Fluoride removal by palm shell waste based powdered activated carbon vs. functionalized carbon with magnesium silicate: Implications for their application in water treatment by Choong, Choe Earn, Wong, Kien Tiek, Jang, Seok Byum, Nah, In Wook, Choi, Jaeyoung, Ibrahim, Shaliza, Yoon, Yeomin, Jang, Min