In water chemical desalination systems, why are anion exchangers installed after cation exchangers?

The reasons are as follows: (l) After the raw water passes through the cation exchanger, the effluent becomes acidic, which facilitates the exchange reaction in the anion exchanger and results in high efficiency in silicon removal. (2) When raw water enters the anion exchanger directly for exchange, insoluble compounds are formed, which block the cross-linked pores within the exchange resin and reduce its exchange capacity. (3) There is a large amount of carbonate in the raw water; it can be decomposed into H2O and CO2 through an cation exchanger. After CO2 is removed by the decarburizer, the total amount of anions entering the anion exchanger is reduced, thereby extending the operating cycle of the anion exchanger and lowering the consumption of regenerants. (4) Anion exchange resins have a weaker ability to resist contamination by organic substances and other factors compared to cation exchange resins; therefore, they should not be used directly with raw water

1. The exchange capacity of anionic resins is low; at low pH, this facilitates the forward progression of the reaction, so they are placed after cations；

There are many books that provide this explanation; just look for relevant books and read them