Comparative effectiveness of activated dolomite phosphate rock and biochar for immobilizing cadmium and lead in soils

Highlights

• Activated DPR (ADPR) and biochar were compared for immobilizing metals in two soils.

• The immobilizing effectiveness of amendments varied with metal and soil type.

• Activation with humic acid increased the immobilizing power of DPR for Cd²⁺ and Pb²⁺.

• The immobilization of metals by ADPR is related to increased soil pH and available P.

Abstract

Sandy soils in Florida are vulnerable to toxic metal pollution, and it is necessary to identify desirable amendments for the remediation of metal contaminated soils. Sorption and incubation experiments were conducted to compare the effectiveness of dolomite phosphate rock (DPR), humic acid activated dolomite phosphate rock (ADPR) and biochar (BC) in immobilizing Cd²⁺ and Pb²⁺ in two representative agricultural soils in south Florida (Alfisol-Riviera and Spodosol -Ankona series). The results showed that the soils had a low sorption capacity for metals with maximum sorption of 0.767–3.30 mg/g. Application of amendments increased the maximum sorption by 4.2–4.8 times for Pb²⁺ and 1.5–2.2 times for Cd²⁺ in Alfisol soil, and 7.1–7.9 times for Pb²⁺ and 1.7–3.1 times for Cd²⁺ in Spodosol soil. ADPR was the most effective amendment for increasing the soil’s sorption capacity for Cd²⁺ and Pb²⁺. 0.01 M CaCl₂ extractable metals in the contaminated soils were significantly decreased by all the amendments, especially ADPR, which reduced extractable Cd²⁺ and Pb²⁺by 87.2 and 76.0% in Alfisol and 91.3 and 76.3% in Spodosol soil as compared to control. The amounts of extractable Cd²⁺ and Pb²⁺ were negatively correlated with soil pH and available P, indicating that the change of soil characteristics by amendments was the dominant mechanism for enhanced immobilization of metals in the contaminated soils. These results indicate that ADPR has great potential for remediating toxic levels of Cd²⁺ and Pb²⁺ in contaminated soils.

Introduction

Soil pollution by toxic metal (loid)s (TMs) is a serious environmental and human health issue (He et al., 2015; Antoniadis et al., 2017). Most soils in Florida are sandy and have a low nutrient retention capacity (He et al., 1999; Yu et al., 2006), and are more vulnerable to TM pollution (He et al., 2006; Zhang et al., 2006). In addition, the water and TMs that may have reached to the top of the subsurface soil hardpan can be transported laterally into the water furrows and then to the distant surface water bodies, resulting in water pollution. Improved management practices are important for sustainable agriculture and to safeguard further contamination of surface fresh water in Florida.

The phosphate industry in Central Florida produces significant amounts of dolomite phosphate rock (DPR) in the mining sites. If the DPR materials can be converted into slow release phosphorous (P) fertilizers and/or soil amendments, it can generate additional profits to the phosphate industry and further, provide agricultural and environmental benefits. P-enriched materials have been increasingly used as soil amendments to reduce the bioavailability of TMs in contaminated soils (Seshadri et al., 2017; Zhao et al., 2017; Huang et al., 2019; Xu et al., 2019). Among these materials, phosphoric acid-derived fertilizers such as triple superphosphate (TSP), diammonium phosphate (DAP), were widely used since they are completely water-soluble, more effective for TMs immobilization but also have greater P leaching potential, causing water eutrophication (Zeng et al., 2017). Direct application of phosphate rock (PR) is cost-effective and environmentally-friendly, but release of P is often too slow to meet the requirements, especially in soils with relatively high pH (>6) (Fayiga and Nwoke, 2016; Xu et al., 2019). Therefore, alternative materials are being sought after for agricultural and environmental applications. Activation of DPR with selected organic molecules resulted in a substantial increase in water-soluble P (He et al., 2017) and the activated DPR (ADPR) could continuously supply P to meet the growth requirements of maize (Zea mays) and millet (Pennisetum glaucum) (Mao et al., 2017). The activation process has also been identified to release less labile TMs, as compared to traditional manufacturing such as acidification with concentrated sulfuric acid (Yu et al., 2019; Wang et al., 2020), and thus activated DPR fertilizers are more suitable for use in those environmentally sensitive regions such as south Florida. Therefore, it is desirable to explore the potential of activated dolomite phosphate rock (ADPR) for remediating TMs contaminated soils.

Biochar has been extensively studied for potential application as a soil amendment to improve physicochemical properties, such as porosity, cation exchange capacity (CEC), and pH, and subsequently, the soil’s holding capacity for water and nutrients, and crop production (Agegnehu et al., 2017; Shaaban et al., 2018; Yu et al., 2019; Guo et al., 2020). Biochar has been reported to be effective for immobilizing toxic elements in contaminated soils (Shaheen et al., 2019; Wang et al., 2019; Bandara et al., 2019). Sandy soils are generally more responsive to biochar additions due to their inherited small water and nutrient holding capacity (Molnár et al., 2016; Blanco-Canqui, 2017; Baiamonte et al., 2019). Therefore, biochar could be a desirable amendment for remediating TM contaminated sandy soils.

However, the effectiveness of soil amendments in remediating contaminated soils is affected by many factors, including soil characteristics, amendment composition, and chemistry and levels of toxic elements (Wang et al., 2019; Dhaliwal et al., 2019; Li et al., 2019). It is crucial to evaluate the effectiveness of different amendments for immobilizing toxic elements in the sandy soils of Florida and elsewhere in the world. This study was aimed to investigate the effectiveness of original DPR, activated DPR and biochar, as soil amendments, for remediating Cd and Pb-contaminated two typical Florida sandy soils (Alfisol and Spodosol) through laboratory thermodynamic and incubation experiments.