The private equity firm Platinum Equity is buying the water treatment chemical company Solenis from BASF and the investment firm Clayton, Dubilier & Rice (CD&R) in a deal valued at $5.25 billion. Platinum intends to combine Solenis with a water treatment company it owns, Sigura, to create a firm with $3.5 billion in annual revenues. CD&R bought Solenis from Ashland in 2014. BASF merged its water treatment unit with Solenis in 2019 in exchange for a 49% stake in the new company. Solenis, which has annual sales of about $2.8 billion, specializes in the wastewater and industrial sectors. Sigura is Lonza’s former pool chemical unit, which Platinum bought in 2019.