Preparation of biochars from coconut fiber and coconut shell and their adsorption for Pb²⁺ in solution

Heavy metal pollution has become more and more serious with the development of industry. Heavy metals can accumulate in plants, and enter into human body through the food chain. Lead (Pb) is one of the most common heavy metal contaminants in the environment, usually discharged with waste water and waste gas. Accumulation of lead in the body can cause physical function disorder, and it is even more harmful to children. Therefore, it is very urgent to control the lead pollution. As a new type of adsorbent for soil heavy metal, biochar has a great advantage compared with other traditional materials. Biomass-derived biochar combines an effective removal of water contaminants with low cost of remediation. Many researchers used waste biomass crops and other materials to make biochar by pyrolysis at high temperature to remove heavy metals. The water movement on the biochar pores is affected by metal adsorption, which is a novel and promising technique to identify the nature of biochar-metal bonds at the solid-liquid interface.However, few researches were reported using common tropical biomass waste such as coconut shell and coconut fiber to prepare biochars. The coconut fiber become thick after matured, and the loose brown fibrous layers are soft and elastic, mainly are composed of hard stratum corneum. The coconut shell and coconut fiber are abundant, cheap and easy to collect in Hainan Province. Therefore, study on the adsorption of lead by biochars prepared from coconut shell and coconut fiber has a great significance to control heavy metal pollution. The objectives of this study were to: 1) characterize the biochars prepared from coconut shell and coconut fiber under different temperatures;2) determine the adsorption characteristics and mechanisms of Pb²⁺ on biochars.Biochars were prepared by pyrolyzing coconut fiber and coconut shell at the temperatures of 300,500 and 700℃ respectively, and their physical and chemical properties were analyzed.The results of Fourier transform infrared spectroscopy and element contents indicated that, the carbonation degree of the biochars increased with the pyrolysis temperature, while the amount of oxygen-containing functional groups decreased. In addition, the surface area, ash content, pH, cation exchange capacity (CEC) and basic functional group content enriched with the pyrolysis temperature. The adsorption of Pb²⁺ by biochars derived from coconut fiber and coconut shell was fitted better with Langmuir model. As the pyrolysis temperature increased from 300 to 700 ℃, the amount of Pb²⁺ adsorbed on the biochars gradually increased. Among the six kinds of biochars, the coconut fiber biochar prepared at 700℃ had the highest adsorption amount of 180.438 mg/g, which was better than many adsorption materials that reported previously. The CEC and ash content were important factors affecting the adsorption deduced from the fit curve. Under the condition of initial Pb²⁺ concentration with 200 mg/L, the saturated adsorption amounts of coconut fiber and coconut shell biochars were 44.89-96.08 mg/g and 15.82-61.77 mg/g, respectively. The saturated adsorption amount of coconut fiber biochars was higher than that of the coconut shell biochars. However, the stable adsorption amounts of coconut fiber and coconut shell biochars were 9.83-13.91 mg/g and 9.68-25.16 mg/g, respectively.In conclusion, the different preparation temperatures can directly affect the physical and chemical properties of biochars. Different raw material sources and pyrolysis temperatures have a great influence on the adsorption capacity of Pb²⁺, and the largest amount of lead adsorption material is YA₇₀₀(180.438 mg/g). In addition, different raw material sources and pyrolysis temperatures have a great influence on the stable adsorption quantity of lead, and the adsorption of Pb²⁺ on the coconut shell biochar is more stable than coconut fiber biochar.