Biosorbents have been explored as potential adsorbents for wastewater treatment for the past two decades. One of the hindrances hampering their commercial use in wastewater treatment is organic leaching from the biosorbent during sorption of adsorbates. This work reports the characterisation of organic leaching from formaldehyde crosslinked local marine seaweed in the sorption of 20 ppm Cu2+ synthetic wastewater under ammonium interference of 2500 ppm-N. Results revealed good percent removal of copper ions at close to 80% removal even under strong ammonium competitive binding interference. In terms of organic leaching, the total organic carbon (TOC) profile remained at low level of 2 ppm, which demonstrated good structural integrity and strength of the treated seaweed in adsorbing low concentration copper from synthetic wastewater. However, the result must be interpreted with caution as possibility exists of higher organic leaching with sorption of higher concentration of metal ions. Overall, treated seaweed did exhibit small amount of organic leaching of 2 ppm of TOC during sorption of 20 ppm copper ions from synthetic wastewater under strong ammonium competitive binding effect.