Influence of pyrolysis temperature on the cadmium and lead removal behavior of biochar derived from oyster shell waste

Highlights

• Pyrolysis temperature significantly influenced the biochar properties.

• OS and OSBs exhibited high Pb²⁺ sorption capacity (923.3–1553.0 mg·g⁻¹).

• OSB900 exhibited high Cd²⁺ sorption capacity (159.8 mg·g⁻¹).

• Precipitation was the main Cd and Pb immobilization mechanism.

• OSB900 was more suitable for remediating Cd- and Pb-contaminated soil.

Abstract

Oyster shells (OSs) were pyrolyzed at 300 °C, 600 °C, and 900 °C to produce oyster shell biochars (OSB300, OSB600, and OSB900, respectively). The physicochemical properties and adsorption mechanisms for the removal of Cd and Pb by the biochars were then investigated. The results indicated that the calcite in OS decomposed into CaO at 900 °C, which may further influence its adsorption capacity. OSBs had a higher affinity for Pb than Cd as they could hydrolyze Pb more easily. OSB900 exhibited superior adsorption performance for Cd (153.8 mg·g⁻¹) in batch adsorption, but the lowest adsorption performance for Pb (923.3 mg·g⁻¹). Furthermore, the Pb adsorbed on OSB900 mainly presented as stable carbonate precipitation (Pb₃(CO₃)₂·Pb(OH)₂). Soil amendment with OSB900 at a 0.5% dosage most effectively decreased the CaCl₂-extractable Cd and Pb by up to 98% and 88%, respectively. These findings suggest that OSBs are suitable for Cd and Pb immobilization in both wastewater and contaminated soil.

Introduction

The globalization of oyster farming for both consumption and pearl production generates vast amounts of waste shells (Bonnard et al., 2020). The value of food or pearls accounts for over 30% of the whole oyster mass (Mo et al., 2018; Silva et al., 2019). According to the FAO, in 2018, 6.1 million tons of oyster were produced worldwide (Food and Agriculture Organization of the United Nations, 2020), with over 4.3 million tons of shell waste (Botta et al., 2020). Waste oyster shells are mostly deposited without control, which may cause sanitation issues and the release of odorous gases, such as H₂S and NH₃, particularly in major oyster-producing regions (Mo et al., 2018; Silva et al., 2019). Considering these environmental issues, sustainable oyster shell waste recycling into value-added products is becoming a global priority (Bonnard et al., 2020).

Natural oyster shells are biogenic materials composed of calcium carbonate (>95%) in association with an organic fraction (approximately 5%; Chilakala et al., 2019). Calcium-enriched oyster shell has been used as a construction material and feed additive, and in wastewater treatment (Chilakala et al., 2019; Silva et al., 2019). According to a recent study, most oyster shell waste is disposed of in landfills or thrown into the sea, and only a small proportion (approximately 10%) is recycled by industry (Silva et al., 2019).

Previous studies have reported that both crab and crayfish shells are excellent feedstocks for Ca-rich biochar production via pyrolysis under an oxygen-limited environment (Hopkins and Hawboldt, 2020). These novel Ca-rich biochars have excellent performance for removing several pollutants from aqueous solutions (Hopkins and Hawboldt, 2020). The malachite green and Congo red adsorption capacities of crab shell-derived biochar in aqueous solutions reached 12.5 and 20.3 g·g⁻¹, respectively (Dai et al., 2018). Additionally, crab shell biochar generated at high temperatures (800–900 °C) could be used in place of industrial Ca(OH)₂ for recovering phosphorus from wastewater (Dai et al., 2017). Crayfish shell biochar was recently employed as an excellent adsorbent for lead removal due to its high sorption capacity (599.70–1166.44 mg·g⁻¹) (Sun et al., 2021). Therefore, calcium-enriched oyster shells may be converted into environmentally friendly materials through pyrolysis.

Heavy metal pollution, including cadmium (Cd) and lead (Pb), is a global issue, particularly in developing countries (Harmesa and Cordova, 2021). Both Cd and Pb easily accumulate in the food chain, severely threatening food security and human health (Dar et al., 2017). Environmentally sustainable materials have been recommended to stabilize Cd and Pb in contaminated soils (Shen et al., 2019b). Owing to their low cost and high availability, biomass waste-derived biochars have been widely used in soil remediation (Zhang et al., 2021). Previous studies have demonstrated that biochar can efficiently remove Cd and Pb from both aqueous solutions and contaminated soil through precipitation, adsorption, or functional complexation (Qiu et al., 2021). However, high biochar application rates ranging from 10 to 110 tons per hectare may result in high costs (Bian et al., 2016; Chen et al., 2018). Calcium, an essential macronutrient, can be used as an exogenous substance to alleviate Cd- and Pb-induced toxicity in plants (Huang et al., 2017; Sakouhi et al., 2016), and ion exchange (Ca²⁺) is the dominant mechanism responsible for Cd and Pb removal by Ca-rich biochars (Sun et al., 2021). Liming materials, including limestone and burnt lime (CaO), have been effectively used to immobilize Cd and Pb in agricultural soils (Chen et al., 2018; Du et al., 2018). However, burnt lime production is energy-intensive, and some commercial lime materials may contain excessive amounts of heavy metals (Bian et al., 2016; Lund, 2007). Therefore, converting oyster shell waste into Ca-rich biochar for heavy metal immobilization is beneficial for waste management and environmental restoration.

In this study, we report a simple and sustainable method for directly preparing Ca-rich biochar (OSB) from oyster shell waste via pyrolysis at various temperatures. This study aimed to investigate the effects of using OSB as a low-cost yet highly efficient heavy metal sorbent. Batch Cd and Pb adsorption was conducted to explore the adsorption kinetics and isotherms, and the physicochemical properties of OSB before and after adsorption were characterized to explore the adsorption mechanism. The potential application of OSB for Cd- and Pb-contaminated soil remediation was further evaluated by conducting an incubation experiment.