A lot vs. Alot: 9 Grammatical Pitfalls
1590s, "act of acknowledging," from acknowledge + -ment. "An early instance of -ment added to an orig. Eng. vb." [OED]. Meaning "token of due recognition" is recorded from 1610s.
alternative form of acknowledgement. OED deems it "a spelling more in accordance with Eng. values of letters."