Adsorptive treatment of landfill leachate using activated carbon modified with three different methods by Erabee, Iqbal Khalaf, Ahsan, Amimul, Jose, Bipin, A. Aziz, M. Manniruzzaman, Ng, A. W. M., Idrus, Syazwani, Nik Daud, Nik Norsyahariati