**4. Regeneration data**

The recycle of adsorbents is a very important and crucial factor to propose an efficient adsorbent. A good adsorbent supposes to have higher removal capacity as well as regeneration efficiency that will reduce the total cost of the wasted adsorbent [73].

Different methods of regeneration were reported in the literature, including two ways, washing of the used samples with different solutions such ethanol, acidic or basic ones to remove the adsorbed dyes as they are [73, 74], or to destroy the adsorbed dyes by thermal treatment at certain temperatures [75], this process will add additional costs to the process due the extra energy consumption. Another method was proposed by other researchers to destroy the adsorbed dyes on the surface of the solids via sulphate radical oxidation. This method was reported to be friendly to the environment since the solution could be used for many regeneration tests [26].

After the step of the retention of the anionic dye, the regeneration and consumption of the waste adsorbents indicates the valuable and the feasibility of their application. The **Figure 9**, show the adsorption efficiency is decreased, between 90% and 85%, regarding the byproduct of a phosphate washing sample for at least four try. Also, the other two samples: Natural Phosphate and phosphogypsum, reveals their retention percentages diminish up to 70% after three regeneration cycles. Overall, the retention efficiency is maintained at 60% for the seventh regeneration cycle.

The study of the adsorption isotherms can be used to study the removal systems. The design objective was to minimize the solid adsorbent for a specific volume of initial concentration.

The study of the adsorption isotherms can be used to study the removal systems. The aim of this study was to reduce the solid adsorbent for a specific volume of initial concentration.

**Figure 9.**

*The percentage removal of AR88 after different regeneration cycles, (a) natural phosphate, (b) phosphogypsum, and (c) phosphate waste rock.*

Consider an effluent containing V (L) of the solution in contact with the colorant and let the dye concentration got reduced from C0 to C1 mg dye L�<sup>1</sup> solution. For a quantity of adsorbent m (g), the solute loading changed from q0 to q1 (mg dye per g adsorbent). When fresh adsorbent is used, q0 = 0, the mass balance for the methylene blue (MB) dye in the single-stage operation under equilibrium is presented in Eq. (11).

$$\mathbf{V(Co - Ce) = m(q\_o \ q\_e) = mqe} \tag{11}$$

In the present case, the removal of AR 88 corresponded well with the Langmuir isotherm. Consequently, the Langmuir equation can be substituted in the Eq. (5), and the rearranged form is given in Eq. (12).

$$\frac{m}{V} = \frac{\mathbf{C}\_o - \mathbf{C}\_e}{q\_e} = \frac{\mathbf{C}\_o - \mathbf{C}\_e}{\frac{q\_m K\_L \mathbf{C}\_e}{1 + K\_L \mathbf{C}\_e}} \tag{12}$$

**Figure 10** represent the plots derived from Eq. (12) to predict the amount of phosphogypsum and PWR required (g) to treat different effluent volumes of the

#### **Figure 10.**

*Predicted mass (m) of PG waste to treat different volumes (V) of AR-88 solutions, at initial concnetration of 100 mg/L.*

*Elimination of Acid Red 88 by Waste Product from the Phosphate Industry: Batch Design… DOI: http://dx.doi.org/10.5772/intechopen.99954*

initial concentration of 100 mg L<sup>1</sup> for 60%, 70%, 80%, and 90% MB removal at different MB solution volumes from 1 to 12 L in 1 L increment. For a single design, the amount of phosphate wastes can be predicted in the range of 11 to 78.78 g for phosphogypsum and 4 to 24.97 g for phosphate waste rock materials. That is to say that the amount required for 90% removal of MB solution of the initial concentration of 100 mg L<sup>1</sup> , was about 78.78 g of PG, and 24.97 g of PWR solids, respectively. The lower mass's values for phosphate waste rock solid were associated to their higher efficiency to remove acid dyes compared to phosphogypsum waste. These data indicated that these materials could be useful as removal agents for anionic dye.
