Let A and B be two connected graded commutative k-algebras of finite type, where k is a perfect field of positive characteristic p. We prove that the quasi--shuffle algebras generated by A and B are isomorphic as Hopf algebras if and only if A and B are isomorphic as graded k-vector spaces equipped with a Frobenius (pth-power) map. For the hardest part of this analysis, we work with the dual construction, and are led to study connected graded cocommutative Hopf algebras H with two additional properties: H is free as an associative algebra, and the projection onto the indecomposables is split as a morphism of graded k-vector spaces equipped with a Verschiebung map. Building on work on non-commutative Witt vectors by Goerss, Lannes, and Morel, we classify such free, `split' Hopf algebras. A topological consequence is that, if X is a based path connected space, then the Hopf algebra H^*(Omega Sigma X;k) is determined by the stable homotopy type of X. We also discuss the much easier analogous characteristic 0 results, and give a characterization of when our quasi--shuffle algebras are polynomial, generalizing the so-called Ditters conjecture.