Company: Pall Corp

Stocks ended lower in thin trading Monday, with the S&P and the Nasdaq posting a loss for the month of April, following a handful of mixed economic reports in addition to news that Spain slipped into a recession.

Filtration equipment manufacturer Pall Corp. has agreed to sell certain operations and equipment used in blood transfusions to health care company Haemonetics Corp. for about $550 million.

Clean tech has become a major destination for investors, from venture capital firms to ordinary shareholders. Click ahead to see 10 select, large ETFs with a strong focus on clean-tech holdings.