Construction of attapulgite decorated cetylpyridinium bromide/cellulose acetate composite beads for removal of Cr (VI) ions with emphasis on mechanistic insights

Eco-friendly and renewable composite beads were constructed for efficient adsorptive removal of Cr (VI) ions. Attapulgite (ATP) clay decorated with cetylpyridinium bromide (CPBr) was impregnated into cellulose acetate (CA) beads, which were formulated through a simple and cost-effective solvent-exchange approach. FTIR, XRD, SEM, Zeta potential, and XPS characterization tools verified the successful formation of ATP–CPBr@CA beads. The composite beads displayed a spherical and porous shape with a positively charged surface (26.6 mV) at pH 2. In addition, higher adsorption performance was accomplished by ATP–CPBr@CA composite beads with ease of separation compared to their components. Meanwhile, equilibrium isotherms pointed out that the Langmuir model was optimal for describing the adsorption process of Cr (VI) with a maximal adsorption capacity of 302 mg/g. Moreover, the D–R isotherm model verified the physical adsorption process, while adsorption data obeyed the pseudo-second-order kinetic model. Further, XPS results hypothesized that the removal mechanism involves adsorption via electrostatic interactions, redox reaction, and co-precipitation. Interestingly, the ATP–CPBr@CA composite beads reserved tolerable adsorption characteristics with a maximum removal present exceeding 70% after reuse for seven successive cycles, proposing its feasible applicability as a reusable and easy-separable candidate for removing heavy metals from aquatic bodies.


Fabrication of ATP-CPBr@CA composite beads
The as-fabricated ATP-CPBr was incorporated into CA beads as follows; dissolving 1 g of CA into DMSO under potent stirring for 60 min.Then, the CA solution added to 1 g of ATP-CPBr composite and agitated for 60 min.The obtained ATP-CPBr@CA composite was added to the distilled water (coagulant medium) by syringe under mild stirring.Finally, after the curing of ATP-CPBr@CA composite beads for 30 min, the beads were separated and washed by distilled H 2 O. Besides, CA beads were prepared via the same reported procedure in "Fabrication of ATP-CPBr@CA composite beads" section except for the step involving the addition of ATP-CPBr composite.Scheme 1 describes the preparation process of the adsorbent composite beads.

Instrumental characterization
The details of characterization techniques are presented in Supplementary Information (S1).

Batch adsorption studies
A series of batch experiments were performed to evaluate the adsorption process of Cr (VI) ions.To pick out the optimal pH, the removal of Cr (VI) by ATP-CPBr@CA adsorbent composite was examined at the pH range of 2-10.The influence of the ATP-CPBr@CA dose was examined at the dose range of 0.01-0.03g.Moreover, the adsorption medium temperature was studied between 25 and 55 °C.The influence of the initial concentration of Cr (VI) was scrutinized at the concentration range of 50-300 mg/L.Besides, the selectivity of ATP-CPBr@ CA towards Cr (VI) was evaluated in the existence of NO 3 − , Cl − and SO 4 2− anions.After intervals of time (up to 180 min), the remaining Cr (VI) concentration was assayed at wavelength of 540 nm using spectrophotometric instrument.The removal (%) and adsorption capacity (q) were calculated as follow 28 : where C o and C t represent the concentration of Cr (VI) ions at initial and at time "t", respectively.V and m are volume of Cr (VI) solution and mass of ATP-CPBr@CA beads, respectively.

Recyclability test
To evaluate the recyclability of the developed composite beads, the beads were subjected for several adsorption-desorption runs.After each adsorption run, samples were collected from the Cr (VI) solution medium and subsequently soaked into a regenerative solution comprising NaCl/C 2 H 5 OH for 60 min.Finally, the regenerated samples were inspected for next adsorption run.The reusability experiment was conducted for seven successive cycles 29 .

Results and discussion
Studying the physiochemical properties of ATP-CPBr@CA FTIR The utilization of FTIR spectra to determine the surface functional groups of the ATP, CPBr, CA and ATP-CPBr@ CA composite before and after adsorption of Cr (VI) ions was revealed in Fig. 1A.As it can be observed that the main bands in the spectrum of ATP are located at 3610 cm −1 (for Si, Mg and Al stretching vibration), 3400 cm −1 (for OH stretching), 1661 cm −1 (-OH hydroxyl groups and adsorbed water), 1034 cm −1 (Si-O-Si stretching) 30,31 .FTIR spectra of CPBr observed the absorption bands at 3403, 2943, 1661, 1466, and 1003 cm −1 which could be assigned to the O-H stretching, asymmetric CH 2 stretching, -OH bending, C-H bending, O-H of the distortion peak of H 2 O, and C-N stretching, respectively 32,33 .The spectrum of CA shows peaks at 1730, 1461, 1351, 1237, 1168 and 1007 cm −1 which corresponded to C=O stretching, C-CH 3 symmetric and asymmetric deformation, C-C-O stretching, C-O-C in cellulose chain, C-O stretching bridge, respectively 26,34 .A comparison between the original and ATP-CPBr@CA composite, FTIR spectra demonstrated that the latter featured peaks is obtained in the composite but there were significant weakened in the intensity which potentially confirmation the reaction between ATP and CA and CPBr and proving the successful fabrication of hybrid composite via electrostatic interaction at the surface (Vander Walls forces).Also, the positions of the peaks is influenced by the change of inter and intermolecular hydrogen bonding, and therefore related to the changes in the chemical surface groups.In summary, FTIR studies indicated that the ATP-CPBr@CA composite has various functional groups including -OH, C-O, C=O and Si-O that provided a significant role in boosting the adsorptive removal of Cr (VI) by Scheme 1. Representation of the ATP-CPBr@CA composite beads fabrication.
Vol:.(1234567890) www.nature.com/scientificreports/ the ATP-CPBr@CA composite beads.Also, there is shift in the position of peaks in ATP-CPBr@CA before and after rejection of Cr (VI) that would be clarified in the section of removal mechanism.

Zeta potential
In order to detect the surface charges of ATP-CPBr@CA hybrid composite beads, the zeta potentials were evaluated.Figure 1C shows that the point of zero charge was 5.7, since the maximum of surface potential was 26.6 mV at pH = 2 and then reduced gradually to -29.28 mV at pH = 12.The surface of ATP-CPBr@CA composite becomes neutral at pH = 5.7.As a result, it displays favourable results in acidic solutions (pH 2-5).When pH increases from 6 to 12, the production of ATP-CPBr@CA species causes the surface charge to turn negative.As a result, zeta potential may have decreased when pH was more than 6 due to the deprotonation of ATP-CPBr@CA surface groups or the adsorption of OH on ATP-CPBr@CA surface.Lower pH values (pH 2-5) rendered the surface of ATP-CPBr@CA to become positively charged which attracted the anionic Cr 2 O 7 2− and HCrO 4 − species by a potent electrostatic interaction.

Morphological characteristics
The morphological characteristics of ATP, CPBr, CA, ATP-CPBr@CA composite are shown in Fig. 2. The prepared ATP has regular shape of individual rods or aggregated of many rods (Fig. 2A), while the SEM image of CPBr showed irregular shape of rough and dense surface (Fig. 2B).CA beads (Fig. 2C) represented as spherical, smooth, and hard beads with a well-defined shape.Although CA beads appeared to be solid without any micro pores on their surface, the cross-section investigation showed a notable porosity at the microspcopic scale in the grooves form (Fig. 2D).After incorporation of ATP-CPBr into CA beads (Fig. 2E), it also observed as a spherical and smooth surface.However, the cross-section of composite beads showed network of microfibers containing beads.The high magnifications revealed that the surface of CA was modified with ATP-CPBr (Fig. 2F).In summary, the SEM images verified that ATP-CPBr composite was successfully incorporated into CA beads.

Surface chemistry characteristics
To gain insight into the formation mechanism of ATP-CPBr@CA composite, the interaction of surface groups was confirmed by XPS spectra.Low resolution spectra show the main characteristic of Al, Si, Mg, C, O, and N at the surface of ATP-CPBr@CA before and after adsorption (Fig. 3A).In details, the peak of O1s is deconvoluted into three signals with centres at 530.9, 531.58, and 532.8 eV.These signals can be attributed to surface lattice oxygen (O 2 ), chemisorbed oxygen species in surface oxygen vacancies, also known as (O-, O 2 − , or O 2 2− ), and oxygen-containing groups like H 2 O, -OH − , or -CO 3 − (Fig. 3B) 36,37 .Related to C 1s deconvolution, it indicated that carbon atoms in different surface functional groups: sp 2 carbon at 284.8 eV (non-oxygenated carbon ring), C-O and C-N at 286.56 eV, and O-C=O at 288.62 eV 38 (Fig. 3C).The deconvolution of N 1s indicating two types of N species are pyridinic at 398.67 eV and graphitic N at 401.81 eV (Fig. 3D) 39 .The binding energy of Mg element was 1303.53 eV that matching with MgO (Fig. 3E), while the binding energy of Al element was 73.89 eV which could be attributed to the existence of Al 2 O 3 (Fig. 3F) 40 .The typical emission peak of Si-O-Si was visible in the XPS spectra of the Si 2p region at 102.02 eV.The signal at 103.24 eV, however, matches well with siloxy species like SiO 2 (Fig. 3G).

Comparison study
The removal efficiency of ATP, CA, ATP-CPBr and ATP-CPBr@CA toward the adsorptive removal of Cr (VI) was investigated, as represented in Fig. 4A.It was recorded that the removal % of Cr (VI) into CA, ATP, ATP-CPBr, and ATP-CPBr@CA were 21.46%, 38.25%, 52.46%, and 71.88%; in addition, their adsorption efficacies toward Cr (VI) were 36.09mg/g, 64.29 mg/g, 88.35 mg/g, and 120.81 mg/g, respectively.These findings denoted the synergistic effect between the authentic components to form higher efficacious ATP-CPBr@CA composite beads.

Effect of pH
Figure 4B displays the findings of an investigation into how the pH of the original solution affected the effectiveness of removing Cr (VI) from an aqueous solution using composite beads made of ATP-CPBr@CA.It can be observed that pH levels greatly influence the Cr (VI) adsorption onto ATP-CPBr@CA composite beads.With an increase in pH from 2 to 12, the adsorption of Cr (VI) ions onto the surface of ATP-CPBr@CA composite beads significantly reduced from 100 to 60 mg/g, in accordance with the literature 41 .At highly acidic conditions (pH = 2), the removal efficiency of Cr (VI) is found to be significantly reduced as pH increases.In an aqueous solution lower than pH = 4, it may to form the various ionic forms of Cr (VI) such as HCrO 4 − , CrO 2− which supports the electrostatic interaction between CrO 4 2− and NH 3 + on the surface of ATP-CPBr@CA 42 .According to the literature, HCrO 4 − is more easily adsorbed than CrO 4 2− as a result of the low adsorption surface free energy 43 .Moreover, at lower pH solutions, the exchange of Br − anion of CPBr with anionic Cr (VI) in the adsorption medium was the main source of Cr (VI).Also, it was hypothesized that the Cr (III) was formed when Cr (VI) was reduced by electrons from ATP-CPBr@CA, resulting in the lower pH that was observed 44 .On the other hands, at pH higher than 6, there are more negative charges on the surface of ATP-CPBr@ CA that support the electrostatic repulsion between ATP-CPBr@CA and Cr (VI), making the adsorption of Cr (VI) onto ATP-CPBr@CA surface is difficult.The solution becomes alkaline when CrO 4 2− was the dominating species, and OH − then competed with Cr (VI) on ATP-CPBr@CA.Moreover, two exchange sites were filled by the CrO 4 2− species, which exchanged with two Cl − molecules, reducing the maximum amount of Cr ions that could be absorbed by ATP-CPBr@CA.All of these factors resulted in reduced Cr (V) adsorption at higher pH levels.As a result, the surface chemistry at the interface may be used to describe the influence of solution pH on the adsorption process 45 .At normal pH range, all forms of Cr (VI) are negatively charged, regardless of how dominant they are.The functional groups on ATP-CPBr@CA have a lone-pair of electrons from N atom, which mainly contribute as an active site for the formulation of ATP-CPBr@CA-Cr complex.
Further specific examples of how pH affects the adsorption process can be provided using the zeta potential.The surface of the adsorbent is positively charged when the pH of the solution is below 6, and negatively charged when the pH is above 6.As a result, it's possible that the positive surface of ATP-CPBr@CA composite beads in an acidic media is more appealing to Cr (VI).By other means, as the pH of the adsorption medium declined, the density of positively charged sites increased, possibly as a result of electrostatic interactions between the anionic Cr (VI) species and the positively charged ATP-CPBr@CA composite.However, as the pH of the solution increased, the anionic binding sites on the ATP-CPBr@CA surface led to an increment in the electrostatic repulsion between the anionic Cr (VI) species and the beads, resulting in a substantial reduction in the adsorption amount of Cr (VI).

Impact of dose
Figure 4C illustrates the adsorption of Cr (VI) as a function of the dose of ATP-CPBr@CA at constant concentration, pH, and temperature.The acquired data depict that the removal efficiency of Cr (VI) ions was significantly boosted with increasing the dosage of ATP-CPBr@CA owing to the ample available binding sites to adsorb Cr (VI) species..The increased rate of Cr (VI) removal can be explained by: (1) the enhanced ATP-CPBr@CA mass transfer caused by the Cr (VI) ions' increased reactivity and diffusion rate, and (2) the thinner boundary layer surrounding the ATP-CPBr@CA, which helps to accelerate the adsorption process 48 .Temporarily, as the reaction between ATP-CPBr@CA and Cr (VI) is an endothermic chemical process, a higher temperature was also advantageous for the elimination of Cr (VI) ions 49 .Above 35 °C, there is a reduction in the adsorption of Cr (VI) which is related to the desorption originated from boosting the thermal energy of Cr (VI) ions 50 .

Kinetics studies
The impact of the initial Cr (VI) concentrations was investigated in the range of 50-300 mg/L during 180 min (Fig. 5A).The highest adsorption capacity of Cr (VI) ions was 281.20 mg/g at an initial Cr (VI) concentration of 300 mg/L, using 0.015 g of the ATP-CPBr@CA beads.The findings revealed that under high concentrations of Cr (VI) ions, the active sites of ATP-CPBr@CA would be effectively used.Further, the adsorption rate rises initially before gradually decreasing the duration of the adsorption time till reaches the equilibrium state after 120 min.It is difficult to occupy any remaining unoccupied adsorption sites due to the repulsive interaction between adsorbed Cr (VI) ions and those exist in the bulk phase after the equilibrium period 51 .
To understand and predict how reaction time would affect the retention and mobility of Cr (VI), the process was scrutinized using the adsorption kinetics.The investigation into the kinetics of Cr (VI) adsorption onto ATP-CPBr@CA at various initial concentrations provided the results shown in Fig. 5B,C for Pseudo first and Pseudo second order, respectively, and Table 1.The adsorption process attained the equilibrium within 120 min.The correlation coefficients of the Pseudo 2nd order model (R 2 = 0.998) are greater than those of the Pseudo 1st order model (R 2 = 0.953).Furthermore, there are an analogy between the computed adsorption capacities by Pseudo 2nd order at varies concentrations of Cr (VI) and the actual adsorption capacities.Such observations denoted the appropriateness of Pseudo 2nd order to model the Cr (VI) adsorption onto ATP-CPBr@CA and the domination of the chemical interactions in the adsorption process 52 .Interestingly, Elovich (Fig. 6A) implied the greater rate of adsorption compared to the rate of desorption, where the values of α are larger than β values at the varied concentrations of Cr (VI) 53 .Moreover, the intraparticle diffusion kinetic model was applied to predict the diffusion pathway of Cr (VI) from the bulk solution to the ATP-CPBr@CA surface.Figure 6B demonstrated that the Cr (VI) migration pathway to the adsorption groups of ATP-CPBr@CA took place throughout three stages; in the first step, Cr (VI) emigrated gradually from their solution and occupied the active sites on the surface of ATP-CPBr@CA.In the second stage, the ions began to get through the pores of ATP-CPBr@CA.Ultimately, in the last step; the Cr (VI) ions permeated the interior pores of ATP-CPBr@CA until attained equilibrium.From Table 2, increasing the concentration of Cr (VI) ions leads to increasing the driving forces that facilitate the intraparticle diffusion of Cr (VI) ions onto ATP-CPBr@CA composite beads.Also, compared to the first zone (related to film diffusion, C1), the second region (related to intraparticle diffusion, C2) had a thicker boundary layer.The plot does not pass through the origin confirming the intra-particle diffusion is not only the rate-controlling step 54 .

Isotherm study
For inferring the interactions' nature between Cr (VI) and ATP-CPBr@CA, the resultant equilibrium data were modelled by Langmuir, Freundlich, Temkin, and D-R (Fig. 7A-D).Generally, the Langmuir model supposes the adsorption of the contaminant species onto the adsorbent surface via the formation of chemical interactions between them, producing a monolayer of the contaminants over the surface of the adsorbent 55 .Moreover, the     www.nature.com/scientificreports/Freundlich model postulates proceeding the contaminants adsorption via the occurrence of multi-layer physical interactions between the contaminant species and the adsorbent 56,57 .On the other hand, the Temkin and the D-R models could identify if the controlling interactions between the contaminant species and the adsorbent are physical or chemical based on b and E values, respectively.In light of the acquired isotherm parameters (Table 3), the Cr (VI) uptake process was well-represented via Langmuir (R 2 = 0.989) and Freundlich (R 2 = 0.985) models.This result denoted the implication of both physical and chemical interactions in the uptake of Cr (VI) onto ATP-CPBr@CA.Moreover, the maximal Cr (VI) uptake capacity by Langmuir was 302.11 mg/g.The n value of Freundlich reflected the surface suitability of ATP-CPBr@ CA to adsorb Cr (VI) species.Notably, the derived b value from Temkin supposed the physisorption of Cr (VI) onto ATP-CPBr@CA since b was lower than 80 kJ/mol.This observation agreed with D-R which also implied the controlling physical interaction on the Cr (VI) adsorption, where E was lower than 8 kJ/mol.

XPS analysis
According to kinetics and isotherms, the Cr (VI) adsorption onto ATP-CPBr@CA occurred via physical and chemical interactions.Hence, XPS spectra were used to predict how these interactions proceeded in detail.The XPS survey of Cr (VI)-loaded ATP-CPBr@CA revealed the distinctive peaks of Cr 2p at 578.68 eV, evincing the occurrence of the uptake process (Fig. 8A).Zeta potential elucidated the abundance of positive active species (26.6 mV) on the ATP-CPBr@CA surface at pH 2. Consequently, the cationic adsorption sites of the beads could capture the anionic Cr (VI) by the potent electrostatic interactions.The peaks shift of the N 1s -spectrum is most likely due to the contribution of the protonated N + of CPBr in the electrostatic interaction (Fig. 8B).Notably, the electrons transferred from the distributed OH onto the surface of ATP-CPBr@CA could reduce the detrimental Cr (VI) to the less toxic Cr (III).Then, the produced Cr (III) ions are attached to the beads via coordinationcovalent bonds.These suggestions were proved from the Cr 2p spectrum (Fig. 8C) that showed the distinguished peaks of Cr (VI) at 589.13 and 580.54 eV and Cr (III) at 585.6 and 577.39 eV.Noteworthy, the amount of adsorbed Cr (VI) and Cr (III) onto the ATP-CPBr@CA was 31.32% and 54.75%, respectively, indicating the significant role of the reduction reaction in the removal of Cr (VI).
Moreover, the partial ion exchange could participate in the adsorption of Cr (VI) in the solution in which the anionic Br 3p could partially replace by Cr (VI) as well as Cr (III) partially exchange with Al 2p , Si 2p , and Mg 1s cations.The XPS survey confirmed the probability of the ion exchange mechanism since there was a noticeable decline in the ratios of Br 3p , Al 2p , Si 2p , and Mg 1s .Furthermore, the possibility of forming outer-sphere complexation between the Cr (VI) and OH groups onto the surface of beads.In addition, the oxygen-containing attapulgite could form inner-sphere complexation with Cr (VI).The peak shifting in the O 1s -spectrum (Fig. 8D) asserted the participation of the oxygenated functional groups of ATP-CPBr@CA in the adsorption of Cr (VI) by outer-and inner-sphere complexations.Interestingly, the interconnect pores structure of ATP-CPBr@CA beads which is the unique feature of the CA beads, provides a pore-filling mechanism during the adsorption process, where the Cr (VI) ions could penetrate the pores.
In one word, the primer adsorption capacity of ATP-CPBr@CA beads toward Cr (VI) that attained 302.11 mg/g is most probably due to the participation of varied powerful physical and chemical interactions in the Cr (VI)/ATP-CPBr@CA adsorption system, comprising electrostatic interaction, reduction reaction, ion exchange, outer-sphere complexation, pore-filling, and inner-sphere complexation (Fig. 9).

Comparison the adsorption performance with other adsorbents
To evaluate the synthesized ATP-CPBr@CA composite beads' has ability to absorb Cr (VI) in comparison to other known adsorbents.The better adsorption behaviour of the produced ATP-CPBr@CA composite beads was www.nature.com/scientificreports/ shown in Table 4.The developed ATP-CPBr@CA composite beads thus provide extremely effective adsorbent material for heavy metal ions decontamination from the polluted wastewater.

Reusability of spent ATP-CPBr@CA
From economic point of view, it is essential to examine the reusability of the constructed adsorbent 58 .Figure 10A elucidated that the ATP-CPBr@CA composite beads still retain better adsorption properties after seven succeeding cycles.It was observed that the composite beads only lost about 4.37% from their initial efficiency, while the  www.nature.com/scientificreports/overall efficiency exceeded 70% after the seventh cycle.These findings prove that the composite beads have high stability in water with acceptable removal reactivity for several adsorption-desorption cycles, suggesting their potential applicability as a reusable adsorbent for Cr (VI) ions with high performance.

Effect of co-existing anions
The effects of co-existing Cl − , NO 3 − and SO 4 2− anions using same concentrations of 5, 10, and 15 mM on the adsorption capability of Cr (VI) ions by the as fabricated ATP-CPBr@CA were examined.Generally, Fig. 10B showed that when the concentration of coexisting anions increased, the rejection efficiency decreased.This may be due to the formation of the complex generated by the interaction of co-existing anions with the ions from ATP-CPBr@CA, which reduces the surface reactive sites of ATP-CPBr@CA, and as a result, reducing the removal percentage of Cr (VI) ions 59 .As demonstrated, each of the studied three anions, all obstructs the adsorption of Cr (VI) to a varying extent.In the ATP-CPBr@CA system, the suppression of reactivity to Cr (VI) adsorption occurs in the following order: SO , resulting in a decrease in the available adsorption sites and subsequently decrease the removal rate of Cr (VI) ions 60,61 .Moreover, the removal of Cr (VI) ions by ATP-CPBr@CA composite beads was inhibited by Cl − and NO 3 − anions.

Conclusion
This study reported the construction and adsorbability of a new ATP-CPBr@CA composite for the adsorptive removal of Cr (VI) ions.The developed composite was formulated in in the form of easy-separable beads via a low-cost and simple technique.The successful formulation of the composite beads was evidenced by several analysis tools.Parameters affecting the adsorption process were explored thorough a series of batch adsorption studies Likewise, several kinetics and isotherms studies were performed to explicate the adsorption process.The removal efficiency of pure CA beads was greatly augmented from 21.46 to 71.84% after incorporation of ATP-CPBr.According to Langmuir model, maximum monolayer adsorption capacity of 302 mg/g was accomplished at pH 2, while Temkin model denoted that the adsorption process of Cr (VI) ions onto ATP-CPBr@ CA composite beads was categorized by a uniform distribution of the binding energies.Kinetically, the gained data obeyed the pseudo 2nd order kinetic model, while the intraparticle diffusion model verified two stages for  diffusion.The removal of Cr (VI) ions primarily involves adsorption, reduction, and co-precipitation.Besides, the reusability results attested the potential proficiency of ATP-CPBr@CA composite beads to adsorb Cr (VI) ions for seven repeated cycles with higher performance.In conclusion, the higher adsorption performance, simple processing, ease-separation, and better renewability strongly recommend the potential usage of the formulated ATP-CPBr@CA composite beads as sustainable candidate for removing anionic Cr (VI) ions from contaminated water.

Figure 4 .
Figure 4. (A) Affinity of beads composition for removal of Cr (VI) ions, (B) impact of pH, (C) adsorbent dosage, and (D) system temperature on the removal (%) and the adsorption capacity of Cr (VI) onto ATP-CPBr@CA composite beads at constant [contact time 60 min, initial Cr (VI) concentration 100 mg/L, and temperature 25 °C].

Figure 5 .
Figure 5. (A) Impact of initial concentration on the adsorption capacity of Cr (VI) onto ATP-CPBr@CA composite beads, (B) pseudo 1st order kinetic model, and (C) pseudo 2nd order kinetic model.

Figure 10 .
Figure 10.(A) Regeneration of ATP-CPBr@CA composite beads in the adsorption of Cr (VI) and (B) the influence of co-existing anions on the Cr (V) adsorption capacity.

Table 1 .
Estimated constants of the kinetic parameters of Cr (VI) on ATP-CPBr@CA composite beads.

Table 2 .
The kinetic parameters from intraparticle diffusion model.
C o (mg/L)

Table 4 .
Comparable investigation for Cr (VI) onto ATP-CPBr@CA composite beads and other reported adsorbents.