Fabrication of surface-engineered superparamagnetic nanocomposites (Co/Fe/Mn) with biochar from groundnut waste residues for the elimination of copper and lead metal ions Dhanya Vishnu, Balaji Dhandapani*, Shankar Ram Ramakrishnan, Prasanna Kumar Pandian, Thiruvikraman Raguraman, 10.1007/s40097-020-00360-y