Please also let me know whether such a topic will fall in HRM category if environmental issues are associated with the topic I choose?
Yes! This is a good way to begin a very focused research project. When anyone takes 2 important concepts and puts them together, it creates a unique outcome.
HRM + environmental responsibility = a unique dissertation
What is the relationship? HRM is about employee relations. Employees are motivated when they know they work for a responsible organization. Would you like to do some case studies on organizations where environmental responsibility was a theme used by HRM to improve employee relations?
OR you can look for organizations committed to environmental responsibility and interview HRM people to determine their roles, best practices, problems they ran into, etc.
Are you interested in any particular industry?