Adsorption of iron and copper with crab biochar

Abstract

The growing human population causes increasing environmental issues from seafood waste. Biochar production through pyrolysis can reduce waste and provide a neutralizing adsorbent to treat acid mine drainage. This study aimed to determine if slow versus fast pyrolysis alters the char’s characteristics and if adding iron to copper containing solution affects copper’s adsorption rate. Snow crab (Chionoecetes Opilio) underwent slow pyrolysis at 500°C producing a porous char with a 55 wt% yield, a specific surface area of 14.73m²/g, and an average pore size of 21.5nm. Elemental analysis indicated a carbon content of 22.69 wt%, 1.24 wt% hydrogen, and 2.72 wt% nitrogen in the char, while proximate analysis determined 12 wt% volatiles, 57 wt% ash, and approximately 27 wt% calcium carbonate. XRD suggested amorphous calcium carbonate and calcite. Slow pyrolysis char contained 1.8 times more volatiles than fast pyrolysis. Batch adsorption experiments included a dosage of 5g/L with iron (II) sulphate and copper (II) sulphate as separate solutions and as mixtures with initial concentrations ranging from 25 to 150mg/L. Copper and iron had 100% removal from separate and mixed solutions. The char’s alkalinity resulted in a pH of 8 and the precipitation of iron hydroxides, reducing iron’s adsorption capacity to 18.4mg/g, while copper was 20mg/g. Separately, iron and copper reached equilibrium within 0.5hr and 2.0hr, respectively. As a mixture, copper’s adsorption rate increased, reaching equilibrium within 0.5h until iron reached 150mg/L due to the solubility limit of Fe(OH)₂. The Pseudo Second-Order Model fitted the adsorption of the metal mixtures, and XPS suggested that metals bind directly to oxygen-bearing groups and hydroxide groups on the char’s surface.