Preparation of magnetic meat and bone meal biochar modified by magnetic fluid and its adsorption properties for Cd2+

Abstract: Aquaculture scale and aquatic products are rapidly rising in recent years, particularly with the increasing intensive level of aquaculture industry in the world. A large number of by-products (waste biomass) are inevitably generated during processing, including abnormally dead aquatic products, residual bait, and feces. Most waste biomass is normally buried or incinerated, resulting in the waste of biomass energy and environmental pollution in the risk of zoonotic diseases. Fortunately, the pyrolysis of waste biomass into the meat-and-bone-meal (MBM) biochar has widely been used for the treatment of heavy metal pollution. Therefore, this study aims to propose a new treatment for the reduction, harmlessness, and resource disposal of waste biomass in the aquaculture industry. Taking the abnormal death of carp as an example, the carp meat-bone-meal biochar (CBC) was prepared. The magnetic fluid was selected to control the type and particle size of loaded iron oxide, thereby modifying the CBC for the magnetic carp meat-bone-meal biochar (MCBC). An investigation was also made on the effects of magnetic fluid modification on the physicochemical properties of MCBC. A series of batch equilibrium and sequential extraction tests were carried out to explore the adsorption characteristics of MCBC for Cd2+ under various microstructural characterization. The results showed that there was a much more uniform surface of Fe3O4 loading on the CBC in the magnetic fluid, while the adsorption efficiency of Cd2+ increased by 160%. MCBC presented a much larger specific surface area, the total pore volume, the average pore size, and surface charge, compared with CBC. The adsorption equilibrium time of MCBC for Cd2+ decreased from 180 to 90 min, as the initial concentration of Cd2+ increased from 50 to 100 mg/L. The adsorption was well fitted with the pseudo-second-order kinetics and diffusion-chemisorption adsorption models. An intra-particle diffusion model demonstrated that the adsorption of Cd2+ by MCBC presented a continuous multi-stage process, including the rapid diffusion of liquid film, slow diffusion of intra particle, and final adsorption/desorption equilibrium, indicating that the intra particle diffusion was not the only rate control step. Freundlich and Temkin adsorption models were utilized to better simulate the isothermal adsorption behavior. The saturated adsorption capacity of MCBC for Cd2+ using Langmuir equation was 74.6-96.2 mg/g at different adsorption temperatures. The adsorption kinetics and thermodynamics showed that the adsorption of Cd on MCBC was a combination of various mechanisms, where the adsorption was mainly using chemical action. The adsorption mechanisms of Cd2+ on MCBC were mainly dominated by ion exchange, precipitation, electrostatic adsorption, and surface oxygen-containing functional group complexation. A sequential extraction test demonstrated that the current forms of Cd2+ on MCBC were exchangeable fraction (48.4%), acid extractable fraction (29.5%), stable fraction (21.8%), and water-soluble fraction (0.3%), indicating that the Cd2+ was predominantly adsorbed on MCBC as exchangeable and acid extractable. The finding can provide a potential theoretical reference for the resource utilization of aquaculture industry wastes and the remediation of heavy metal pollution.