The utility of green crab shell waste in the chitin form as a biosorbent material for metal removal from waters and drinking waters
Green crab is a recent evasive species that has entered the eastern Atlantic waters. It has been reported that the green crab’s voracious appetite has ruined habitats of important commercially viable crab species, lobster and fish, by destroying the ocean floor and their important spawning areas. Green crab are about one tenth the size or less of a snow crab, and in of themselves, have little economic food potential; though some have tried to make a fish-stock base for flavouring soups and other entrees from them. However, that is not to say that the green crab does not have its use. Perhaps the most important aspect of the green crab is that its shell contains a readily available source of a potentially useful chemical compound called chitin. The chemical Chitin, (Figure 1), which is a naturally occurring polysaccharide, is commonly found in the shells of shrimp and crab, including green crab. Chitin can be extracted fairly easily from the crab shells by a succession of boiling under acidic conditions, separation and grinding. One important aspect of chitin is that the material contains acetamido chemical groups which can bind to heavy metals. This research area attempts to explore the utility of extracted chitin from green crab shells in order to use it as a potential biosorbent for the removal of heavy metals from waste waters. Such examples of it use, include the reclaiming of mine tailing water waste that may contains difficult to extract or environmentally poisonous metal loads, or use in factory effluent or public waste water systems where metal concentrations are of concern. In order to determine the viability of chitin as a biosorbent for this purpose, a series of tubes containing the extracted chitin were prepared. A series of metals to include: silver (Ag+), chromium (Cr3+), copper (Cu2+) and zinc (Zn2+) of known metal concentrations were added to the extracted chitin biosorbent tubes. Chitin’s extraction performance (the ability to extract particular metals) were compared to that of identical tubes made with a commercially available chitin obtained from a chemical manufacturer (Aldrich Chemicals, analytical standard) and a practical grade chitosan (Aldrich Chemicals, analytical standard) in order to assess the amount of metal uptake.
Comparison trials were carried out over an acidic to basic range (pH 2-9) to find optimal pH binding conditions for each of the metals to the three different biosorbents. The analysis technique Flame Atomic Absorption Spectroscopy (FAAS) was used to determine the metal amount left in the water samples after the doped metal water samples had passed through the tubes. Simple subtraction from the original amount of metal concentration added to the tube allowed one to determine the amount of metal held by the different biomaterials. It was found that for all biosorbents Cr3+ and Zn2+ adsorbed most efficiently at a neutral pH of 7. Cu2+ in chitosan had relatively even adsorption between pH 7 and 8 while neutral water (pH 7-8) performed the best for commercial chitin. Ag+ adsorbed efficiently at virtually all pH levels with chitin, pH 3 and 8 for green crab, and at pH 7 with chitosan.
Another technique infrared spectroscopy (FTIR) was used to determine the purity of extracted chitin from the green crab and to determine to which functional groups on the chitin the metals were binding.
Work is continuing to ascertain the maximum binding of particular metals at pH =7 and to determine concentration plots and maximum saturation, based on single phase model. Further work will reveal if the utility of extracted chitin will hold potential, for a cheap, accessible, and “green” chitin metal removing biosorbent. A naturally occurring substance, like chitin, might be an economic way for a business to pay for removal of green crab or in the use of a by-catch through the usage of its shell waste. Initial studies are showing promise. The project, under investigation of different aspects of chitin, is ongoing. Previous students that have contributed are: Johnathan Grandy (2012), Denys Ploughman (2013), and Jessica Norris (2014). We are currently looking for funding and industrial partnerships.