Selective removal of lead ions from aqueous solutions using 1,8-dihydroxyanthraquinone (DHAQ) functionalized graphene oxide; isotherm, kinetic and thermodynamic studies

An anthraquinone – graphene structure was fabricated and applied for the removal of lead(ii) from aqueous solution. The equilibrium occurred in about 10 min revealing the high adsorption rate at the beginning of the process. The maximum Pb(ii) adsorption capacity of the Fe3O4@DHAQ_GO nanocomposite was about 283.5 mg g−1 that was observed at 323 K and pH 5.5. The Pb(ii) adsorption ability increased with the increasing pH. The isotherm and kinetic studies indicated that the Sips isotherm model and the linear form of the pseudo-second kinetic model had a better fit with the experimental results. The positive value of ΔH0 indicated endothermic interactions between Pb(ii) and Fe3O4@DHAQ_GO. The negative ΔG0 indicated that the reactions are spontaneous with a high affinity for Pb(ii). The positive ΔS0 values indicated increasing randomness at the solid–solute interface during the adsorption process. The selective removal of Pb(ii) by the nanocomposite confirms the presence of higher-affinity binding sites for Pb(ii) than Cd(ii), Co(ii), Zn(ii), and Ni(ii) ions. Furthermore, the Fe3O4@DHAQ_GO nanocomposite revealed an excellent preferential adsorbent for Pb(ii) spiked in drinking water samples containing natural ion matrices. EDTA-2NA 0.01 N was found to be a better elution agent than HCl 0.1 M for the nanocomposite regeneration. After five adsorption/desorption cycles using EDTA-2NA 0.01 N, more than 84% of the adsorbed Pb(ii) was still desorbed in 30 min. Capturing sub-ppm initial concentrations of Pb(ii) and the capability to selectively remove lead from drinking water samples make the Fe3O4@DHAQ_GO nanocomposite practically convenient for water treatment purposes. High adsorption capacity and facile chemical synthesis route are the other advancements.


Introduction
Lead ions are a severe environmental concern and can contaminate drinking water resources. 1,2The maximum contaminant level (MCL) of Pb 2+ for drinking water is 10 ppb set by EPA and national standard organizations. 3,4The strict limitations on discharge effluents containing Pb 2+ into natural water bodies are due to the high toxicity potential for vital organs such as brain and kidney. 2 Different methods are currently applied for the removal of high concentrations of lead ion that can be found in industrial wastewaters; [5][6][7][8] whereas only a few methods e.g. using functionalized adsorbents 9,10 and membrane technologies 11 can be adapted for the capturing of low concentrations (around 1 ppm) commonly occurring in drinking water sources.Furthermore, avoiding alteration of the natural ion matrices of drinking waters during the removal of a certain target contaminant is a consideration especially for membrane-based water treatment technologies. 12,13New generation adsorbents such as graphene oxide and carbon nanotubes show metal adsorption capacities much more than those of traditional adsorbents. 14For example, the ordinary adsorption capacity of activated carbon is less than 70 mg g À1 , whereas graphene oxide nanosheets are capable of reaching an adsorption capacity of 4000 mg g À1 . 15raphene oxide is an emerging carbon-based nanomaterial that has revealed the promising adsorptive properties.Despite the graphene (G) and reduced graphene oxide (RGO), the graphene oxide (GO) creates a highly stable aqueous dispersion. 16his property leads to increase the effective contacts with target contaminants without vigorous mechanical mixing.The dispersability properties of GO is attributed to the plenty hydrophilic functional groups covering the GO akes. 17The GO ake surface contains various functional groups including epoxy and hydroxide, whereas the edge of akes mainly contains the carboxylic groups. 18n recent years, using Pb 2+ selective membrane electrodes (ISE) have been extensively studied with different active materials to determine lead ion concentration in water and wastewater. 19,20The active materials are mainly consisting of ligands or Schiff bases, which are known as ionophores. 21It has been revealed that some ionophores such as anthraquinone, [22][23][24] methacrylate, 25 and nucleic acids 26 have the selective affinity to lead ion.The main drawback regarding to the most of ionophores is their hydrophobic nature which makes them unusable to create aqueous solution for the lead ion removal. 27Using GO akes as the aqueous dispersion agents can provide an appropriate platform for the attachment of ionophores and producing a water dispersible GO-ionophore composite.
1,8-Dihydroxyanthraquinone (DHAQ), namely Dantron is a dye intermediate and a medicine. 27,28Furthermore, some works report the high affinity of DHAQ as a ligand to form stable complexes with Pb 2+ . 20,24In this study, DHAQ was used as an ionophore agent in the structure of Fe 3 O 4 @SiO 2 -GO to form the Fe 3 O 4 @DHAQ_GO nanocomposite and aimed to have Pb 2+ selective removal property from aqueous solutions.

Preparation of Fe 3 O 4 @SiO 2 _GO
Our previous work reported the fabrication of graphene oxide (GO), Fe 3 O 4 magnetic nanoparticles, Fe 3 O 4 @SiO 2 _NH 2 nanoparticles, and Fe 3 O 4 @SiO 2 _GO nanocomposite. 1 The preparation of GO was based on using sulfuric acid as digestion agent, and H 2 O 2 for the oxidation of graphite. 29Co-precipitation method was used to prepare Fe 3 O 4 magnetic nanoparticles. 30hen, NH 2 -groups were applied as linkers to create covalent bonds between Fe 3 O 4 magnetic nanoparticles and GO.
Consequently, covering APTES and TEOS on the Fe 3 O 4 magnetic nanoparticles produces Fe 3 O 4 @SiO 2 _NH 2 . 31,32inally, a condensation reaction between the carboxylic groups (COO-) of GO and the amine groups (NH 2 -) of Fe 3 O 4 @SiO 2 was prepared for the fabrication of Fe 3 O 4 @SiO 2 _GO nanocomposite. 323.Preparation of Fe 3 O 4 @DHAQ_GO 200 mg DHAQ was added into 50 mL DMF followed by mild stirring for 3 hours.Then, 200 mg EDS and 100 mg NHS were added and pH was adjusted between 4 to 6 followed by vigorous mixing for 2 hours at room temperature.Aer that, 0.5 g Fe 3 -O 4 @SiO 2 _GO was dispersed into the mixture and mixing was continued up to 12 hours.Finally, dispersed solid was separated via centrifuge (12 000 rpm, 10 min), washed with deionized water, and dried to obtain Fe 3 O 4 @DHAQ_GO.Schematic of the synthesis path applied for the fabrication of Fe 3 O 4 @DHAQ_GO nanocomposite was presented in Fig. 1.As revealed, 1,8-dihydroxyanthraquinone attaches to amine group linked with Fe 3 O 4 nanoparticle.

Characterization
A Hitachi-S4160 scanning microscope were used to provide SEM images (Tokyo, Japan).The AFM measurements were obtained by using a Nanoscope V multimode atomic force microscope (Veeco Instruments, USA).Samples prepared for the AFM measurements contained dispersions of GO/methanol solutions (70 mg mL À1 ) smeared on a fresh mica surface and allowed drying in the air. 33

Adsorption experiments
A typical adsorption experiment was established by adding 10 mg Fe 3 O 4 @DHAQ_GO into a 100 mL solution containing Pb 2+ ions at room temperature.Varied initial concentrations of Pb 2+ , from 1 mg L À1 to 10 mg L À1 , were used and for all the Pb 2+ aliquots, the pH value was kept on 7 applying buffer solutions.The mixing rate was constant at 150 rpm for the all solutions.
An external magnetic eld was used for the removal of adsorbent aer the adsorption time.The equilibrium adsorption capacity (q e , mg g À1 ) of Pb 2+ was determined by the following equation.
where, C 0 and C t are the Pb 2+ initial and nal concentrations (mg L À1 ), x ads is the adsorbent mass (g), V is the reactor volume (L), and 1000 is a conversion factor.A Spectro Arcos ICP-optical emission spectrometer (SPEC-TRO Analytical Instruments, Kleve, Germany) was used for the measurement of Pb 2+ concentrations.
The parameters of isotherm and kinetic equations were determined by applying Solver engine of Microso Excel spreadsheet soware 34 based on nonlinear forms of the equations.

Selectivity study
Two independent studies were conducted to investigate the capability of Fe 3 O 4 @DHAQ_GO nanocomposite for the selective capturing of Pb 2+ from water: binary ion study; including aliquots contained binary ion matrices (Pb 2+ /Cu 2+ , Pb 2+ /Cd 2+ , Pb 2+ /Zn 2+ , and Pb 2+ /Co 2+ ) and selective removal of Pb 2+ from natural water samples; including drinking water samples spiked with Pb 2+ ions.The concentration of metal ions was measured by using ICP-OES.The distribution coef-cient K d (mL g À1 ), selectivity coefficient k, and the relative selectivity coefficient k were determined by eqn ( 2)-( 4), respectively.
where, C i and C f are the initial and nal concentrations of metal ions, respectively.K d(Pb(II)) and K d(M(II)) are the distribution coefficient of Pb 2+ and metal (M) ions, respectively.k MGO-DHAQ and k MGO are the selectivity coefficient of Fe 3 O 4 @DHAQ_GO and Fe 3 O 4 @SiO 2 -GO, respectively.

Desorption and regeneration
Pb 2+ in solution (25 mL, 2.45 mg L À1 ) was adsorbed onto Fe 3 -O 4 @DHAQ_GO (30 mg L À1 ) at pH 7 for 1 h and then the adsorbents were separated by applying an external magnetic eld and the residual quantity of metal ions was determined by ICP-OES.Aer that, the adsorbents were regenerated in 25 mL Erlenmeyer ask containing 10 mL 0.02 mol L À1 eluent to completely leach metal ions at room temperature for 6 h.The concentration of metal ions released from adsorbent into the aqueous phase was measured by ICP-OES.Desorption ratio (D) was determined by using the following equation: where, H de (mg L À1 ) is the amount of metal ion desorbed into the elution medium.H ad (mg L À1 ) is the amount of metal ion adsorbed onto the Fe 3 O 4 @DHAQ_GO nanocomposite.

Results and discussion
It is well known that various derivatives of anthraquinone are able to form stable complexes with a variety of metal ions in some non-aqueous solvents 35,36 and anthraquinone-lead(II) complexes are among the most stable ones. 37,38Applying the graphene oxide provides the active sites for the anthraquinone that can be covalently bonded and produced a hydrophilic property which is appropriate for the adsorption of Pb 2+ in the aqueous solution.

Characterization studies
The FT-IR spectra for GO, Fe 3 O 4 @SiO 2 -GO, and Fe 3 O 4 @-DHAQ_GO are presented in Fig. 2. The stretchings C-O (1055 cm À1 ), C-OH (1226 cm À1 ), C-O carbonyl (1733 cm À1 ), and O-H hydroxide (3419 cm À1 ) 39-41 can be observed in the FT-IR spectrum of GO (Fig. 2(a)).The stretching assigned to the adsorbed water molecules is observed at 1621 cm À1 assigning also to the skeletal vibrations of un-oxidized graphite. 42,43n Fig. 2(b), the spectrum of Fe 3 O 4 @SiO 2 -GO is depicted.It shows the vibration of Fe-O stretching at 591 cm À1 and an intense stretching around 3400 cm À1 .Although, it can be attributed to the remaining water on the surfaces of Fe 3 O 4 nanoparticles. 44ig. 2(c) depicts the FT-IR spectrum of Fe 3 O 4 @DHAQ_GO.As shown, a vibration is observed at 3401 cm À1 assigning to the N-H stretching.Furthermore, the peak at 1733 cm À1 , observed in Fig. 2(a), is disappeared and a new wide peak of C]N stretching is appeared at 1641 cm À1 .The vibration of C-N stretching is appeared at 1230 cm À1 . 45The obvious peaks at 802 and 1110 cm À1 can be attributed to the Si-O vibrations.The FTIR spectra conrmed that APTES functionalized Fe 3 O 4 has been bonded covalently to GO nanosheets via the amide linkage. 46ig. S1 † depicts eld emission SEM images of GO, Fe 3 O 4 @-SiO 2 -GO, and Fe 3 O 4 @DHAQ_GO nanoparticles.From Fig. S1(a), † it can be observed that GO is partially transparent and 2-or 3-layered graphene oxides are formed. 47,48From Fig. S1(b), † the spherical Fe 3 O 4 @SiO 2 -NH 2 nanoparticles having 50-60 nm diameters are identied, which nally have been enveloped by GO layers producing aggregated morphologies of Fe 3 O 4 @DHAQ_GO as shown in Fig. S1(c) †.
Fig. 3 illustrates the tapered mode AFM topography scan.A single platelet of GO laid on a freshly cleaved mica surface can be observed in Fig. 3(a) and (b) represents a frequency histogram of platelets thicknesses having the mean thickness of 3.21 nm.Height prole of the green line (Line 1 in Fig. 3(a)) presents a height of 0.732 nm in cross-section A-A as shown in Fig. 3(c).
Fig. S2 † presents thermal gravimetric analysis (TGA) of Fe 3 O 4 magnetic nanoparticles, Fe 3 O 4 @SiO 2 -GO, Fe 3 O 4 @DHAQ_GO, and graphene oxide.As revealed, major weight losses were occurred between 150 and 350 C attributing to CO, CO 2 released from labile functional groups. 48,49Slower rate of mass loss was detected between 350 and 650 C assigning to the removal of some stable oxygenated functional groups.Similar trends of weight loss were observed in Fe 3 O 4 @SiO 2 -GO and Fe 3 O 4 @DHAQ_GO.The Fe 3 O 4 @DHAQ_GO weight loss was 13.5% more than those of Fe 3 O 4 @SiO 2 -GO in 740 C attributing to the presence of 1,8-dihydroxyanthraquinone in the structure of Fe 3 O 4 @DHAQ_GO. 50ig.S3 † shows the XRD patterns of GO and Fe 3 O 4 @SiO 2 -GO.GO sharp diffraction peaks observable at 2q ¼ 12.24 and 42.83 are attributed to the reections of (002) and (101), respectively.Furthermore, six typical peaks at about 2q ¼ 30.4,35.6, 43.1, 54.1, 57.7 and 62.5 are observed for Fe 3 O 4 @SiO 2 -GO, attributing to indices (220), (311), (400), (422), (511) and (440), respectively.Appropriate match of intensities and positions of above mentioned diffraction peaks conrming by pure magnetite JCPDS card (75-1610). 51As represented in XRD patterns corresponding to Fe 3 O 4 @SiO 2 -GO, the reection peak (002) belonging to GO was disappeared.It is assumed that the GO sheets cover the Fe 2 O 3 nanoparticles and it hinders the stacking of sheets to create a crystalline arrangement. 52he vibration sample magnetization (VSM) was used to determine the magnetic characteristics of fabricated materials contained Fig. 4 presents the nitrogen adsorption isotherm of Fe 3 -O 4 @DHAQ_GO nanocomposite.The surface area of 215 m 2 g À1 was obtained that is relatively lowered than those reported for pristine GO. 53 It seems that the agglomeration of Fe 3 O 4 NPs and GO nanosheets tend to an shrinking effect on the GO nanosheets causing the decrease of free surface area 48 as observed in Fig. S1.† The average pore size of Fe 3 O 4 @DHAQ_GO was determined to be about 9.26 nm identifying the mesopore structure of the adsorbent.

Adsorption experiments
Adsorption isotherm.The isotherm models Langmuir (eqn (6)), Freundlich (eqn (7)), and Sips (eqn (8)) were applied to investigate the effect of equilibrium concentrations of Pb 2+ (C e ) on the equilibrium adsorption capacities (q e ) of Fe 3 O 4 @DHAQ_GO nanocomposite. ) ) where, K L is the Langmuir adsorption constant (L mg À1 ) and q m represents the maximum adsorption capacity attributing to the complete monolayer coverage of the adsorbent (mg g À1 ).Furthermore, K F (mg g À1 ) and n F (unit less) are the Freundlich constants.K S (L g À1 ) and n S are the Sips equation parameters denoting the affinity constant and surface heterogeneity, respectively. 54,55s represented from Table 1, the R 2 values indicated that Sips model has better t with the experimental results then Langmuir and Freundlich models.Fig. 5 depicts the nonlinear functions of Sips model tted to the experimental points obtained from the batch studies in different temperatures.The Sips equation containing three parameters having the capability to apply for both the homogeneous and heterogeneous systems. 56The surface heterogeneity of adsorbent should be considered if the deviation of n S values from 1 is observed. 55,57However, the Sips isotherm reach a constant level at high concentrations while a pattern of Freundlich model can be observed at low concentrations. 57s revealed from Table 1, the Pb 2+ adsorption capacities of Fe 3 O 4 @DHAQ_GO nanocomposite were increased by the increasing of temperature assigning to decrease water viscosity along with the increasing of Pb 2+ collisions between the sites of nanocomposite and Pb 2+ ions.The maximum adsorption capacities (q m ) obtained by Langmuir isotherm were overestimated (e.g.243.1 in 323 K) while those of Sips model (e.g.164.3 in 323 K) represents a good t to the experimental data (also, see Fig. 5) which can be due to the heterogeneity characteristic considered in the Sips model. 58Increasing the deviations of n S and n F values from unity can be assigned to develop the nanocomposite surface heterogeneity over raising the temperature. 57.2.2.Kinetic studies.The sorption capacities (q t ) of Fe 3 -O 4 @DHAQ_GO exposed with Pb 2+ initial concentrations 2.5, 5, and 10 mg L À1 were studied over corresponding times.The kinetic models; Lagergren-rst-order (eqn ( 9)) and pseudosecond-order (eqn (10)) were applied for determining the appropriate function to describe the kinetic behavior of the batch systems.
where, q t and q e are the sorption capacity (mg g À1 ) at time t and at the equilibrium time, respectively.k 1 and k 2 correspond to the pseudo-rst-order and pseudo-second-order rate constants, respectively. 59,60ig. 6 illustrates tting the non-linear forms of pseudosecond kinetic model to the experimental points.As shown, the equilibrium was took place sooner for the batch systems underwent lower Pb 2+ initial concentrations.
Table S1 † presents kinetic parameters of Pb 2+ removal obtained by using the non-linear forms of pseudo-rst and pseudo-second kinetic models (eqn ( 9) and ( 10)).][63] 3.2.3.Thermodynamic parameters.Changing in free energy (DG 0 ), enthalpy (DH 0 ), and entropy (DS 0 ) can be determined by the following equations: where, R is the gas constant (8.314J mol À1 K À1 ), K c (q e /C e ) is equilibrium constant at different temperatures, and T is the absolute temperature (K).Eqn (11) calculates DG 0 values assigning to the obtained temperature shown in Table 2.
][66] Table 2 represents that DG 0 has negative amounts assigning to different temperatures.So, it can be concluded that Pb 2+ adsorption on Fe 3 O 4 @DHAQ_GO nanocomposite proceeds spontaneously.
(2014) reported that the changes of free energy DG 0 at 298 K are À6.46 kJ mol À1 .As shown in Table 2, DG 0 is À19.73 kJ mol À1 at 323 K having an appropriate agreement with the ndings of Fan et al. (2013).[70] As represented in Table 2, increasing the temperature tends to lower values assigned to DG 0 conrming that the adsorption is more efficient at the higher temperatures. 71,72The enthalpy (DH 0 ) value was 24.07 kJ mol À1 having the positive value of DH 0 that indicates the endothermic nature of the adsorption.The entropy (DS 0 ) was obtained with a positive value proving the increase of randomness during Pb 2+ adsorption process. 73,743.Selectivity study Two independent studies were conducted to evaluate the selectivity properties of Fe 3 O 4 @DHAQ_GO nanocomposite for the separation of Pb 2+ ions from aqueous ion matrices.The rst one was capturing Pb 2+ ions from four different aqueous solutions so that each solution contains Pb 2+ and one other divalent metal ion.Consequently, four binary ion matrices were prepared, including Pb 2+ /Cu 2+ , Pb 2+ /Cd 2+ , Pb 2+ /Zn 2+ , and Pb 2+ / Co 2+ .
The second study was conducted for the assessment selective removal of Pb 2+ in drinking water samples containing natural ion matrices.Certain amounts of Pb 2+ ion were spiked into 30 different drinking water samples collected from various groundwater sources.Batch experiments were conducted based on the optimized values of variables pH, dosage, temperature, and the initial concentration.
3.3.1.Selective removal of Pb 2+ from binary ion matrices.The above mentioned aliquots containing binary ions were exposed to the functionalized (Fe 3 O 4 @DHAQ_GO) and pristine (GO@SiO 2 -Fe 3 O 4 ) nanocomposite through independent batch experiments.Table 3 shows the results of distribution coefficient K d (mL g À1 ), selectivity coefficient k, and the relative selectivity coefficient k obtained from eqn (2)-( 4    shows the results of Fe 3 O 4 @DHAQ_GO regeneration study in an aqueous ion matrix consisting of ve divalent metals.This experiment aims to investigate the presence of four coexistence ions (Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ ) in the case of their effect on lead removal and to assess the capability of the nanocomposite for the retaining of lead adsorption capacity aer several washing steps in the presence of other cations.As observed, the removal capacities of Pb 2+ ion was remained more than 111 mg g À1 over ve regeneration steps using the desorption agent EDTA-2NA 0.01 N. Furthermore, increasing the sorption capacity assigned to the four coexistence ions were almost negligible conrming the notable stability of the nanocomposite structure over several regeneration experiments.
The stability of the Pb 2+ and Fe 3 O 4 @DHAQ_GO complex was conrmed via the adsorption/desorption experiments.The conventional methods for evaluating the regeneration and reusability of adsorbents are according to the consecutive adsorption/desorption steps in batch volumes containing deionized water solution inoculated with the target pollutant.Consequently, the effects of coexistence ions are neglected, especially when the reusability of adsorbents having selectivity properties is considered. 77re, we put forward a facile approach to investigate the reusability of Fe 3 O 4 @DHAQ_GO in aqueous ion matrices containing different competitor divalent cations (Fig. 8).Yu et al. reported applying EDTA-2Na 0.015 N as washing agent over three cycles regeneration steps.Results showed the notable interference of Cd 2+ (ref.78) while, in our work, the minimum interfering of the coexistence cations was observed.

Conclusions
In this work, a novel hydrophilic nanocomposite based on GO was synthesized comprising an anthraquinone derivative having selective removal capability for lead.Fe 3 O 4 nanoparticles was used as a magnetic agent to facilitate the separation of nanocomposite from aqueous solution.Also, GO was used as a dispersible platform to obtain the hydrophilic property for the nanocomposite and preparing enough surface area to proceed the adsorptive mechanisms.The morphology and structure of the obtained adsorbent was characterized by UV-Vis, FT-IR, SEM, XRD, and TGA.The synthesis rout was simple and DHAQ was an environmental friendly compound without toxic effect.The selectivity characteristics of the nanocomposite was  evaluated through two different methods including controlled ion matrices and the natural ion matrices obtained from drinking water samples.Furthermore, the regeneration and reusability studies were conducted in the presence of coexistence ions.It seems that Fe 3 O 4 @DHAQ_GO nanocomposite can be a promising selective removal agent for the removal of lead from polluted waters and industrial discharges.

Fig. 3
Fig. 3 Tapered mode AFM topography scan.Exfoliated graphene oxide deposited on a freshly cleaved mica surface (a), histogram of platelet thicknesses from images of 138 platelets (the mean thickness is 3.21 nm) (b), height profile through the green line (Line 1) presented in (a).Crosssection A-A through the sheet shown in (a) exhibiting a height of 0.732 nm (c).

Fig. 6
Fig. 6 Nonlinear forms of pseudo-second kinetic model fitted on experimental points at different Pb 2+ initial concentrations (adsorbent dosage 100 mg g À1 ; volume of solution 100 mL; pH 7; T ¼ 298 K).

3. 4 .
Fig.8(a) depicts the repeated adsorption/desorption of Pb 2+ ions using batch experiments exposed with Fe 3 O 4 @DHAQ_GO nanocomposite in single ion aqueous solution.As shown, aer 5 consecutive regeneration steps, the nanocomposite could remove 86 percent of Pb 2+ ions so that only 12 percent of removal loss was observed.Fig.8(b)shows the results of Fe 3 O 4 @DHAQ_GO regeneration study in an aqueous ion matrix consisting of ve divalent metals.This experiment aims to investigate the presence of four coexistence ions (Cu 2+ , Ni 2+ , Co 2+ , Cd 2+ ) in the case of their effect on lead removal and to assess the capability of the nanocomposite for the retaining of lead adsorption capacity aer several washing steps in the presence of other cations.As observed, the removal capacities of Pb 2+ ion was remained more than 111 mg g À1 over ve regeneration steps using the desorption agent EDTA-2NA 0.01 N. Furthermore, increasing the sorption capacity assigned to the four coexistence ions were almost negligible conrming the notable stability of the nanocomposite structure over several regeneration experiments.The stability of the Pb 2+ and Fe 3 O 4 @DHAQ_GO complex was conrmed via the adsorption/desorption experiments.The conventional methods for evaluating the regeneration and reusability of adsorbents are according to the consecutive adsorption/desorption steps in batch volumes containing deionized water solution inoculated with the target pollutant.Consequently, the effects of coexistence ions are neglected, especially when the reusability of adsorbents having selectivity properties is considered.77
), respectively.As observed, the values of selectivity coefficient k is more than 19 for all binary ion comparisons.It means that Fe 3 O 4 @-DHAQ_GO nanocomposite has a more notable preference for capturing Pb 2+ ions compared with that of coexistence ions.For instance, Fe 3 O 4 @DHAQ_GO nanocomposite could capture Pb 2+ ions 19.66 times more selectively than Cu 2+ ions.Cai et al. reported a k value of 11.66 for Pb 2+ /Cu 2+ binary ions.Furthermore, Msaadi et al. and Zhu et al. reported similar ndings for Pb 2+ ions removal using ion-imprinted nanocomposites. 75,763.3.2.Selective removal of Pb 2+ from drinking water samples.Table S2 † shows a set of multiple regression models

Table 2
Thermodynamic parameters for the adsorption of Pb 2+ onto the Fe 3 O 4 @DHAQ_GO nanocomposite (adsorbent dosage 55 mg L À1 , contact time 60 min, pH 7) Effect of temperature on the adsorption of Pb 2+ ions by Fe 3 -O 4 @DHAQ_GO nanocomposite.
View Article Online ranked according the Akaike's Information Criterion (AIC).Table4represents the coefficients of the model obtained rank 1 in TableS2.† As observed, cations formed the drinking water matrices (Na + , K + , Ca 2+ , Mg 2+ ) obtained negative values con-rming their competition with Pb 2+ ion to occupy the active sites of Fe 3 O 4 @DHAQ_GO nanocomposite.The large value assigned to the intercept (105.47)ensured notable preference of Fe 3 O 4 @DHAQ_GO nanocomposite for the separation of Pb 2+ ion from drinking water.

Table 4
Ranking list of linear multiple regression models applied to describe the effect of main natural water ions on mercury removal efficiency by Akaike's Information Criterion (AIC) Multiple R 2 : 0.81, adjusted R 2 : 0.73. a