Complex formation is the reaction of a metal ion and ligands through electron pair sharing and the resulting product is called the metal coordination compound. The metal ion is the electron-pair acceptor, and the ligand is the electron-pair donor. The metal ion serves as the central ion, and the organic ions are coordinated around it in a first coordination sphere.
Covelo et al., 2008 studied the heavy metal sorption and desorption behavior of a Fibric Histosol with that of its organo mineral fraction (OMF) and reported that greatest sorption capacity was for lead and copper. Urrutia et al., 2013 studied the physico-chemical characteristics of diverse types of humic-metal-phosphate acid complexes and reported that molecular size distribution of humic-metal-phosphate as a function of pH is similar to that of potassium or sodium humates and metal-humate complexes.
Eshwar et al., 2017 studied the stability constant of Fe(II), Cu(II), Mn(II) and Zn(II) metal ion complexes with HA/FA at pH 7 and at 30 ⁰C and reported that for humic acid the log k values was highest for Cu (II) and in case of fulvic acid the log k values was highest for Mn (II). Mukhopadhyay and Sanyal, 2004 studied the release potential of arsenic (As) from arsenic-humic/fulvic complexes by soluble phosphate nitrate salt and examined it in terms of the appropriate exchange isotherms and reported that phosphate had a greater degree of exchangeability with arsenic than did nitrate, at both the lower and higher concentrations and also reported that stability constant of synthetic HA/FA were greater than that of natural HA/FA.
Sinha and Bhattacharyya, 2011 studied the release isotherm of As from the HA/FA complexes extracted from vermicompost and FYM in presence of molybdate, nitrate, phosphate, sulfate and borate and reported that the greatest tendency to displace As from the complexes was shown by sulphate, molybdate, and nitrate. Datta et al., 2001 studied the stability constant of complexes formed by the humic acid with Cd2+ ions in aqueous phase and reported that humic phenolic-OH group was involved in the formation of Cd2+ -humic complex
Shahid et al., 2012 studied the effects of different organic ligands like ethylene diamine tetra-acetic acid (EDTA), low molecular weight organic acids, humic substances on lead speciation and reported that EDTA forms soluble, stable, and phytoavailable Pb-chelates due to high binding lead affinity. He et al., 2016 studied the binding behavior of Cu+2 to all ten HA fraction and reported that the FTIR and SEM-EDX image of HA fraction revealed that carboxyl and phenolic groups were responsible for the Cu2+ sorption on the ten sequentially extracted HA fractions process.
Higher molecular weight humic acids bind heavy metals more strongly than low molecular weight acids, and the study of soil organic fractions that can sorb heavy metals moderating its toxicity in the soil/crop system and preventing its entry in the food chain should be explored in future.