Transport number of Cd2+ in CdI2 appears abnormal at higher concentration due to the presence of a secondary process called the iodine migration effect.

Explanation:
The transport number is a measure of the efficiency with which an ion carries current in an electrolyte. It is defined as the fraction of the total current carried by a particular ion in a solution. In the case of Cd2+ in CdI2, the transport number of Cd2+ appears abnormal at higher concentrations due to the presence of the iodine migration effect.

Iodine Migration Effect:
The iodine migration effect is a secondary process that occurs in concentrated solutions of CdI2. It involves the migration of iodine ions (I-) towards the anode, creating an additional current that is not carried by Cd2+ ions. This effect is more pronounced at higher concentrations of CdI2.

Reasons for Abnormal Transport Number:
1. Higher Concentration: The abnormal transport number of Cd2+ in CdI2 is observed at higher concentrations. This is because the iodine migration effect becomes more significant as the concentration of CdI2 increases.

2. Migration of Iodine Ions: In concentrated solutions of CdI2, the iodine ions (I-) migrate towards the anode due to their higher mobility compared to Cd2+ ions. This migration creates an additional current that is not carried by Cd2+ ions, leading to an abnormal transport number.

3. Competing Ion Movements: The migration of iodine ions towards the anode creates a counterflow of Cd2+ ions towards the cathode. This counterflow reduces the transport number of Cd2+ as some of the Cd2+ ions are displaced by the migrating iodine ions.

4. Concentration Polarization: The migration of iodine ions creates concentration gradients near the electrodes, leading to concentration polarization. This polarization affects the transport of Cd2+ ions, further contributing to the abnormal transport number.

Conclusion:
The abnormal transport number of Cd2+ in CdI2 at higher concentrations is primarily due to the presence of the iodine migration effect. This effect involves the migration of iodine ions towards the anode, creating an additional current that is not carried by Cd2+ ions. The higher concentration of CdI2 and the competing ion movements contribute to the abnormal transport number.