Effective degradation of Di-n-butyl phthalate by reusable, magnetic Fe₃O₄ nanoparticle-immobilized Pseudomonas sp. W1 and its application in simulation

Highlights

• DBP degradation efficiency of W1 was improved after immobilization by PDA-IONPs.

• W1-PDA-IONPs retained 99.6% of DBP degradation efficiency after three cycles usage.

• DBP in 100% landfill leachate was efficiently degraded by W1-PDA-IONPs.

• Landfill leachate at proportion ≤50% promoted DBP degradation by W1-PDA-IONPs.

• W1-PDA-IONPs were magnetically separated at aeration rates from 60 to 600 mL min⁻¹.

Abstract

Di-n-butyl phthalate (DBP), one of the most widely used plasticizers, has been listed as a priority pollutant because of its toxicity to both humans and animals. In this study, Pseudomonas sp. W1, isolated from activated sludge, was capable of degrading 99.88% of DBP (1000 mg L⁻¹) within 8 days. We immobilized the W1 strain using Fe₃O₄ iron nanoparticles (IONPs) coated with poly-dopamine (PDA), and further evaluated its DBP degradation efficiency. The DBP degradation performance of W1 was improved by immobilization, exhibiting 99.69% of DBP degradation efficiency on the 6th day, which was 25.68% higher than un-immobilized W1. After three cycles of magnetic recycling and utilization, W1-PDA-IONPs retained 99.6% of their original efficiency. W1-PDA-IONPs were then used to degrade DBP in landfill leachate. When the proportion of raw leachate was ≤50%, DBP could be all degraded by W1-PDA-IONPs within 6 days. In 100% landfill leachate, DBP degradation efficiency after 10 days of incubation reached 66.40%. Furthermore, W1-PDA-IONPs cells in a simulated aeration system could be effectively magnetically separated at aeration rates from 60 to 600 mL min⁻¹. These results highlight the potential of W1-PDA-IONPs in the bioremediation of DBP-contaminated waste water.

Introduction

As the most widely used plasticizers, phthalate esters (PAEs), which are mutagenic, teratogenic, and carcinogenic, have aroused considerable concern because of their threat to human and animal health (Fang et al., 2009b; Xiao et al., 2018). Given their capabilities of enhancing plasticity and flexibility, PAEs are often used in various daily products, including plastic products, cosmetics, personal care products, paints, pesticides, and medical products (Wang et al., 2013). As a result, large quantities of wastes containing PAEs are produced. Currently, residual household wastes have not been strictly controlled in most countries, and waste materials containing PAEs are usually disposed of with other municipal solid waste (MSW) to landfill (Fudala-Ksiazek et al., 2018). After landfill stabilization, organic materials are released from the waste into the leachate, which is mostly generated through the penetration of precipitated water. Because PAEs are physically rather than chemically bound to the polymer chains, they are easily released from waste (Gao and Wen, 2016). Moreover, PAEs have become the most common organic pollutants, ubiquitous in river water, seawater, soil, sediments, and biota (Liang et al., 2008).

Di-n-butyl phthalate (DBP), a low-molecular-weight phthalate often used to impart flexibility in plastics, is one of the most abundant PAEs present in the environment and has been listed as a priority pollutant by the US Environmental Protection Agency (Cheng et al., 2018). As a reproductive toxicant, DBP has adverse effects on both humans and animals. Long-term exposure to DBP may cause multiple neuritis, spinal neuritis, and multiple cerebral neuritis (Liu et al., 2018). Due to its low water solubility and high octanol/water partition coefficient, DBP is relatively stable in the natural environment (San et al., 2004). Moreover, DBP has been found at high concentrations in landfill leachate (Fang et al., 2009a; Liu et al., 2010). Microbial degradation is known to play a major role in mineralizing phthalate esters in the environment. Many bacterial genera, such as Rhodococcus sp., Corynebacterium sp., and Pseudomonas fluoresences, and some fungal genera, such as Aspergillus, Fusarium, and Penicillium, isolated from different sources including wastewater, activated sludge, mangrove sediment, and soil, have been reported to be able to degrade DBP (Cheng et al., 2018).

Bioremediation has become a low-cost, environmentally safe approach to remove organic pollutants, including DBP and many other PAEs, from the environment (Zhuang et al., 2015). However, the application of PAE-degrading microorganisms in the environment, especially in wastewater treatment, depends on their survivability in complex environments (Fang et al., 2015). To maintain an acceptable degradation capacity of functional microorganisms in practical applications, it is of great importance to enhance the tolerance and stability of the target bacteria using specific technology. Studies have shown that microbiological immobilization technology can enhance the specificity and tolerance of the wastewater treatment system, while the microbes themselves become more adaptable and stable (Memon et al., 2018). Traditional bacterial immobilization methods, such as entrapment in alginate beads, adsorption on support materials, and cross linkage with carriers, have many disadvantages, including substrate diffusion and poor cellular recovery (Kefeng et al., 2012). Magnetic nanoparticles (MNPs), especially iron oxides, have become one of the most useful materials since their discovery and have been used in numerous applications, such as catalysis, magnetic fluids, magnetic resonance imaging, and in the environmental disciplines (Li et al., 2005). In recent years, Fe₃O₄ iron nanoparticles have attracted significant attention and have begun to be used as effective carriers for microorganism immobilization because of their ease of separation and delivery, large surface area, controllable synthesis and surface modification, high biocompatibility, recyclability, and low toxicity (An-Hui et al., 2010; Bai et al., 2018). However, very few studies have focused on the potential of Fe₃O₄ iron nanoparticles as a carrier for the immobilization of bacteria to remove PAEs in wastewater.

In this study, a bacterial strain capable of degrading DBP was isolated from the activated sludge of an industrial wastewater treatment plant. The bacterial strain was then immobilized by polydopamine (PDA)-coated Fe₃O₄ iron nanoparticles (IONPs). We investigated the DBP degradation ability of the immobilized bacterial strain in landfill leachate and its availability in a simulated wastewater treatment system.