**5. Exchange and adsorption reactions**

Cation exchange and adsorption reactions involving cations and their hydrolytic products are dominant soil processes. Aide and Aide [9] reviewed REE reactions in the soil environment, including REE adsorption. Numerous studies cited in this review produced similar REE adsorption conclusions, including: (i) cation exchange reactions are largely associated with basal planar surfaces and pH-dependent silanol and aluminol reactions at edge positions, (ii) predominance of outer-sphere complexes occur at pH levels less than 4 and an increasing degree of inner sphere complexes at pH levels greater than 5, (iii) cation exchange was consistent with one electrostatic and non-specific site and one specific complexation site involving edge aluminol groups, (iv) REE affinity was reduced by increases in the ionic strength, (v) REE complexation affinity was greater at higher pH intervals. Conversely Tertre et al. [31] demonstrated the inner-sphere nature of aluminol sites on kaolinite and montmorillonite. Tang and Johannesson [32] noting that REE adsorption was more pronounced at greater pH intervals. At lower pH intervals, adsorption was attributed to REE3+ species whereas at greater pH intervals adsorption was attributed to REE3+ and REE-carbonate species. The adsorption constants increased regularly with an increase in REE atomic number.
