Biosorption of Ni(II), Cr(III), and Co(II) from Solutions Using Acalypha hispida Leaf: Kinetics, Equilibrium, and Thermodynamics

Biosorption studies were conducted to study the removal of Ni(II), Cr(III), and Co(II) from aqueous solution of Acalypha hispida leaf. e FTIR spectral characteristics of Acalypha hispida leaf revealed the presence of ioniazable groups that could participate in the binding of metal ions in solution. e kinetic, equilibrium, and thermodynamic studies of the biosorption of the metal ions were investigated using various physicochemical parameters; each parameter was found to affect the biosorption process. e kinetic studies showed that the biosorption process was best represented by pseudo-second-order kinetics among four kinetic models tested. Equilibrium data were better represented by Freundlich isotherm among Langmuir and Freundlich adsorption isotherms. e study on the effect of dosage showed that the dosage of the biomass signi�cantly affected the uptake of the metal ions from solution. ermodynamic parameters such as standard Gibbs-free energy (ΔGG), standard enthalpy (ΔHH), standard entropy (ΔSS), and the activation energy (AAA were calculated. e order of spontaneity of the biosorption process was found to be Cr(III)>Ni(II)>Co(II). e activation energy for the biosorption of each of the metal ions was less than 42 kJmol at 323K indicating that each was a diffusion-controlled process.


Introduction
Environmental pollution by toxic metals is a worldwide problem due to increased industrialisation.e metal ions are particularly problematic due to their accumulation in the food chain and their persistence [1].Among the metal ions are Ni(II), Cr(III), and Co(II).ere is the need to regulate their levels in the environment before they enter food chain.Of recent, biosorption of metal ions using biological materials has been identi�ed as a potential technique for remediation of metal bearing effluents [2][3][4][5][6][7][8][9][10][11].Biosorption has the merits of low cost and environmental friendliness over the conventional methods of removing metal ions from solution [6].Acalypha hispida (Red Hot Cattails) is a plant that is native to the �outh Paci�c (New Guinea and the �alay Archipelago).Acalypha hispida is a tropical shrub in the spurge family (Euphorbiaceae).is interesting plant gets its common name from the exotic �owers that look like strands of chenille yarn.Other descriptive common names include foxtails, monkey tail, red-hot cat's tail or chenille plant.It is usually grown as a houseplant for its unusual tassel-like �owers.is evergreen plant can grow to 15-feet tall and 8feet wide in suitable climates but in containers will remain much smaller.Acalypha hispida �ower has been reported to contain three anthocyanins, cyanidin 3-O-(2 �� -galloyl-6 �� -O--rhamnopyranosyl--galactopyranoside) (5%), cyanidin 3-O-(2 �� -galloyl--galactopyranoside) (85%), and cyanidin 3-O--galactopyranoside (5%) [12].Phytochemical screening of the leaf extracts revealed the presence of tannin, alkaloid, steroid, saponin, and �avonoid [13].It was on this basis that this readily available weed was investigated for its potential 2. Methodology 2.1.Biomass Preparation.Acalypha hispida leaves were harvested from the mini campus of Olabisi Onabanjo University, Ago-Iwoye, Ogun State, Nigeria.e leaves were carefully collected inside a transparent nylon, sun dried immediately, and kept dry till time of usage.

Preparation of Solution.
All chemicals used in this study were of analytical reagent grade and were used without further puri�cation.Standard solutions of Ni(II), Cr(III) and Co(II) used for the study were prepared from NiCl 2 ⋅6H 2 O, Cr(NO 3 ) 3 ⋅9H 2 O, and CoSO 4 ⋅7H 2 O, respectively.e working solutions with different concentrations of the metal ions were prepared by appropriate dilutions of the stock solution immediately prior to their use with distilled water.e initial pH of the solution was adjusted accordingly with a pH meter.ermostated Water bath (Haake Wia Model) was used as the medium for the process.e concentration before and aer biosorption of each metal ion was determined using a Perkin-�lmer Analyst 700 �ame atomic absorption spectrophotometer (AAS) with deuterium background corrector.Fourier transform infrared (FTIR) spectra of dried unloaded biomass and metal loaded biomass were recorded at 400-4000 cm −1 using a Shimadzu FTIR model 8400 S spectrophotometer.

Batch Biosorption
Study.e biosorption study was determined by batch experiments by contacting 0.5 g of the Acalypha hispida leaf with 25 mL of each metal ion solution under different conditions for a period of time in a glass tube.e biosorption studies were conducted at 27 ∘ C using thermostated water bath to determine the effect of pH, contact time, and initial metal ion concentration on the biosorption.e residual metal ion of the supernatant was analyzed using AAS.e amount of metal ion biosorbed from solution was determined by difference and the mean value calculated.

Effect of pH on Biosorption
. e effect of pH on the biosorption of the metal ion was carried out within pH 1-7 to prevent the precipitation of metal ions.is was done by contacting 0.5 g of Acalypha hispida leaf with 25 mL of 100 mgL −1 metal ion solution in a glass tube.e pH of each solution was adjusted to the desired value by drop wise addition of 0.1 M HNO 3 and/or 0.1 M NaOH.e glass tubes containing the mixture were le in a water bath for 6 hours.e biomass was removed from the solution by decantation.e residual metal ion concentration in the supernatant was analyzed.e optimum pH was determined as the pH with the highest biosorption of each metal ion.

Effect of Contact Time on
Biosorption.e biosorption of the metal ions by Acalypha hispida leaf was studied at various time intervals (0-360 min) and at the concentration of 100 mgL −1 .is was done by contacting 0.5 g of Acalypha hispida leaf with 25 mL of 100 mgL −1 of metal ion solution at optimal pH.e leaf was le in solution for different periods of time.At predetermined time, the glass tubes were withdrawn from the bath, and the residual metal ion concentration in solution was determined using AAS.e amount of metal ions biosorbed was calculated for each sample.
2.6.Effect of Initial Concentration on Biosorption.Batch biosorption study of metal ion was carried out using a concentration range of 10-100 mgL −1 .is was done by contacting 0.5 g of the leaf with 25 mL of each solution at optimal pH.Two glass tubes were used for each concentration.e tubes were le in a thermostated water bath maintained at 27 ∘ C for the predetermined optimum time.e leaf was removed from the solution, and the concentration of residual metal ion in each solution was determined.

2.7.
Effect of Temperature on Biosorption.e batch biosorption process was studied at different temperatures within the range 20-50 ∘ C in order to investigate the effect of temperature on the biosorption process.is was done by contacting 0.5 g of Acalypha hispida leaf with 25 mL of 100 mgL −1 of metal ion solution at the optimal pH and time.

Statistical
Analyses.e curve �ttings of the data obtained were performed using Microcal Origin 6.0 soware.
Phytochemical screening of the leaf extracts revealed the presence of tannin, alkaloid, steroid, saponin, and �avonoid [13,14].erefore, knowledge of the surface functional groups would give insight to the biosorption capacity of the biomass.ese groups would form active sites for sorption on the material surface.e FTIR spectra of dried unloaded and metal-loaded leaf were taken to obtain information on the nature of possible interactions between the functional groups of Acalypha hispida leaf biomass and the metal ions as presented in Figure 1.Several peaks were observed from the spectra indicating that Acalypha hispida leaf is composed of various functional groups which are responsible for binding of the cations.e IR spectra pattern of the biomass showed distinct and sharp absorptions indicative of the existence of the -OH, -NH, -C-O-, and C=O groups as shown in Figure 1.ese bands are due to the functional groups of Acalypha hispida leaf that participate in the biosorption of Ni(II), Cr(III), and Co(II).On comparison, there are clear band shis and, decrease in intensity between the bands of the unloaded biomass and that of the metal-bound biomass as presented in Table 1.e FTIR spectra of the Acalypha hispida leaf biomass indicated slight changes in the absorption peak frequencies due to the fact that the binding of the metal ions causes reduction in absorption frequencies.ese observed shis in absorbance imply that there were metal binding processes taking place on the active sites of the biomass.Analysis of the FTIR spectra showed the presence of ionizable functional groups (O-H, NH 2, C-O, C=O,) which are able to interact with cations [1,6,[15][16][17].is implies that these functional groups would serve in the removal of positively charged ions from solution.

Effect of Solution pH on Metal Ion
Biosorption.e pH of the solution usually plays an important role in the biosorption of the metal ions [18].It is an important parameter governing the uptake of heavy metals by biosorption process as it not only affects metal species in solution, but also in�uences the surface properties of biosorbents in terms of dissociation of binding sites and surface charge [19].e net charge of the sorbate and that of the sorbent are dependent on the pH of the solution.At low pH, the metal ion uptake is inhibited by net positive charge on the sorbent and the competition between the metal ions and the hydrogen ions in solution.
As the pH increases, the negative charge density on biomass increases as a result of deprotonation of the metal binding sites on the biomass, consequently, the biosorption of the metal ions increases.Figure 2 shows the variation of the metal ions biosorbed on Acalypha hispida leaf at various solution pH values.For the three metal ions, the biosorption increased as the pH increased from pH 1 to pH 5. e increase observed in the biosorption with increase in pH implies that ionexchange process is involved.e reaction involved the biosorption of metal ion (represented as   ) from the liquid phase to the solid phase, the biosorbent with lone pair of electron (represented as Ä) and can be considered as a reversible reaction with an equilibrium being made between the two phases as schematically shown below for a divalent metal ion in solution: 3.3.Biosorption Kinetics.Figure 3 illustrates the dynamic biosorption process of the three metal ions on Acalypha hispida leaf.It is observed that the biosorptive quantities of the three metal ions on the leaf increase with increasing contact time.In each case, biphasic kinetics are observed: an initial rapid stage (fast phase) where biosorption is fast and contributes to equilibrium uptake and a second stage (slow phase) whose contribution to the metal ion biosorbed is relatively smaller.e fast phase is the instantaneous biosorption stage, it is assumed to be caused by external biosorption of metal ion to the biomass surface.e second phase is a gradual biosorption stage, whose is diffusion rate is controlled.Finally, the biosorption sites are used up, the uptake of the metal ion reached equilibrium.is phase mechanism has been suggested to involve two diffusion processes, external and internal, respectively [20].e biosorption of each of the three metal ions eventually achieves equilibrium although their rates of uptake and times of reaching equilibrium are different.is might be due to the differences in hydrated ionic sizes of the metal ions [21].
In order to establish the mechanism of the biosorption of Ni(II), Cr(III), and Co(II) on Acalypha hispida four kinetic models were applied to the kinetic data obtained.ese are the pseudo-�rst-order, the pseudo-second-order, the Elovich kinetic model, and the Intraparticle diffusion model equations.One of such models is the Lagergren pseudo-�rstorder model which considers that the rate of occupation of the biosorption sites is proportional to the number of the unoccupied sites [17]: Which can also be written as Integrating between the limits   = 0 at  = 0 and   =   at  = , we obtain is can be rearranged to obtain a linear form: where  1 is the Lagergren rate constant of the biosorption (min 1 );   and   are the amounts of metal ions sorbed (mg g 1 ) at equilibrium and at time , respectively.e plot of log(     ) versus  for the biosorption of metal ions on the biomass at initial concentration of 100 mgL 1 did not give a straight line implying that the process does not follow �rst-order kinetic model.e data was equally sub�ected to the pseudo-second-order kinetic model.e pseudo-secondorder kinetic model is represented as On integrating between boundary conditions, we have On rearrangement, we have where  2 is the equilibrium rate constant of pseudo-secondorder biosorption process (g mg −1 min −1 ).In the three metal ions under study, the pseudo-�rst-order kinetics did not �t the data obtained.However, plots of  versus   showed good �tness of experimental data with the pseudo-secondorder kinetic model for different initial concentration of the three metal ions as presented in Figure 4. e data were also subjected to the Elovish kinetic model given by and the intraparticle diffusion equation given as e intraparticle diffusion equation has been used to indicate the behaviour of intraparticle diffusion as the rate limiting step in the biosorption process. is the percent metal biosorbed,   is the intraparticle diffusion constant,  is the contact time, while  is the gradient of the linear plot.In the linear form, (10) turns to og  =  og   og   .
e correlation coefficients obtained were found to be highest for the pseudo-second-order kinetics as it was found to be in excess of 0.9 as presented in Table 2.
On comparison of the values of  2 for the experimental points, the pseudo-second-order kinetic model is the best kinetic model to predict the dynamic biosorption of Ni(II), Cr(III), and Co(II) on Acalypha hispida leaf similar to what was reported for banana leaf [22].e result shows that the rate of biosorption of the metal ions is of the order Ni(II) > Co(II) > Cr(III).e biosorption capacity is in the order Cr(III) > Ni(II) ≈ Co(II).e differences observed in the rate of biosorption as well as in the biosorption capacity may be accounted for in terms of the differences in ionic charges and hydrated ionic sizes of the ions in solution [21].

Biosorption
where  and 1 are the Freundlich constants related to the biosorption capacity and biosorption intensity of the biosorbent, respectively.e linear form of the Langmuir equation is expressed as where Γ, Γ  , and   are the Langmuir parameters.e results show that the regression coefficients obtained for Freundlich isotherm are higher than for Langmuir isotherm.is implies that the biosorption is assumed to be a monolayer sorption with a heterogeneous energetic distribution of active sites, accompanied by interactions between biosorbed molecules  F 6: Efficiency plot for the biosorption of Ni(II), Cr(III), and Co(II) ions using Acalypha hispida leaf.[1].e Freundlich isothermal parameters for the biosorption are presented in Table 3.

Biosorption Efficiency.
e result of the study on the effect of initial metal ion concentration on biosorption efficiency is shown in Figure 6.e plots show that the biosorption efficiency of the biomass reduces with increase in the initial metal ion concentration of Cr(III) and Co(II) which might be due to the �xed number of binding sites in the biosorbent having more ions than at lower concentration.On the other hand, the biosorption efficiency increased with the increase in initial metal ion concentration for Ni(II).e biosorption efficiency () for each metal ion was calculated as where   and   are the initial and the equilibrium metal ion concentrations (mgL − ), respectively.
3.6.Effect of Biomass Dosage on Biosorption.e effect of biomass dosage on biosorption capacity is reported in Figure 7. e general trend of increase in metal ion biosorbed with increase in biomass dosage indicates an increase in uptake due to more binding sites on the biomass available for biosorption.It was found that biosorption capacity increases with increase in dosage of the biosorbent.is is due to the fact that increase in biomass dosage leads to increase in the number of active sites available for biosorption.Hence, the amount of metal ions available for biosorption per gram of biosorbent will be less when the amount of biosorbent is increased.e difference in biosorption capacity  (mg g − ) at the same initial metal ion concentration and contact time may also be attributed to the difference in their chemical affinities and ion exchange capacity, with respect to the chemical functional group on the surface of the biosorbent.is trend has been reported for other biosorbents [23].
3.7.Biosorption ermodynamics.e biosorption of metal ions may involve chemical bond formation and ion exchange since temperature is a major parameter affecting them.e variation of temperature affects the biosorption of metal ions onto solid surfaces of biomass since the biosorption process is a reversible one.e nature of each side of the equilibrium determines the effect temperature has on the position of equilibrium.e side that is endothermic is favoured by an increase in temperature, while the contrary holds for the exothermic side.e corresponding free energy change was calculated from the relation [16,24]: where K) is the absolute temperature.e equilibrium constant (  ) was calculated from the following relationship: where   and  ad are the equilibrium concentrations of metal ions (mgL − ) in solution and on biosorbent, respectively.Consequently, the thermodynamic behaviour of the biosorption of Ni(II), Cr(III), and Co(II) onto Acalypha hispida leaf was evaluated through the change in free energy (Δ ∘ ), enthalpy Δ ∘ ), and entropy (Δ ∘ ).e thermodynamic parameters like enthalpy and entropy were calculated using van't Hoff equation [6,10].e change in free energy is related to other thermodynamic properties as where  is the absolute temperature (K);  is the gas constant (8.314J⋅mol −1 ⋅K −1 ).Δ ∘ (J⋅mol −1 ) and Δ ∘ (J⋅mol −1 ⋅K −1 ) were calculated from the slope and intercept of the linear plot of ln   versus 1/, respectively.e thermodynamic parameters obtained for this study are presented in Table 3. e thermodynamic parameters (free energy (Δ ∘ ), enthalpy (Δ ∘ ), and entropy (Δ ∘ )) for the biosorption of the metal ions were determined by the application of ( 15) and ( 16) as presented in Table 4.In general, the change of standard free energy for physisorption is in the range of −20 to 0 kJ mol −1 and for chemisorption varies between −80 and −400 kJ mol −1 [25,26].In the present study, the overall Δ ∘ (as shown in Figure 8) has values ranging from −1.5 to 2.25 kJ mol −1 .ese results correspond to a spontaneous physical adsorption of the metal ions, indicating that this system does not gain energy from external resource [25,27].e decrease in Δ ∘ with increase in temperature indicates more efficient biosorption at higher temperature.e order of spontaneity of the biosorption process was found to be Cr(III) > Ni(II) > Co(II).e positive value of enthalpy change (Δ ∘ ) indicates the endothermic nature of the biosorption process, while negative value implies exothermic process.e positive values of Δ ∘ for the biosorption of the three metal ions suggest an endothermic nature of each biosorption process.is is also supported by the increase in the value of biosorption capacity of the biosorbent with rise in temperature.e positive value of Δ ∘ indicates the presence of an energy barrier in the biosorption process.Similarly, the Δ ∘ values for Ni(II) and Co(II) are positive indicating increase in randomness during the biosorption process for these metal ions.ese positive values of Δ ∘ observed for the biosorption of the three metal ions showed an increase in randomness at the solid/solution interface during their biosorption.e positive values of Δ ∘ suggest an increase in randomness at their solid/liquid interface, and it implies that signi�cant changes occur in the internal structure of the adsorbents through the adsorption [25].Furthermore, the magnitude of activation energy () gives an idea about the type of adsorption which is mainly diffusion-controlled process (not diffusivity of solute through micropore wall surface of a particle) or chemical reaction processes [28].Energies of activation, , below 42 kJ/mol indicate diffusion-controlled processes, and higher values give chemical reaction-based processes.erefore, energy of activation, , has been calculated as per the following relation: e values of  at two different temperatures have been tabulated in Table 4.In this study, the activation energy () values were less than 42 kJ mol −1 indicating diffusioncontrolled adsorption processes.

Conclusions
In this work we have studied the biosorption of Ni(II), Cr(III), and Co(II) by Acalypha hispida leaf under various conditions.e biosorption of each was in�uenced by each of the parameters investigated.e pH has much effect on the biosorption of these metal ions from aqueous solutions.e rate of the biosorption of these metal ions followed pseudosecond-order kinetic model.e sorption isotherms of these metal ions onto the biosorbent are better represented by the Freundlich isotherm model.e thermodynamic study shows that the biosorption of each of Ni(II), Cr(III), and Co(II) was spontaneous in the order Cr(III) > Ni(II) > Co(II).e order of disorderliness is Cr(III) > Ni(II) ≈ Co(II).

F 3 :
Contact time dependence pro�le for the biosorption of Ni(II), Cr(III), and Co(II) using Acalypha hispida leaf.

F 7 :
Effect of biomass dosage on the biosorption of Ni(II), Cr(III), and Co(II) using Acalypha hispida leaf.
T 1: e FT-IR spectral characteristics of Acalypha hispida leaf before and aer biosorption of Ni(II), Cr(III), and Co(II).
F 1: FTIR spectra of the free and metal-bound Acalypha hispida leaf.