You can use a nasty trick to do the job using hex codes.
For example in default UTF-8 ascii you can print
\x27 which is the hex code of single quotes.
echo -e "\x27hello\x27"
The copyright symbol is not part of default ascii table but belongs on the extended ascii codes. You can also print extended ascii as a combination of hex codes
(as advised by http://www.fileformat.info/info/unicode/char/a9/index.htm)
echo -e '\xc2\xa9'
Similarilly , you can use those hex codes in sed
sed 's/\xc2\xa9/??/' <<<"$a" ##replacing copyright symbol with two questionmarks
If \xc2\xa9 which stands for copyright symbol are not working, you can try \xe2\x92\xb8 which is the "CIRCLED LATIN CAPITAL LETTER C".
\o251 (use a lowercase letter 'o', not a zero).