Interpretations

Abstract

Percentage removal for metal ions is calculated in terms of sorption efficiency of the respective biosorbents under various experimental conditions, viz., biomaterial dosage, contact time, metal concentration, optimum particle size, volume, and pH. The data are to be handled with appropriate statistical treatment and tabulated. The concentration of the removed metals may be represented in terms of μmol, μg, and ppm. The representative tables exhibiting the sorption efficiency of particular biosorbents used for the decontamination of toxic metals from water bodies are given below. The influence of each variable is taken into account for its effect on the sorption phenomenon.

Single Metal Sorption

Percentage removal for metal ions is calculated in terms of sorption efficiency of the respective biosorbents under various experimental conditions, viz., biomaterial dosage, contact time, metal concentration, optimum particle size, volume, and pH. The data are to be handled with appropriate statistical treatment and tabulated. The concentration of the removed metals may be represented in terms of μmol, μg, and ppm. The representative tables exhibiting the sorption efficiency of particular biosorbents used for the decontamination of toxic metals from water bodies are given below. The influence of each variable is taken into account for its effect on the sorption phenomenon (Tables 1, 2, 3, 4, and 5).

Table 4 This table explains Cr(III) ion concentrations (μM) after adsorption on Zea mays cob powder (ZMCP) as functions of contact time and biomaterial dosage at volume (200 mL), particle size (105 μm), and pH (6.5)

Table 2 This table explains Cr(VI) ion concentration (μM) after adsorption on Saraca indica powder (SILP) as functions of contact time and biomaterial dosage at volume (200 mL), particle size (105 μm), and pH (2.5)

Table 3 This table explains soluble Ni(II) ion concentration (μM) after adsorption on shelled Moringa oleifera seeds (SMOS) as functions of contact time and biomass dosage at volume (200 mL), particle size (105 μm), and pH (6.5)

Table 5 This table explains Pb(II) ion concentrations (μM) after adsorption on S. indica leaf powder (SILP) as functions of contact time and biomaterial dosage at volume (200 mL), particle size (105 μm), and pH (6.5)

Table 1 This table explains Cd(II) ion concentration (μM) after adsorption on L. leucocephala seed powder (LLSP) as functions of contact time and biomaterial dosage at volume (200 mL), particle size (105 μm), and pH (6.5)

Results may be incorporated in the following terms.

Effect of Particle Size on Metal Sorption

The particle size is an important factor from the biosorption point of view and has a significant influence on the kinetics of adsorption. Figure 1 explains the role of size of the particle of the biosorbent [Leucaena leucocephala (LLSP)] on the sorption phenomenon.

Fig. 1

Effect of particle size on metal sorption at LLSP (Time: 40 min, biomaterial: 4.0 g)

The figure explains that as the size of the particle goes on decreasing there is an increase in the sorption efficiency of the metal ions. The reduction in particle size of the biosorbent results in an increase in surface area. With increased availability of the surface area, more adsorption sites are available for metal removal.

Effect of Contact Time on Metal Sorption

Time-dependent studies offer data about the changes in metal sorption related to time. In these studies, the minimum time necessary for the biomaterial to be in contact with the metal ion is identified. The variation of contact time with biosorbent [Z. mays cob powder (ZMCP)] may be interpreted in the following fashion:

It has been observed that initially there is increase in sorption with the increase in time and finally attaining a maximum value; however, any further increase in contact time may not result in increase in the sorption efficiency (Fig. 2).

Fig. 2

Effect of contact time on metal sorption at ZMCP (Conc. initial: 25 mg/L for Cd(II), Cr(III), and Ni(II) and 50 mg/L for Cr(VI); biomaterial: 4.0 g)

Effect of Biomaterial Dosage on Metal Sorption

The amount of biomaterial seems to influence the extent of uptake of metals. Hence, this factor needs to be taken into consideration in application of any biomaterial as biosorbent. Therefore, sorption efficiency of biosorbents for different metals may be evaluated as a function of biomaterial dosage leading to standardization of optimum amount of biomaterial required. Percentage sorption versus biomaterial dosage indicates, in general, sorption efficiency of different metal ions increased with increase of biomaterial dosage attaining a maximum value; however, further increase in dosage does not result in increase in the sorption efficiency. Figure 3 shows the effect of biomaterial dosage on metal sorption.

Fig. 3

Effect of biomaterial dosage on metal sorption at (SMOS) (Conc. initial: 25 mg/L for Cd(II), Cr(III), Ni(II) and 50 mg/L for Cr(VI); time: 40 min)

The basis of attainment of equilibrium between adsorbate and adsorbent at the existing operating conditions is likely to render the adsorbent incapable of further adsorption.

Effect of Concentration on Metal Sorption

The sorption capacity of the biomaterial at a fixed amount for different metal ions present in the various concentration ranges can be calculated. Percent sorption of biosorbents is to be calculated in each case.

Biosorption efficiency, in general, increases initially; however, further increase in concentration may not result in the sorption efficiency (Fig. 4).

Fig. 4

Effect of metal concentration on metal sorption at SILP (Time: 40 min, biomaterial: 4.0 g)

Although actual wastewater treatment systems have to deal with a mixture of heavy metals, most research activities are single metal sorption oriented and not realistic; therefore, the assessment of the biosorption performance becomes less accurate. Multi-metal biosorption studies are, therefore, particularly important for evaluating the degree of interference with a biosorption process of common metal ions in wastewater.

Mechanistic Aspects of Sorption

The pH condition of the solution is an extremely important parameter in metal biosorption. It governs a series of phenomena like site dissociation, solubility, mobility, and chemistry of the metals ions. pH affects the selectivity of the biomaterial to bind a variety of metals. At different pH values, binding sites are different. Heavy metals tend to bind the biomaterial at acidic pH than the pH at which the metal precipitates in hydroxide form. Record of pH profile becomes necessary since unknown reactions between the metal ions and the biomaterial might occur, modifying the extent of normal metal behavior.

Keeping the above views in mind, pH profile for metal ion binding is to be recorded for each metal. Consider the example of biosorbent [Ficus religiosa leaf powder (FRLP)] which may be discussed as follows. The percentage sorption of Cd(II) and Ni(II) on seed biomaterial increases as the pH of the solution increased from 2.5 to 6.5. No significant difference in sorption behavior was noticed with further increase in pH up to 7.5. The pH profile for Cd(II) and Ni(II) sorption on FRLP shows that metal sorption is a function of pH, exhibiting maximum removal efficiency at pH 6.5.

Investigation on pH variation beyond 7.5 yielded an apparent increase in sorption up to pH 8.5, which might be due to precipitation carryover of Cd(II) and Ni(II) starting at pH 7.5 onward. Cd(II) and Ni(II) precipitation is undistinguishable from sorption phenomenon at pH 7.5 (Fig. 5).

Fig. 5

pH profile and metal ion binding (Conc. initial: 25 mg/L for Cd(II) and Ni(II), biomaterial: 4 g, time: 40 min). Error bar represents standard deviation for three replicates

The biosorption of different oxidation states of metals like Cr(III) and Cr(VI) by sorbent taking the example of L. leucocephala seed powder (LLSP) in the pH range 2.5–8.5 has been considered. The biosorption efficiency of Cr(III) increased gradually with rise in pH from 2.5 to 6.5 attaining optimum sorption at 6.5. However, percent sorption is found to be almost constant with further rise in pH up to 7.5.

Increase in pH from 7.5 to 8.5 results in Cr(III) hydroxide precipitation. Anionic metallic species [Cr(VI)] showed high sorption tendency in acidic pH range (2.5–3.5). Further increase in pH from 3.5 to 8.5 resulted in a sharp decreasing trend of sorption (Fig. 6).

Fig. 6

pH profile and metal ion binding (Conc. initial: 25 mg/L for Cr(III) and Cr(VI), biomaterial: 4 g, time: 40 min). Error bar represents standard deviation for three replicates

A possible mechanism for metal binding to the biosorbent may be designed. The aqueous solution of biosorbents, in general, is a heterogeneous complex mixture having various functional groups: protein, fat, carbohydrate, ash, and high amounts of free amino acids.

Amino acids have been found to constitute a physiologically active group of transporters, working even at low concentrations, which because of ability to interact with metal ions is likely to increase their mobility. These proteinaceous amino acids have a variety of structurally related pH-dependent properties of generating appropriate atmosphere (positively and/or negatively charged sites) for attracting the cationic and anionic species of metal ions.

pH profile and sorption behavior of various metals provide an insight into the mechanistic aspects of sorption process. Maximum sorption of metals (cationic species) is found to be in the pH range 6.5–7.5 for Cd(II), Cr(III), and Ni(II) and 2.5–3.5 for anionic species Cr(VI). Cr(III) exists in cationic forms such as Cr³⁺, Cr(OH)²⁺ in the pH range 4.0–6.0, whereas Cr(VI) exists in anionic forms such as Cr₂O₇ ²⁻, HCr₂O₇ ⁻, HCrO₄ ⁻, and CrO4²⁻ at low pH values 1.0−4.0.

The majority of amino acids present in biosorbents have isoelectric points in the pH range 4.0–8.0. In this range of pH, over 90% of the amino acid molecules are in ionized state, i.e., they have both positively charged amino groups and negatively charged carboxylate ions. Sorption tendency of cationic metallic species is very less at lower pH values. It may be because of lower pH; binding sites (amino acid moieties) in the biomaterial are generally protonated and thus repulsion occurs (Fig. 7).

Fig. 7

Amino acid–metal interaction

Sorption of cationic metallic species increases with rise in pH, attaining a plateau around 6.5–7.5. At relatively higher pH (above 4.5), the carboxylic groups are deprotonated and as such are negatively charged. These negatively charged carboxylate ligands are likely to attract the cationic metallic species.

The solution chemistry of Cr(VI) clearly shows its existence as an oxoanion in several stable forms such as Cr₂O₇ ²⁻, HCr₂O₇ ⁻, HCrO₄ ⁻, and CrO₄ ². Optimum sorption efficiency of biosorbents for Cr(VI) is found to be at pH 2.5. At lower pH (2.5), the sorbent is positively charged due to protonation of amino groups, while the sorbate, dichromate ion, exists mostly as an anion leading to electrostatic attraction between sorbent and sorbate. This fact results in increased sorption efficiency of biosorbents for anionic metallic species at low pH.

This is in support of the school of thought that metal binding is likely to be caused by interactions with functional groups such as carboxyl and amino groups located on the cell surface of the biosorbent.

In addition to metal–amino acid interactions responsible for biosorption several other hypotheses based on metal–polyphenols, metal–hydroxyl, metal–sulfhydryl, and metal–carbohydrates have also been mentioned in the literature.