Anion and Molecular Retention

The anions of concern to agriculture include Cl-, HCOJ, NOJ, SO, HPO -, H2PO4, OH-, and F-. In addition, some micronutrients (HjBOij-, MoO -, and HAsO -) and heavy metals (CrO -) exist as anions in soils, as do some pesticides, such as the dissociated phenoxyacetic acids (2,4,5-T and 2,4-D). Molecular species of interest include NH3, undissociated weak acids such as H3BO3 and H4Si04, and the undissociated forms of many pesticides (DDT, 2,4,5-T, ant 2,4-D.) The study of anionic and molecular retention by soils has been the subject of increasing research in recent years. 

The approach in this chapter is to describe various retention mechanisms and to cite examples of their involvement in the retention of specific anionic and molecular species. Several of the mechanisms are general and apply to many of the species listed above. Adsorption isotherms are also discussed because of their widespread use to describe anion and molecular retention by soils. 

Anion and molecular retention Acid and salt-affected soils... 

Certain mechanisms are active in the retention of all species (cationic, anionic, and molecular), while other mechanisms are active in the retention of only certain species. Explain, giving examples of species retained predominantly by each specific mechanism. 

Molecular retention nowadays mostly refers to the widespread interest in the retention of organic pesticide molecules by soils. These molecules are mostly electrically neutral overall but have functional groups (PO -, Cl-, NOj, etc.) that interact with organic and inorganic soil solids. Positively and negatively charged molecules behave somewhat like simple cations or anions. 

Onabe [84] analyzed the drainage behavior of pulp suspensions with a two-component system, with attention to polyion complex formation. With cationic and anionic polyacrylamides (molecular weights 3,000,000 and 1,000,000 g/mol) added individually, a polyion complex with an irregular three-dimensional stmcmre was precipitated on the fiber surface. This reduced the homogeneity of the paper sheets. In contrast to this, a pre-mixed system of an anionic and cationic polyacrylamide [83] resulted in an improvement in pulp retention and paper quality (paper strength). Because of the very high molar mass and thus the big coil diameter, the formed PECs are very big and are therefore good flocculants. 

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