Private-equity firms are again betting on China's environmental sector as the government adopts a new sense of urgency in cleaning up the nation's filthy air and water.After a decline in 2012, the firms' investments in environment-related businesses in China last year totaled $1.2 billion, a record-high 7.1% of all private-equity investment in the country, according to the Hong Kong-based Centre for Asia Private Equity Research.