but the 1st topic is actually harder to write. Coagulation would involve discussing in detail the multiple organic chemical reaction steps and I would probably have to cut my paper down to fit 20 pages. Heavy metal toxicity is a much briefer topic, but I agree its more novel.
Kind of along my point. Second topic allows you to focus on one relatively narrow topic and delve in depth. First one's going to require a lot broader strokes (or just very selective focus) to fit the topic in 20 pages; you'll end up somewhere between an encyclopedia article and an actual research paper.