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(a) Significant contributions to science and/ or technology development by the nominee
based on the work done in India during most part of last 5 years:
Metal Speciation is a multidisciplinary study of the occurrence, mobility and detection of metal species within different environments as well as their interaction with life. Dr. Chakraborty initiated metal speciation study (after establishing the FIRST metal-speciation laboratory in India (at CSIR-National Institute of Oceanography) in 2008) to understand the role of metal-natural ligands interactions in controlling distribution, fate, mobility, and bioavailability of trace/heavy metals in marine environments (estuarine, coastal and open ocean). His research succeeded in revealing one of the closely guarded secrets of nature: how the metal-buffering action of natural ligands (Fe/Mn-oxyhydroxide, sedimentary organic matter, dissolved organic carbon, humic substances etc.) makes them a vital web in the complex fabric of homeostasis (1-5). He discovered that metal (trace/heavy) loading in sediments (6-8) trace metal competitions (4, 6) , major cation‘s screening (9, 10), , quantity and quality of sedimentary natural ligands (2,11,12) play key roles in controlling the concentration of labile metal-complexes and their dissociation rate constants (a good indicator of metal‘s bioavailability) in marine sediment system (13-16). Ligand Field Stabilization Energy, water exchange rate, Jahn-Teller distortion of transition metals has been used by Dr Chakraborty as useful tools to understand metal-natural ligands interactions in estuarine and coastal sediments. Results of his research offer a new way to explain metal–natural ligands interactions in marine environments (17, 18). His research provides a better description on the patterns of trace/heavy metals distribution and the processes that control metal speciation in the continental shelf sediments around India (19, 20). An important research achievement of Dr Chakraborty has been to advance the science of metal speciation by identifying the analytical timescale of measurement as the critical parameter for defining the chemical species in natural environments (3, 21, 22). Dr Chakraborty‘s research in perennial oxygen minimum zone shows that varying redox condition of the overlying water column can influence the stability and lability of metal-complexes in sediment (23-25). He proved that sediment-water exchange fluxes at low oxygen environment may alter bioavailability of some heavy metals in water column which may have tremendous impact on biological species in the overlying water column. Dr Chakraborty has also been investigating the speciation and fate of different metal complexes in hydrothermally altered deep sea sediments and polymetallic nodules from the Central Indian Ocean Basin (26). His aim is to assess the probable impacts of deep sea mining on speciation and bioavailability of metal complexes in the Central Indian Basin and their impact on deep sea environments. Dr Chakraborty‘s research showed that low thermodynamic stability (with fast dissociation rate constants) and high concentration of labile metal complexes are responsible for their high bioaccumulation in sessile organisms (15, 16). Dr. Chakraborty succeeded in finding out a mechanistic linkage between trace/heavy metal speciation, bioavailability in marine system (13-16). In view of the Arsenic/mercury posing health problems in our country, Dr. Chakraborty also developed simple, reliable and inexpensive techniques for arsenic and mercury speciation in marine sediments/water systems (27, 28)).
(b) Impact of the contributions in the field concerned:
Dr Chakraborty has made unique and outstanding contributions to the field of Environmental Geochemistry which has facilitated the understanding of the metals-natural ligands interaction in marine environment. His study always helps us to know different geochemical processes in the Indian Ocean region. The major contributions made by Dr Chakraborty in the field of Environmental Geochemistry (in the field of trace and heavy metal speciation in marine systems) are described below: 1. Since the marine systems are dynamic in nature and never at chemical equilibrium, Dr Chakraborty realized that chemical-equilibrium-based approach to do metal speciation would give wrong estimates of labile metal concentrations (which are widely correlated with ecotoxicity) in marine sediments. Dr Chakraborty developed kinetics-based approaches to understand metals speciation and their bioavailability in estuarine/coastal and deep sea systems. Dr Chakraborty‘s research focusses on developing and applying a wide range of kinetic methods for chemical speciation in marine/estuarine sediment systems, each with its own characteristic timescale of measurement. An important research achievement of Dr Chakraborty has been to advance the science of metal speciation by identifying the analytical timescale of measurement as the critical parameter for defining the chemical species in coastal and marine sediment system. The ability of the speciation techniques (multi-method approach) to provide a chemically significant description of the kinetics of metal-natural ligands interaction in sediment suggests that the metal and ligand exchange reactions precede mainly by the disjunctive mechanism (complete dissociation of ML), a fundamental process in coastal marine systems. Dr Chakraborty showed that kinetics-based approach provides the actual metal speciation instead of the current practice of chemical equilibrium speciation. This kinetic based is expected to promote wider acceptance of the approach by regulatory agencies.
2. Dr Chakraborty has demonstrated the effects of Ligand Field Stabilization Energy, water exchange rate, Jahn Teller distortion of transition metals on their speciation and bioavailability in estuarine and coastal sediments. This is for the first time that these effects have been described in sediments. This research is directly relevant for researchers in geochemistry as it
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will help provide a better understanding of metal–natural ligand interactions in marine environments. 3. By realizing the global catastrophic risk in near future, many developing countries have already signed the treaty to reduce Hg use and its release to control environmental Hg pollution. However, how to reduce the impact of climate change on Hg pollution is not truly known. Climate change is anticipated to increase Earth's average temperature and influence overall patterns and amounts of precipitation. Climate change has been proclaimed to increase Hg pollution even if anthropogenic Hg emission remains constant. Therefore, increasing anthropogenic Hg release with climate change is expected to intensify the detrimental effects of Hg in the developing countries. Dr Chakraborty took the challenge to understand Hg-natural ligands interaction with an aim to reduce Hg pollution in marine systems. He showed that sedimentary Hg concentrations around India are low and not alarming.
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