Excretion of any anion, causing net negative charge of urine, can pull H+ along with it. The anion acts as a base for excretion of acid without having to be in protonated state. The primary anion contributing negative charge in urine is chloride, charge from chloride is more than an order of magnitude more than from any other anion. Therefore, electrostatically, the primary way to excrete acid is with chloride, by retaining cations other than H+ (in the kidney using Na/H exchange), forcing the excretion of H+ along that electrical gradient. Chemically this is analogous to hydrochloric acid being main way to excrete acid, just like how HCl is used to excrete acid into gaster.
