**4. Results and discussion**

The findings of the experiments suggest that the highest Ni(II) adsorption by BGMA may be attained at a pH of 6 and a biomass loading of 2 g of BGMA. BGMA has an adsorption capability of 42.056 mg/g. During the continuous 24 hours of contact time, the agitation rate of 120 rpm is maintained.

**Figure 1** depicts the visual effect of Ni(II) starting metal ion concentration on equilibrium metal absorption and % clearance. The largest percent elimination of Ni (II) metal ions is observed at low starting metal ion concentrations. The diminishing trend in Ni(II) metal ion removal is observed with an increase in initial metal ion concentration due to an increase in the ratio of the initial number of metal ions to the fixed number of active sites. Furthermore, for a certain number of active sites, the amount of substrate metal ions accommodated in the interlayer gap rises, resulting in decreased metal ion removal. An increase in the initial metal ion concentration causes a decrease in the ionic strength of the solution, which helps to improve metal absorption. As a result of the lowering ionic strength, a rise in initial metal ion concentration raises the equilibrium metal uptake (qeq).

#### **4.1 FTIR-characterisation of BGMA biomass**

**Figure 2** depicts an FTIR spectroscopic investigation of BGMA prior to Ni(II) adsorption. The dNH stretching is shown by the wide adsorption bands at 3696.36 cm<sup>1</sup> , 3620.77 cm<sup>1</sup> , and 3408.94 cm<sup>1</sup> . The dCH2 stretching is measured at 2928.18 cm<sup>1</sup> . The wide adsorption band at 1643.81 cm<sup>1</sup> might be attributed to the carboxylic C]O group, whereas the carboxylate group is represented by the adsorption band at 1427.97 cm<sup>1</sup> . Furthermore, the band at 1039.87 cm<sup>1</sup> shows CdN amide stretching. **Figure 3** depicts an FTIR spectroscopic investigation of BGMA following Ni(II) adsorption. The shifts of peaks at 3695.90–34696.36 cm<sup>1</sup> , 1643.81– 1644.54 cm<sup>1</sup> , and 1041.07–1039.87 cm<sup>1</sup> after Ni(II) adsorption indicate that the amide dNH bonding, CH stretching, carboxylic acid, and hydroxyl groups are the main functional groups involved in the adsorption of Ni(II) metal ions.

#### **4.2 Adsorption isotherms**

**Figure 4** depicts the experimental adsorption behaviour of Ni(II) from its synthetic aqueous solutions onto BGMA, which is particularly important in distinguishing

**Figure 1.**

*Effect of initial metal ion concentration of equilibrium metal uptake and % removal for adsorption of Ni(II) onto BGMA.*

*Removal of Divalent Nickel from Aqueous Solution Using Blue Green Marine Algae… DOI: http://dx.doi.org/10.5772/intechopen.103940*

**Figure 2.** *FTIR spectra of BGMA biomass after adsorption of Ni(II).*

**Figure 3.** *FTIR spectra of BGMA biomass before adsorption of Ni(II).*

the form of the isotherm [32, 64]. According to Giles et al. [65], the isotherm of Ni(II) onto BGMA is detected as an L curve pattern. As a result, it is determined that there is no considerable rivalry between the solvent and the adsorbate for the active sites of BGMA. Also According to Limousin et al., [66], BGMA has a restricted sorption capability for Ni(II) adsorption at the circumstances used in this study.

### *4.2.1 One parameter model*

The experimental data for the adsorption of Ni(II) onto BGMA is fitted to Henry's law (one parameter) model. This model's parameter values and regression coefficient R2 are shown in **Table 2**. The model fails to match the experimental data under equilibrium conditions due to the small R<sup>2</sup> value.

#### **Figure 4.**

*Experimental results of adsorption of Ni(II) onto BGMA.*


#### **Table 2.**

*Parameter values of one parameter (Henry's law) model for adsorption of Ni(II) onto BGMA.*

#### *4.2.2 Two parameter models*

**Table 3** shows the regression coefficients and parameter values of two parameter adsorption isotherm models for Ni(II) adsorption onto BGMA. The R<sup>2</sup> values for the Dubinin-Radushkevich, Hill-de Boer, Fowler-Guggenheim, Halsey, Harkin-Jura, Elovich, and Kiselev models are weak and negative when compared to the experimental data. Though the R2 values of the Temkin and Flory-Huggins models suggest their relevance, the parameter values found in both models (bT and nFH) appear to be too high and negative, which are not physically realisable.

Only four models, namely Freundlich, Henry's law with intercept, Jovanovic, and Langmuir, are considered to carry out the following discussion. **Figure 5** depicts a plot of Ceq (mg/L) vs. qeq (mg/g) for the two models, as well as experience data.

With experimental data, the Freundlich isotherm model performs better. Its R2 score indicates its relevance to a large extent. Adsorption sites are stimulated via the surface exchange process, resulting in enhanced adsorption. Because the value of nF is in the 1–10 range, It means that Ni(II) adsorption from its synthesised solution onto BGMA is favourable. The value of 1/nF is calculated as 0.5608, which is closer to zero, assuring that the active sites of BGMA for Ni(II) adsorption on its surface are more heterogeneous.

Henry's law with intercept model has a high R<sup>2</sup> value, implying its importance. The incorporation of the intercept term considerably improves the linear connection between qeq and Ceq.

Langmuir gives improved agreement (R2 = 0.9432) with experimental adsorption data when followed by the Freundlich and Henry's law with intercept model. It denotes monolayer coverage of the Ni(II) at the BGMA's outer surface. The value of bL is


*Removal of Divalent Nickel from Aqueous Solution Using Blue Green Marine Algae… DOI: http://dx.doi.org/10.5772/intechopen.103940*

**Table 3.**

*Parameter values of two parameter adsorption isotherm models for adsorption of Ni(II) onto BGMA.*

0.05284 mL/g, which quantifies the affinity of Ni(II) and BGMA. The computed value of RL is 0.2534, indicating that the adsorption of Ni(II) onto BGMA is favourable.

However, the qmax of BGMA calculated by this model (55.75 mg/g) differs from the observed qmax value (42.056 mg/g). The variation cannot be significant. The concordance between experimental adsorption data and the Jovanovic isotherm model is quite substantial. It is demonstrated by its R2 value (0.9359) and qmax value (42.04 mg/g).

Because the R2 values of the four models are high and provide strong mathematical agreement with the experimental results, it cannot be stated that the four isotherms or processes are suitable for adsorption of Ni(II) onto BGMA across the whole concentration range studied. **Figure 6** confirms this by comparing the four models to experimental equilibrium metal uptake and demonstrating the amount of concordance.

#### **Figure 5.**

*Comparison of experimental values of equilibrium uptake of Ni(II) with two parameter model values.*

The Freundlich isotherm mechanism clearly indicates maximum satisfaction with the equilibrium experimental data based on the R<sup>2</sup> , SSE, and RMSE values.

#### *4.2.3 Three parameter models*

**Table 4** shows the parameter values for three parameter adsorption isotherm models for the adsorption of Ni(II) onto BGMA. Hill, Redlich-Peterson, Langmuir-

