in the Uv region that includes 205 nm,232 nm,300 nm.The band at 300 nm in SAL-E is the characteristic band of SAL indicates that SAL is adsorbed onto the surface of calcite derived from eggshell, DHB-E composite exhibited three Uv bands as 216 nm,250 nm, and 293 nm.The peak at 293 nm in DHB-E complex assigned the characteristic band of DHB, ASP-E composite showed three absorption bands in Uv region that includes 215 nm,233 nm, and 296 nm, the peak at 296 nm in ASP-E complex indicates a characteristic band of ASP.The extra bands that are present in humic acid-modified calcite but absent in virgin calcite indicate the adsorption of humic acids onto the surface of calcite and developed highly functionalised calcite surfaces that remediate the Cu heavy metal.It affirms that Uv absorption peaks of salicylic acid-modified CaCO 3 were repositioned to higher wavelengths from 295 nm →300 nm and 231→232.The actual literary texts, on the precept of UV spectroscopy, conveys that a compound can absorb UV light more attributable to the occurrence of either or both conjugated pi(π)bonded organic molecules (π →π* transition due to C=C of the aromatic ring of SAL and lone pair containing organic moiety (n→π* transition attributable to the occurrence of lone pair present on oxygen in SAL).The UV absorption aspect exists when electrons are moved from lower energy orbital's (π, n, and ∂ ) and higher energy orbital's (π*, ∂ *).There is a definite energy gap between lower energy bonding orbital and high energy anti-bonding orbital.Wavelength depends on the distance between lower energy orbital and higher energy orbital when this distance is changed the wavelength is also changed drew on this, it is contemplated that as a result of supremacy SAL-E, the energy gap between π→π* and n→π* in pure SAL probably changed, which leads to the repositioning of wavelength in SAL-E as compared to that of virgin SAL.

DRS spectroscopy
The occurrence of SAL and DHB at the calcite surfaces is instantly verified by diffuse reflectance spectroscopy.The powdered SALs are applied as gauges, the spectra display the UV absorption band at around a wavelength of 300 nm classically for the protonated phenol.Although the inspected bath chromic shift is an outcome of the interplay of salicylate molecule with the solid surface of calcite derived from eggshell waste.Regardless of the reality that the presented kubelka-Munk diffuse reflectance spectra are not applicable for perceptible intentness, the increasing absorbance at 300 nm 296 nm of SAL.The DRS spectra of SAL-E composite was presented in Fig S4.

SEM
The morphological structure exploration of calcinated eggshell and their composites with SAL and DHB can conform from the SEM images, the SEM images of E-500, SAL-E, and DHB-E were displayed in Fig S1 .It was found that the SAL-E composite was a well-defined pore structure as compared to that of E-500 and DHB-E.The crystalline structure of E-500, SAL-E, and DHB-E showed hexagonal designs.

Raman spectroscopy of E-500, SAL-E, and DHB-E
Raman spectroscopy is a superior tool competent in exact and immediately identifying CaCO 3 obtained from eggshell waste via the calcination method and their composites with humic acids (SAL, ASP, and DHB).This technique is notably powerful at differentiating the CaCO 3 obtained from eggshell waste and their composites with SAL, and DHB.Moreover, it was found that the adsorption of humic acids (SAL, and DHB) onto the surface of calcite increased the Crystalline nature of CaCO 3 investigated by XRD, hence the composites of humic acids with CaCO 3 are highly crystalline as compared to that of virgin calcite.The liability of Raman spectroscopy to differentiate between crystalline and amorphous phases at high resolutions.The Raman spectra of CaCO 3 derived from eggshell and their composites with humic acids (SAL and DHB) were displayed in Fig S7 .Raman spectra of CaCO 3 contain only four peaks at various wavenumbers such as 3618 cm −1 ,1085 cm −1 ,713 cm −1 and 375 cm −1 274 cm −1 the peak appears at 1085 cm −1 indicates the C-O stretching of carbonate and the peak at 713 cm −1 corresponding to the C-O in-plane (bending) of carbonate group and the peaks below 400 nm −1 includes 357 cm −1 and 274 cm −1 belongs to vibrations among the molecules in lattice.The SAL-E composites contains more functional groups as compared to that of virgin calcite the extra-functional groups that are not present in virgin calcite, but present in SAL-E composite includes 1600 cm −1 ,1466 cm −1 ,1303 cm −1 ,1250 cm −1 , 821 cm −1 and 3054 cm −1 are belongs to SAL .The peak that appears at 1600 cm −1 corresponding to C=O stretching, the peak at 3054 assigned the C-H stretching of SAL, which indicates SAL was successfully adsorbed onto the surface of calcite, a peak that develops at 1303 cm −1 attributed to the COO − symmetric stretching.

pH changes during the adsorption
The pH of the solution was affected frequently as the adsorption progressed.The pH alterations (while the first 10 min) of a 50 mL solution accommodating 25mgL −1 of Cu (II) and 20 mg of porous adsorbent material were analysed.The pH of all the three porous adsorbent materials dropped immediately throughout the first few minutes and the pH levels of the SAL-E, DHB-E, and E-500 solution endured at a fastened value of about 2, 4,6 minutes Figure 1(c).As noticed above, the ion exchange and complex formation were investigated as intended notable mechanisms for the remediation of heavy metals from aqueous solution, accordingly, the discharge of H + from the porous adsorbent materials and the decline in Cu (II) presumably happens in correspondence with the following equation: Since NaOH is a very strong base that can abstract a proton from oxygen-containing functional groups present onto the surface of salicylic acid-modified eggshell develops negatively charged surface, which can remediate the Cu (II) more efficiently via the functional group complexation and electrostatic interaction process as shown in FTIR spectra displayed in supplementary material.To explore the effect of pH on the adsorption nature of heavy metals and knock out the intervention of precipitation, at various pH range was elected.For Cu (II), an initial pH ranging from 1 to 7 was elected.With increasing of pH discharge of H + takes place that develops negatively charged surface, that leads to increase the remediation of heavy metals, hence with increasing pH value, the adsorption capacity of biosorbent increased, SAL-E composite remediates the hard heavy metals more efficiently as compared to that of soft heavy metals .This could be elucidated by Pearson theory [31] that hard ligand-like salicylic acid is more acceptable to bind with hard heavy metals such as Cu(II) The surfaces of all the three porous adsorbent materials have various acidic functional groups including carboxylic group, hydroxyl groups, few of the functional groups discharge H + and deliver negatively charged group to bind with Cu (II).As presented in Figure 1(a), the porous adsorbent materials uptakes large amount of Cu(II) at high pH values, the Cu(II) adsorption enlarged from 1.35% to 10%,2.5% to 60%, and 3.2% to 99% for E-500, DHB-E, and SAL-E respectively, although the earliest concentration of Cu(II) was 25 mg/L.As cited earlier, the adsorption process is admired at optimum pH Figure 1(a) but the disintegration of salicylic acid becomes a dire issue with increasing pH .Accordingly, the desorption ratio of the composites at various pH values were explored.The results are presents in Figure 1(a) .In the present study, the maximum desorption ratio of SAL-E was 6.96% at pH 7 followed by DHB-E 6.5% at pH 6.In the current study, this desorption ratio was assumed reliable.

Equilibration time
At the ideal pH value (pH7) the sorption process of Cu (II) on three porous adsorbent materials goes on immediately.The equilibration times for SAL-E, DHB-E, and E-500 were 15, 10, 5, respectively, Figure 1(b), the kinetic models needed various adsorption data during equilibration time, but attaining adequate data for kinetic modelling in such a little while it was very tough to study the kinetics of the SAL-E, DHB-E, and E-500.Eggshell powder put in service to adsorb heavy metals is rare in the literature on kinetic modelling.The only few adsorptions kinetic process of the eggshell membrane have been published.Usually, the equilibration time adopted was between 30 min and 60 min, hence the binding of heavy metals with calcinated eggshell powder was a fast process.The excellent adsorption velocity of calcinated eggshell in such a case may be assigned to its particle size that made the calcinated eggshell powder competently react with Cu (II) in solution consequently increasing the application rate of the adsorbent

Adsorption mechanism
The feasible mechanism of adsorption of Cu (II) onto the SAL-E composite was searched out.The Cu (II) cation present in an aqueous medium can form complexes with hydroxyl, carbonyl, and carboxyl functional groups present onto the surface of salicylic acid-modified calcinated eggshell waste, because the oxygen-containing functional groups act as outstanding ligands, it was inspected that SAL-E composite exhibited strong-binding capacity towards Cu (II) compared to that of DHB-E and E-500.The sequence of adsorption adequacy of all three adsorbent materials is like that; SAL-E>DHB-E>E-500 as it may be elucidated by the acid-base interaction.Furthermore, the establishment of a complex between Cu (II) and SAL-E composite could be born out by the creation of doublets in the Ir spectra of SAL-E composite after adsorption of Cu (II) was also noticed from the region 3540 to 3035 cm −1 a zone peculiar of OH stretches.This could be specified that OH stretches of groups obtained from phenol or carboxylic group engaged together to form complexes with Cu (II) [32].The disappearance of certain peaks after Cu(II) adsorption includes 1598 cm −1 of C=C of the aromatic ring was involved in Cu(II) remediation via Cu(II) -π interaction,1163 cm −1 of C-OH stretching vibration of hydroxyl group, and 1248 cm −1 of C-OH vibration stretching of the carboxyl group indicates that the hydroxyl, carboxyl, were involved in surface complexation.The peak that appears near 1400 cm −1 of carbonate group in pre-adsorption of Cu(II) disappears after Cu(II) adsorption assigns that the carbonate group was involved in adsorption.This was affirmed by post-adsorption ftir result analysis shown in supplementary material.This was persistent with already published research articles on calcite [33].The Intraparticle diffusion adsorption occurred through pores which would increase the adsorption capacity of porous adsorbent materials to some extent.This was affirmed by the intraparticle diffusion kinetic data presented in Table 2.The adsorption of SAL onto the pure calcite increased both pore volume as well as surface area, the remediation of Cu (II) from aqueous solution by SAL-E occurs via surface complexation Cu (II)-π interaction of aromatic C=C, Intraparticle diffusion (pore capture adsorption), Electrostatic interaction, cation -exchange, and carbonate group, but in case of virgin calcite the adsorption occurs only via the cation -exchange and carbonate group

Adsorption isotherm
To specify the adsorption process and explore the technique of adsorptions, the different Cu (II) adsorptions isonomous to different equilibrium Cu (II) concentrations were resolved.The data attained Figures 2-4 and, S8 was then examined employing different adsorption isotherms that include Langmuir, Freundlich, Tempkin, and Kahn isotherms, different isotherm parameters were evaluated via non-linear regression analysis, conforming to the parameters, the correlation coefficient of Langmuir Isotherm was found to be high.Langmuir isotherm is assigned for monolayer adsorption of a species on a homogeneous surface for which adsorption energy is the same for all active sites, irrespective of the level of coverage.Simultaneously, the Freundlich isotherm is indicated for multilayer adsorption on a heterogeneous surface where the number of emplacements coupled with free adsorption energy decline exponentially with the rise in free energy.The adsorption process intervening the Cu (II) ions and the functional groups on the surfaces of SAL-E, DHB-E, and E-500.Langmuir and Freundlich were selected like that

Equation (10) can be rearranged into linear form
In Equations ( 9) and ( 10), Ce and qe are the equilibrium concentration (mg/L) and quantity of Cu (II) heavy metal (mgg −1 ) accumulated at equilibrium, sequentially, K L assigns the intensity of interaction between adsorbate and the surface of the adsorbent, Qmax is the optimum quantity of Cu (II) per unit mass of adsorbent to from complete monolayer on the surface of solid adsorbent material, emulate the adsorption capacity (mgg −1 ).The plot between LogQe and logCe is surely resolved by its slope and intercept, from Equation (11) K f and 1/n are the adsorption capacity and adsorption intensity respectively, K f is the fixed for the system allied to the binding energy.kf can be stated as adsorption and distribution coefficient and signifies the amount of Cu (II) accumulated onto the adsorbent.The slope extending within 0 and 1, expedient for the surface heterogeneity.A plot log Qe v/s logCe empowers the observational constants K f and 1/n to be intent by the intercept and slope of the linear regression analysis, Table S4 and Table 1 exhibits the conclusions of Langmuir and Freundlich fit by availing the adsorption capacity data at different temperature.It was found that the Langmuir model affords a considerably fabulous, fit than the Freundlich model, as emulated with correlation coefficient (R 2 ) 0.999 v/s 0.97 and 0.999 v/s 0.94.The adsorption isotherms affirm the SAL-E, DHB-E, and E-500 composites can only intake 177.74,128 and 51.10 mgg −1 of Cu (II), respectively.As also elucidated in Table 1 and S4 the value of 1/n is less than 1 that assigns an admiring adsorption isotherm.Indeed, its statical adsorption capacity with relevant biochar has been tabulated in S3 table [34][35][36][37][38][39].The freundlich parameter K f was larger for SAL-E (105.61L/mg) than that of DHB-E (95.21 L/mg), and E-500(18.90L/mg).The large value of K f commonly suggests the good affinity of Cu (II) to the porous adsorbent material.The important property of a Langmuir can be viewed concerning dimensionless constant R L which can be explained according to the below equation The R L parameter assigns the figure of the isotherm, which can be categorised into four types (R L = 0) irreversible, (0˂ R L ˂1) favourable (R L = 1) linear (R L >1) unfavourable.The value of R L in the zone of 0-1 suggests the thermodynamically favourable adsorption of Cu (II).Figures 3-5, and S8 presents the plots of different isotherms that include Freundlich, Langmuir, and Temkin of Cu (II) onto SAL-E, DHB-E, and E-500.The estimated isotherm parameters were presented in Table 1

Temperature changes during adsorption of Cu (II) onto calcinated eggshell and modified eggshells
As reported by Yeddou and Bensamilia (2007) [40], one of the main thermodynamic parameters; free energy might be estimated from the equilibrium constant.Moreover, they declared that free energy indicates the spontaneity of the adsorption process, in which a negative value reveals a higher energetically agreeable adsorption.So, this displays the usefulness of the adsorption process in which free energy of adsorption is estimated by the following equation [41]  By plotting graph between LnK D and 1=Tusing linear regression analysis, the value of ΔH 0 and ΔS 0 can be estimated.Where ∆G is Gibbs free energy, T is temperature, R is Universal gas constant, and K D Langmuir constant.Comprehensively, it might be acclaimed that free energy value for calcinated eggshell (SAL-E) is higher as compared to that of DHB-E, and SAL-E.The positive value of free energy at different temperatures revealed the non-feasibility of the process and the decline of the positive value of ΔG 0 with increasing in temperature assigns that the system slowly transfers towards feasibility.Tempkin isotherm -takes into discussion the interaction of adsorbate (Cu +2 ) and adsorbent (modified eggshells).The results of the study showed that the heat of adsorption was higher for SAL-E(62.8J/mole) as compared to that of DHB-E(31.37J/ mole), and E-500(21.82J/mole) hence the heat of adsorption is more than 20 Kjmol −1 B ˃ 20 Kjmol −1 that suggests the ion exchange and Chemisorption lead over the physisorption.The remediation of Cu (II) by SAL-E composite was mostly occurring via surface complexation, between Cu (II) and functional groups present on the surface of SAL-E .In addition to that C=C of aromatic ring are also responsible for the Cu (II) remediation via Cu (II) -π interaction takes into consideration of Dubinn-Raduskevich isotherm model, the mean-free adsorption energy of all the three porous adsorbent materials was more than 8jmol −1 which is an indication of chemosorption.The surface complexation, ion exchange and Cu (II)-π interaction dominates the physisorption.The results summarised in Figure 5(d) and Table S4 present the impact of temperature on the adsorption capacity of the calcinated eggshell and modified eggshells.Furthermore, it was found that the adsorption of all adsorbent materials increases slightly with increasing temperature from 19.85°C to 38.85°C, consequently implying that the adsorption is thermodynamically feasible.Although the finest Cu (II) adsorption onto the calcinated eggshell and modified eggshells happen agreeably at high temperature, this is affirmed by the positive value of ∆H 0 which affirms that the adsorption is endothermic and probably to be influenced by the chemisorption process associated with strong forces.The best adsorption capacity of adsorbent material at a high temperature can be associated with the strong forces, and rising nature of chemisorption with rise in temperature.The positive value of ∆S 0 of all adsorbent materials reveals a rise in randomness at its own solid/solution interface.The deviations in the degree of positive values of ∆G 0 of all adsorbent materials when the temperature is increased are revealed that the adsorption is fast and more feasible at higher temperatures.The values of thermodynamic parameters were displayed in Table 2.

Adsorption kinetics
Figure 5(e,a,b,c) presents the graph plots of the pseudo-second-order and pseudofirst-order models of Cu (II) adsorption onto the calcinated eggshell and modified ones, respectively.The estimated kinetic parameters were summarised in Table 3. Figure 5(a,b,c) revealed that the first-order kinetic plots of all the porous adsorbent materials could not display a straight line but exhibit an understandable recognition into a few regions.This controversy is confirmed by the small value of the correlation coefficient (R 2 0.97) consequently, it is implied that the adsorption of Cu (II) on these porous adsorbent materials completely does not follow the pseudo-first-order model in all adsorbent materials.On the contradictory, the results displayed were well-fitted to the second-order model in all cases with exceptionally R 2 > 0.999.A good compromise of the pseudo-second-order model was verified by the close values of estimated adsorption and experimental adsorption in all cases.The good fit to the pseudo-second-order model suggests that the adsorption mechanisms confide in the adsorbent and adsorbate [42].Commonly, when adsorption steps do not base on one another then the plots of qt versus t 0.5 will provide more than two intersecting lines based on the actual mechanism [43].The intraparticle diffusion graph plots of all cases give a linear line equation, even so, nil of the lines cross through the origin of the plots I ≠ 0 suggests that the intraparticle diffusion is engaged in the adsorption process still not the only ratedetermining step.Specifically, one more mechanism including surface complexation, ion-exchange [44].The large value of intercept in the case of SAL-E composite presents that the boundary layer diffusion influences the adsorption too few degrees as compared to that of DHB-E, and E-500 where the values of intercepts are small.The intraparticle diffusion parameters were summarised in Table 3. Figure 5(f) presents the plot of intraparticle diffusion kinetics for adsorption of Cu (II).Moreover, it was found that the interaction between SAL-E composite and Cu (II) was very fast for their adsorption rate constants (K 2 ) were less than > [45].

Conclusion
In the present study, the CaCO 3 derived from eggshell waste via calcination method was modified with different derivatives of humic acids (SAL and DHB) enhanced the heavy metal adsorption onto the surface of calcite.The adsorption of humic acids onto the surface of calcite developed a highly functionalised calcite surface and well-defined pore property that brings sufficient improvement in adsorbents performance, for remediation of heavy metals from wastewater and aqueous solution.The extreme Cu (II) adsorption of the SAL-E composite at pH 7 was 177.74.mgg −1 four times more than the virgin calcite.Higher concentrations of humic acids to put in service to synthesise SAL-E ought to create exceptional better adsorbent.Modifying E with SAL and DHB increased the crystalline nature, surface area, and pore property of the adsorbent.However, SAL-E showed superior adsorption property towards Cu (II) compared to that of DHB-E and E-500.The adsorption process attained equilibrium during few minutes, furthermore, the adsorption process was in exemplary accord with the Langmuir model.FTIR, Raman, and UV DRS evaluations proclaimed the introduction of functional groups that can be rendered more emplacements for binding Cu (II).Surface alteration by humic acid is an efficient method for enhancing the Cu (II) adsorption capacity of calcinated eggshell without affecting any damage of pollution in water samples.The Langmuir isotherm model is well equipped with the data of Cu (II) biosorption.Salicylic acid-modified eggshell carried out good adsorption capacity (177.74mg/g)followed by 2, 4-dihydroxy benzoic acid-modified eggshell (128 mg/g), and calcinated eggshell (51.10mg/g).The positive value of ∆H 0 revealed that the adsorption is endothermic in nature and dominates by chemisorption

Figure 1 .
Figure 1.Effect of solution pH on Cu(II) uptakes of porous adsorbent materials and desorption ratios of salicylic acid adsorbed onto SAL-E composite (a) Effect of contact time on Cu(II) adsorption (b) Effect of pH on Cu(II) adsorption (c) Adsorbance of Salicylic acid onto calcite as a function of the initial salicylic acid concentration.