Recent investigations have revealed the harmful health effects of elevated calcium levels in drinking water. Chemically activated biochar is the most popular option for low-cost cation adsorption. However, most of these chemicals are hardly available for rural communities, corrosive, and difficult to handle by unskilled personnel. This study aimed to determine the ability of natural coconut vinegar, a common mild acid, to activate Trema orientalis wood biochar pyrolyzed at 300ºC (BC) as a facile, safe, and low-cost approach for aqueous calcium ion (Ca 2+ ) removal. Column tests showed a two-fold increase of Ca 2+ adsorption capacity and a ten-fold increase of Ca 2+ retaining capacity of BC after activation with vinegar. The isotherm results were well correlated with the Langmuir model. The maximum Langmuir adsorption of the activated biochar (BC-A) was 9.96 mg/g. Ca 2+ amount was determined using flame photometry. EDX analysis showed that the O/C ratio of BC increased from 0.07 to 0.13 after activation. FTIR and wettability studies showed increased oxygenated functional groups on the BC-A surface. The authors suggest a possible acid-catalyzed hydration of the C-O-C bridges of the biochar, introducing new hydroxyl/carbonyl/ester/carboxylic/lactone groups to the biochar surface due to the vinegar activation, enhancing Ca 2+ adsorption through chemisorption.