**Acknowledgements**

Higher *Kf* value for GACA indicates a higher adsorption capacity for methylene blue

**Langmuir isotherm model Freundlich isotherm model**

 57.47 0.2107 0.9957 17.7174 2.862 0.9819 82.64 0.0588 0.9745 7.66655 1.725 0.9635 78.74 0.0968 0.9825 10.7226 1.873 0.9691 68.03 0.1786 0.9851 15.01067 2.293 0.9097 67.11 0.2569 0.9925 18.02603 2.455 0.8879

**/mg)** *R***<sup>2</sup>** *Kf* **(mg/g) (dm3**

**/mg)** *N R***<sup>2</sup>**

The main goal of desorption studies is the competitiveness of adsorbents reusability in the multiple adsorption or desorption cycles and their beneficial potential in practical and economical applications. Desorption studies were performed with 0.01 M, 0.05 M and 0.1 M HCl. Typically, 1 g of PMC saturated with 100 mg/L of MB was placed in different desorption solutions and constantly stirred in a water bath at 200 rpm for 2 h. The adsorbent solutions were centrifuged and analysed using UV-VIS spectrophotometer. **Figure 9(a)** demonstrates the effect of eluent concentrations on MB dye desorption efficiency. It was observed that desorption efficiency increased with increase in the eluent concentration even though the shift is small in percentage. The maximum desorption percentage was found at 0.1 M HCl (99.8%) whereby 0.01 M HCl showed the minimum desorption efficiency (98.8%). An increase in HCl concentration resulted in an increase in H<sup>+</sup> ions concentration which led to a subsequent increase in dye desorption efficiency.

Regeneration shows the competitiveness of the adsorbent where it expresses the good reusability and recycling abilities. **Figure 9(b)** demonstrates the possibility of regeneration and reusability of the grafted pine magnetite composite with acrylamide. Adsorption-desorption reaction cycles were repeated 4 times using 0.1 M HCl

*(a) Desorption characteristics of MB adsorbed and (b) regeneration characteristics of MB adsorbed.*

and a value of *n* > 1 indicates favorable adsorption conditions [27, 28].

**3.10 Desorption and regeneration**

**Table 2.**

**Figure 9.**

**168**

**Temperature (K)** *Qm* **(mg/m)** *KL* **(dm3**

*Waste in Textile and Leather Sectors*

*Isotherm parameters for methylene blue dye adsorption on GACA.*

We are grateful to Prof E.N. Nxumalo and Mr S.A. Zikalala for their contributions on the study. Vaal University of Technology and National Research Foundation of South Africa are thanked for financial support.
