SELECT s.name
FROM sys.schemas s
WHERE s.principal_id = USER_ID('TestUser')

ALTER AUTHORIZATION ON SCHEMA::db_owner TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_accessadmin TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_securityadmin TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_ddladmin TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_backupoperator TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_datareader TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_datawriter TO dbo; 
ALTER AUTHORIZATION ON SCHEMA::db_denydatareader TO dbo;
ALTER AUTHORIZATION ON SCHEMA::db_denydatawriter TO dbo;