email sending issue

Please check the documentation about the Email class.
You will find that the to() method will only accept a string or an array.
The string can contain just one single e-mail address, or a comma delimited list of addresses.
The array must contain e-mail addresses as well.

The examples in the documentation page make clear how it works:

PHP Code:

$this->email->to('[email protected]');
$this->email->to('[email protected], [email protected], [email protected]');
$this->email->to(
       array('[email protected]', '[email protected]', '[email protected]')
);