**7.4 Comparative study of the selectivity of heavy metals adsorption on raw chitin in relation to other adsorbent cited in the literatures**

From the previous results, we can classify the retention capacity of heavy metals by raw chitin according to the following sequences:

> Ccre Pb > Cu > Cd > Zn Ccra Pb > Zn > Cu > Cd Clan Cu > Cd > Zn > Pb

These results are in agreement with some studies, indeed Haug et al. [67] showed that the addition of metal ions to a solution of Na alginate prepared from *Laminaria digitata* leads to the order of the next affinity: Pb > Cu > Cd > Zn. Nicolas [21] found the affinities of the following heavy metals:

> Zn > Cu = Ni > Cd (pyrophosphates +61 mg/g polysaccharides) Cu = Ni > Cd > Zn (pyrophosphates +222 mg/g of proteins)

Melchor [38, 39] showed that pure chitin crabs exhibits affinity vis-à-vis heavy metals in the following order: Cu > Cd > Pb.

In general, the chitin selectivity sequence depends, on the one hand, on the origin of this chitin (algae, fungi, crustaceans, etc.) and on the other hand, on the nature of the ions, in particular the 3d orbitals. It does not depend in any case on their Cu 0.72 Å, Pb 1.2 Å, Cd 0.97 Å, and Zn 0.74 Å sizes. However, the adsorption selectivity of metal ions on mineral surfaces varies greatly from one medium to another. We give the following sequences as an example: Pb > Cu > Zn = Cd and Pb > Cd > Zn > Cu respectively for montmorillonite and Kaolinite [68].

## **8. Test of metallic ion adsorption in series**

In order to study the effect of the number of treatments on the percentages of elimination of metal ions and to solve the problem of competitive adsorption, we proceeded to a series treatment starting with an initial concentration of 100 mg/l from M2+. After each adsorption test, the measured filtrate is again adsorbed on the material. This operation is repeated four times for the four metals. In parallel, we do a simple treatment using the same quantity used previously. The results found will be compared to FAO standards.

The results of the monitoring of the evolution of the metal ion contents during the series treatment and after the simple treatment of the synthetic rejection are shown in **Table 10**.

This table shows that the equilibrium concentration decreases after each adsorption test. Thus, we go from 100 to 0.032 mg/l after the first test and from 0.032 to only 0.002 mg/l after second test for Pb. This value is 25 times lower than the FAO standard of 0.050 mg/l; it is a remarkable reduction of the Pb ion.

*Trace Metals in the Environment - New Approaches and Recent Advances*

effluents rich in heavy metals.

Lead: Ccre > Ccra > Clan Cadmium: Ccre > Clan > Ccra Copper: Clan > Ccre > Ccra Zinc: Ccra > Clan > Ccre

heavy metals.

**Table 9.**

following order:

**7.3 Interpretation and comparison of results**

*Values of the maximum adsorption capacity K (mg/g) of heavy metals.*

it follows that the CAP can be substituted by the raw chitin for the treatment of the

CAP 58.69 9.01 4.35 3.38 Ccre 9266.86 34.81 42.25 10.82 Ccra 265.07 7.38 17.02 23,74 Clan 3.59 18.92 204.67 11.49

**Pb2+ Cd2+ Cu2+ Zn2+**

According to this study, we can draw strong conclusions about the existence an affinity between biosorbent materials based on shells of crustaceans and some

Raw shrimp chitin exhibits a strong affinity for Pb. The adsorption capacity of zinc on the raw chitin of crabs is twice as great as that on the raw chitin of shrimp. The selectivity of the metal for each of these materials was defined by the plot of the adsorption isotherms. The metal ions are retained by these materials in the

The kinetic study has shown that the adsorption process is relatively fast compared to the adsorption on the supports described in the bibliography (of mineral origin). More than 50% of these ions are adsorbed before equilibrium is reached (20 minutes). The adsorption kinetics has also shown that the mineral part of the raw chitin is partly responsible for the retention of heavy metals. The hardness of the shells has a negative effect on the kinetics of the adsorption process, given the time required for equilibrium. Indeed, the adsorption of Pb on the raw chitin shrimp requires only 30 minutes;

Studies cited in the literature have shown that to obtain a given abatement, it is necessary to introduce a sufficient quantity of the material. Thus, our treatment tests have shown that to achieve a removal rate of 99% copper contained in

while the raw chitin lobsters the equilibrium time is 60 minutes.

*Evolution of the average percentages of elimination according to the supports adsorbents.*

**252**

**Figure 12.**

#### *Trace Metals in the Environment - New Approaches and Recent Advances*


#### **Table 10.**

*Values of equilibrium concentration in mg/l of metal ions after single treatment and after each test during serial treatment.*


#### **Table 11.**

*Percentages of reduction of metals according to the number of tests.*

In the case of Cd after the fourth test, the Cd ion content is below the FAO standard. In the case of Cu, the concentration ranges from 100 to 0.780 mg/l after third test and from 0.780 to 0.151 mg/l, after the fourth test, the equilibrium concentration is therefore lower than the FAO standard for Cu2+.

Finally, in the case of Zn2+, only two adsorption tests are required to reach the FAO standard of 5 mg/l, which is relatively higher than the standards for other metals given the biological role played by Zn2+ in living organisms.

After a simple treatment, using the same quantity of the adsorbent used in the series treatment, it is noted that the quantities of metal ions remaining at equilibrium are greater than those remaining after the fourth serial treatment test; this shows that serial processing is more effective than simple treatment.

In **Table 11**, we calculated the percentage of abatement after the first test, the fourth trial, and simple treatment.

According to this table, the series treatment is very effective to reduce the amount of metal ions and thus increase the reduction efficiency. For example, for Zn, we go from a percentage of 68.52% after the first test and 99.27% after fourth adsorption test.
