Question

A sediment sample taken from a lignite strip-mine pit contains highly alkaline (pH 10) water. Cations were displaced from the sediment by treatment with HCl. A total analysis of cations in the leachate yielded, on the basis of millimoles per 100 g of dry sediment: 160 mmol Na⁺, 10 mmol K⁺, 30 mmol Mg²⁺, and 70 mmol Ca²⁺. What is the cation exchange capacity of the sediment in milliequivalents per 100 g of dry sediment? Why does H⁺ not have to be considered in this case?

Answer 1

100 g of dry sediment contains

                       160 millimoles of Na⁺ = 160 /1 = 160 meq of Na⁺ [ since millimoles/valency = milliequivalents]

                       10 millimoles of K⁺   = 10 /1 =   10 meq of K⁺

                        30 millimoles of Mg2+ = 30 /2 = 15 meq of Mg²⁺

                        70 millimoles of Ca²⁺ = 70/2 = 35 meq of Ca²⁺

                                                                 ---------------------

                                               Total            220 meq of exchangeable cations/100g of dry sediment

Thus the answer CEC = 220 meq/100 g of dry sediment

since the sediment is highly alkaline H⁺ not have to be considered in this case.