Characterization of Cassava Root Husk Powder: Equilibrium, Kinetic and Modeling Studies as Bioadsorbent for Copper(II) and Lead(II)

This study was aimed to investigate Pb(II) and Cu(II) ions removal ability from aqueous solution by cassava root husks (CRH) as a cheap, sustainable and eco-friendly bioadsorbent. The CRH was characterized by Fourier Transform Infrared (FTIR) spectroscopy which indicated the availability of various functional groups for metal coordination and the result was supported by elemental analysis studies. UV-Visible spectral studies indicated the presence of oxalate ( 2 2 4 C O − ) and it could possibly interact with metal ions to give rise to a stable chelated coordination complex which affects metal ions removal efficiency. Bioadsorption process was carried out as a function of metal concentration, contact time, pH of the solution, particle size, and dosage of the adsorbent. Experimental results indicated the optimal adsorption condition of pH 4 for both Pb(II) and Cu(II) ions, dosage of 0.1 g/0.1L and 1 g/0.1L for Pb(II) and Cu(II) ions respectively, adsorption equilibrium time of 2 and 25 minutes for Pb(II) and Cu(II) respectively, and concentration of 0.5 mg/L for both metal ions. Kinetic data best fitted pseudo-second-order model and not the pseudo-first-order model. Equilibrium data best fitted the Freundlich model than the Langmuir model. Specific surface area and pore volume studies indicated that CRH is non-porous and hence rapid adsorption kinetics is expected. Supporting the experimental results, molecular modeling studies performed using Schrödinger software predicted several sites in the structure capable of docking with metal ions.


Introduction
Among water pollutants, synthetic dyes and heavy metals pose severe health threats and the latter is currently a concern in many parts of Papua New Guinea (PNG) especially in urban and mining areas [1] [2]. Many reviews are dedicated to identifying and presenting low-cost sorbents for heavy metal removal and recovery [3] [4]. Therefore it is important to find an economically viable treatment method for drinking water as well as wastewater to minimize the concentrations of heavy metals in water bodies. The objective of this research was to investigate the effectiveness of the root husk of cassava (Manhiot esculenta) to remove Pb(II) and Cu(II) ions from solutions through the process of bioadsorption.
Among several techniques, adsorption using solid materials, such as modified silica, alumina, activated carbon, and resin, is considered as the superior methods for metal ion extraction process [5] [6] [7]. However, these materials are expensive to acquire in many developing countries [8] including PNG. According to McKay [6], the criteria for adsorption as an efficient treatment process is to first search for an adsorbent with high selectivity, high adsorbent capacity, abundant availability at low cost. Bioadsorption using agricultural wastes have the advantage of being readily available in tones and can bind metals through various mechanisms such as chelation/coordination, ion exchange, precipitation, and sorption by other physical and chemical forces [9]. Cassava is a common food crop in many countries around the world and managing and dumping of their root husks become a burden so they can be utilized effectively for purifying water as well as for trapping valuable metals.
FTIR is an important tool used in identifying the functional groups available on biomaterials which are responsible for the heavy metal ions coordination.
FTIR uses an interferometer and flourier transformations to obtain a spectrum corresponding to wave numbers that can be assigned to certain functional groups of a biomaterial. Elemental analysis (CHNS) was used to determine the percentage elemental composition of carbon, hydrogen, nitrogen, and sulphur. This provides the basis for identifying the species of elements that can either act as Lewis base and anions for metal sequestration. Bioadsorption studies are usually carried out in batch processes as a function of experimental perimeters such as pH, dosage, contact time, concentration, and agitation speed. The data from concentration and contact time are used to generate the equilibrium and kinetic data respectively. Data from equilibrium and kinetic studies are related to the CRH characterization studies to explain in detail the metal ion adsorption mechanism. Computational studies were done to further explain the interaction between the metal ions and the CRH with Schrodinger software, by carrying out docking studies using Glide. Schrodinger is a suite of molecular modeling packages that take advantage of the latest technological advances in computational chemistry [10]. Additionally, pH studies were also carried out to account for the adsorption behaviors.
Equilibrium studies provide data on the capacity (or affinity) of the adsorbent Journal of Encapsulation and Adsorption Sciences for the solute and hence the dosage of absorbent required to remove a unit mass of solute from the solution. Mathematically equilibrium is expressed in isotherms. Two commonly used isotherms are Langmuir and Freundlich [11] [12] [13] and their applicability is judged with correlation coefficient (R 2 ). The Langmuir isotherm assumes mono-layer adsorption on a uniform surface with a finite number of adsorption sites and maximum adsorption occurs when a saturated mono-layer of adsorbate is present on the adsorbent surface [5] [6]. The linear form of Langmuir isotherm model [14] is given in Equation (1). Here, K ads is the equilibrium constant that increases with increasing molecular size (Langmuir constant), q max is the adsorption capacity (mg/g), q e is the equilibrium adsorption (mg/g) and C e is the equilibrium (residual) concentration of metal ion The dimensionless separation factor R L is given in Equation (2) which is an essential feature of Langmuir isotherm [15]. If R L = 0, the process is said to be irreversible, if R L = 1, it is linear, if R L > 1, it implies an unfavourable adsorption and if 0 < R L < 1, it's a favourable adsorption. C i is the initial concentration of metal ions (mg/L).
Freundlich isotherm is an empirical model and describes equilibrium on heterogeneous surface and does not assume mono-layer adsorption. It is an indicator of the extent of heterogeneity of adsorbent surface [6]. The linear form of Freundlich isotherm [14] is represented in Equation (3), where K is Freundlich constant or adsorption capacity and 1/n is adsorption intensity.

log
log log e e q K C n = + Adsorption kinetics describes the adsorption mechanism as well as estimates the time required for obtaining equilibrium concentration of metal ions onto the biomass [6]. It measures the efficiency of the process and shows a large dependence on physical and/or chemical characteristics of the sorbent material [15].
Commonly used are pseudo-first-order and pseudo-second-order models and their linear forms are given in Equations (4) and (5) respectively [15].
Here q t is the adsorption capacity at time t (mg/g) and k 1 is the rate constant of pseudo-first-order (min −1 ) and k 2 is the rate constant of pseudo-second-order (g·mg −1 ·min −1 ). Journal of Encapsulation and Adsorption Sciences

Preparation and Characterization of CRH of Bioadsorbent
Cassava samples were harvested from Markham, Morobe Province, PNG (−6˚38'29.51"S and 146˚51'37.55"E). The CRH were peeled from the pulp, cleaned and washed thoroughly with tap water and rinsed with distilled water.
The CRH were then cut into smaller pieces, sun dried for 7 days and then ground and sieved to get particle sizes ≤ 50 µm. A summary schematic representation of the process is depicted in Figure 1.

Infrared Spectral Studies
The FTIR spectra of CRH are provided in Figure 2. The presence of hydroxyl and carboxylic acid groups, methionine, and tryptophan, aromatic molecules such as thiamine and other amino acids [16] [17] make the band assignments quite complex and an attempt was made to assign the observed bands (Table 1) Figure 2. FTIR spectrum of CRH powder. based on available literature [19]. It is very clear that functional groups like acid (-COOH), amine (primary-NH 2 and secondary-NHR), imine (>C=NH), alcohol (-OH), sulphydryl (-SH), mono-substituted and para-disubstituted benzene rings are available in CRH. Availability of these functional groups supports chemisorption process. [20].
Based on the infrared spectral studies, a better depiction of the ability of CRH to coordinate with metal ions is provided in Figure 3. Coordination could be considered as a heterogeneous phenomenon where CRH acts as a solid phase while the aqueous phase contains metal ions. Equilibrium was set up between the two species at the solid-solution interface. The abstraction of metals from aqueous solution is simply the coordination of metal ions to active ligating sites of CRH. This correlates with the results from pseudo-second-order kinetics studies indicating chemisorption as the driving force for adsorption.

UV-Visible Spectrophotometry
The oxalate (   of oxalate in the solution. Oxalates are characterized by two prominent absorptions at 190 nm due to n → σ* and ~300 nm due to n → π* transitions [19]. Oxalate is a polydentate ligand (bidentate) which gives rise to stable chelates upon sequestering M(II) [22]. Because the complex was present in the filtrates which were subjected to ICP-OES analysis for metal estimations, some of the expected behaviors in the adsorption studies were not seen as reported by Jorgetto et al. [18].

Elemental Analysis
CHNSP analysis of CRH clearly supported the presence of sulphur, nitrogen and phosphorus containing groups which directly or indirectly aided in metal coordination. Table 2

Surface Area and Pore Volume
The BET surface area and the pore volume of CRH was found to be 0.9039 m 2 /g and 0.002101 cm 3 /g (based on single point adsorption on pores < 1019.662 Å radius) respectively. The result clearly corroborates with earlier findings and the surface area was found to be <1 m 2 /g [18]. The data implies that the material is non-porous and hence rapid adsorption kinetics is expected which agreed well with the kinetic studies (Table 3).

Experimental Parameters
The pH of the solution plays a very significant role in the adsorption process. For both Cu(II) and Pb(II), at lower pH, the adsorption of both metals was found to be low ( Figure 4). This could be due to the diminished exchange of acidic sites of CRH with metals on the basis of Le Chatelier principle. At low pH, the hydrogen ion concentration is very high thus favoring the reverse reaction of Equation (6) Beyond pH 7, the adsorption decreases, probably due to metals precipitating as hydroxides (Equation (7)). The maximum adsorption of Pb(II) and Cu(II) occurs at around pH 4 with removal of 95.26% and 78.66% respectively ( Figure  4). However, the results are in contrast with the literature reported data [14] where the Cu(II) adsorption to cassava husk was found to be rapid from pH 1 -3, beyond which the adsorption stabilizes to a plateau.
The effective dosage for Pb(II) was found to be 0.   that the nature of metal ion is very vital for adsorption process because higher dosage (10 times more for Cu(II) compared to that of Pb(II)) as well as lower adsorption was observed for Cu(II).
The removal of Cu(II) and Pb(II) with CRH was fast ( Figure 6). The adsorption equilibrium time for Pb(II) and Cu(II) was found to be 2 and 25 minutes respectively. The adsorption of both Pb(II) and Cu(II) onto CRH was high at lower concentration and decreases as the concentration increases ( Figure 7).
With 0.5 mg/L, Pb(II) and Cu(II) adsorption was found to be 94% and 76% respectively. Effects of pH, dosage, contact time and concentration on the adsorption of Cu(II) and Pb(II) ions to CRH are provided in Figures 5-8. Table 4 below gives the concentration values (initial concentration, C i , and equilibrium concentration, q e ) for Cu(II) and Pb(II) solutions which were used to plot the Langmuir and Freundlich Isotherms. Table 4 gives the values of C i and q e for the metals which were used to plot the isotherms. The Langmuir isotherm model for both metal ions is presented in Figure 9. The maximum adsorption capacities (q max ) for completely adsorbed metal monolayer coverage were found to be 26.74 and 16.50 mg/g of CRH for

1) Langmuir Isotherm
Pb(II) and Cu(II) ions respectively (Table 5). K ads is related to the affinity of the binding sites and lower values of K ads [0.0652 and 0.0175 for Pb(II) and Cu(II) respectively] indicated the particle radius for CRH was smaller towards adsorption Journal of Encapsulation and Adsorption Sciences      [9]. The separation factors (R L ) for both Pb(II) and Cu(II) adsorption to CRH lie very close to 1 ( Table 6) which supported its low R 2 value for less favorable adsorption.

2) Freundlich Isotherm
The experimental data for Pb(II) and Cu(II) adsorption onto CRH showed a very good linear relationship with Freundlich isotherm model ( Figure 10). This indicated that the adsorption reached equilibrium on a heterogeneous surface or the adsorption energies of CRH particles vary. Fitting the Freundlich isotherm also means the adsorption has multi layers of metals adsorbed onto CRH. As indicated in Table 7, the adsorption of Pb(II) and Cu(II) to CRH is favorable because it fulfills the condition of n > 1 [18]. This data correlated the adsorption capacity, K value which was found to be 1.5485 and 0.2780 mg/g for Pb(II) and Cu(II) respectively. A very good linear relationship has indicated the strong binding of Pb(II) and Cu(II) ions to CRH [23].

Adsorption Kinetic Studies 1) Pseudo-First-Order Kinetics
The experimental data for the adsorption of Pb(II) and Cu(II) did not fit well Journal of Encapsulation and Adsorption Sciences Table 6. Comparison of separation factors (R L ) between Pb(II) and Cu(II) for adsorption with CRH.   with Pseudo-first-order kinetic model (R 2 = 0.2619 and 0.1993 respectively) as shown in Figure 11. No correlation was found for comparing the experimental equilibrium adsorption capacity (q e.exp ) and calculated equilibrium adsorption capacity (q e.cal ) ( Table 8). It can be concluded that the adsorption process did not follow pseudo first order model [23].
2) Pseudo-Second-Order Kinetics Journal of Encapsulation and Adsorption Sciences Figure 11. Pseudo-first-order kinetics for adsorption of Pb(II) and Cu(II) to CRH. Both the adsorption of Pb(II) and Cu(II) to CRH has shown a very good linear relationship (R 2 = 1 for both cases) with the pseudo-second-order kinetic model ( Figure 12). Furthermore, q e.exp agreed well with q e.cal for Pb(II) but not so well with Cu(II) adsorption (Table 9). This agreed data implies that the adsorption depends on both the adsorbent and adsorbate and the rate limiting step was chemisorption process, involving valency forces through sharing or exchange of electrons between the adsorbent and adsorbate [16]. The availability of functional groups on CRH ( Figure 2) very well supports chemisorption of Pb(II) and Cu(II) through exchange or sharing of electrons.

Molecular Modeling Studies
A thorough literature search was undertaken to identify that there was no reported structure for CRH. Hence, data sets for 2600 unique protein sequences were screened to get a reasonable starting geometry for docking (coordination) studies. Molecular modelling studies were performed using Schrödinger software; a sophisticated chemical simulation software used in materials research [24]. However, cassava protein sequences were reported [25] in an overview of protein identification studies. These sequences were used as the starting point for the modelling studies. Vanderschuren et al. [26] identified over 2600 unique proteins in the cassava root and provided data sets for these proteins. These data sets were screened to get the best possible match against the protein data bank to Journal of Encapsulation and Adsorption Sciences  Docking studies using Glide predicted several sites in the structure capable of docking with metal ions (Figure 13).

Coordination Ability of CRH to Metals
The properties of metals that influence the coordination possibilities with the ligating atoms and hence the stability of coordination complex formed are 1) Nature; 2) Size, charge and charge density; 3) Electronic configuration; and 4) Hardness [27]. The properties of ligating atoms that decide the ease with which they coordinate with the metal are 1) Lonepair availability on the ligating atoms (N, O, S, etc.) and hence the basicity; and 2) Hardness of ligating atoms [27].
According to Pearson's hard and soft acids and bases (HSAB) concept [28]

Conclusion
The adsorption study was carried out using CRH powder by varying the parameters such as pH, adsorbent dose, contact time, and concentration of metals.
The study brought out the fact that this bioadsorbent was very effective in removing Pb(II) from aqueous solutions compared to Cu(II) ions and this was attributed to the soft nature of the former metal and the very high charge density of the latter. This research clearly has concluded that the metal removal processes by adsorption are very much dependent on the nature, surface charge density, and hardness of the metals, and the nature of sites available on the bioadsorbents for binding. This study has conducted trials pertaining to metal removal from aqueous solutions using naturally available bioadsorbents and correlates the results with the coordination ability of metals and the availability of ligating sites on bioadsorbents chosen. Attempts were made to assign FTIR spectral bands which also indirectly provide evidences for the presence of ubiquitous amounts and types of amino acids in the chosen bioadsorbent.

Funding
This research was funded by Papua New Guinea University of Technology Research Funds.