There are three groups of thought on this.
Maximum and maximal are synonyms when they function as adjectives.
Maximum is a noun only. Maximal is the adjectival form of maximum.
From wordwizard.com - View topic - Optimum / optimal
... it seems that ‘maximal,’ has come to be used as ‘the greatest possible under the circumstances’ and may or may not refer to an absolute maximum – “For maximal results, you need calcium and exercise.” However, the ‘maximum’ results humanly possible under controlled laboratory conditions may be far higher than any ‘maximal’ results ‘little old us’ might ever achieve.
Choose the one you like.