Adsorption of copper by crab shell biochar

Hopkins, David T. (2021) Adsorption of copper by crab shell biochar. Masters thesis, Memorial University of Newfoundland.

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Abstract

In Atlantic Canada, fisheries and seafood processing represent major industries and contribute heavily to the regional economy. By-products from seafood processing, such as shells and bones, make up a significant waste stream from this industry and are currently difficult to valorize. Pyrolysis, a thermochemical process involving biomass in the absence of oxygen, is a simple and effective means of valorizing biomass, producing three product streams: a biogas, a condensable bio-oil, and a solid residue called biochar. In this thesis, the properties of biochar produced from the snow crab (Chionoecetes Opilio) shell are studied, along with its capacity to remove copper (Cu²+) and sulfate (SO₄²-) from water. Chapter 2 of this thesis includes a review of the literature on the use of plant-based, or lignocellulosic biochar for the removal of metals from water and simulated Acid Mine Drainage (AMD), along with marine-based biochar. The findings indicate that while marine-shell biochar is a seldom studied field, these biochars have metal adsorption capacities that are generally well in excess of those from lignocellulosic feedstocks, owing largely to their minerality (primarily in the form of calcite (CaCO₃)) and their alkalinity. In Chapter 3, we synthesize a crab shell biochar (CSB) from snow crab shell and then characterize its properties using a range of techniques to analyze its surface morphology, proximate analysis, surface zeta potential, and surface chemistry. These results indicate that the CSB is highly alkaline (pH of 11.75) and porous, with a BET surface area of 20.71 m²/g and pore width ranging from 3-10 nm. Zeta potential analysis indicates that the CSB has a primarily negative charge in solution across the equilibrium pH values studied (pH values of 7-11.5). Spectroscopic analysis indicates the biochar is mostly made of calcite, with some residual organic carbon groups, with some oxygenic and nitrogenic groups involved. Chapter 4 then analyzes the adsorptive performance of the CSB for Cu²+ and SO₄²-, common constituents of AMD. This is performed through analysis of required dosage, effect of solution pH, adsorption kinetics, adsorption isotherms, and adsorption thermodynamics. The results indicate that the biochar can adsorb Cu²+ effectively at a dosage of 5 g/L, while adsorption of sulfate species is limited unless in the presence of Cu²+, at this point individual studies on SO₄²- are stopped. Cu2+ adsorption is further unaffected by acidity in solution under the initial pH values of 2-7, perhaps due to the alkalinity of the biochar. The adsorption capacity of the CSB is found to be 184.8±10.2 mg/g for Cu²+, with adsorption kinetics best fitting the Pseudo First Order Model. Thermodynamic analysis of Cu²+ adsorption demonstrates that adsorption capacity increases with increasing solution temperature, with adsorption from a 700 mg/L Cu²+ solution increasing from 52.2±3.0 mg/g at 5 ºC to 122.2±2.0 mg/g at 30 ºC. Mechanistic analysis demonstrated that the adsorption of Cu²+ was due to precipitation in the form of posnjakite (Cu₄[(OH)₆SO₄]·H2O), as well as some influence from residual organic groups. Overall, this research demonstrates that biochar from crab shell is a highly effective adsorbent for Cu²+, with a good potential for use in AMD owing to its alkalinity.

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