Bio-remediation of acephate–Pb(II) compound contaminants by Bacillus subtilis FZUL-33

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Abstract

Removal of Pb2 + and biodegradation of organophosphorus have been both widely investigated respectively. However, bio-remediation of both Pb2 + and organophosphorus still remains largely unexplored. Bacillus subtilis FZUL-33, which was isolated from the sediment of a lake, possesses the capability for both biomineralization of Pb2 + and biodegradation of acephate. In the present study, both Pb2 + and acephate were simultaneously removed via biodegradation and biomineralization in aqueous solutions. Batch experiments were conducted to study the influence of pH, interaction time and Pb2 + concentration on the process of removal of Pb2 +. At the temperature of 25°C, the maximum removal of Pb2 + by B. subtilis FZUL-33 was 381.31 ± 11.46 mg/g under the conditions of pH 5.5, initial Pb2 + concentration of 1300 mg/L, and contact time of 10 min. Batch experiments were conducted to study the influence of acephate on removal of Pb2 + and the influence of Pb2 + on biodegradation of acephate by B. subtilis FZUL-33. In the mixed system of acephate–Pb2 +, the results show that biodegradation of acephate by B. subtilis FZUL-33 released PO43 +, which promotes mineralization of Pb2 +. The process of biodegradation of acephate was affected slightly when the concentration of Pb2 + was below 100 mg/L. Based on the results, it can be inferred that the B. subtilis FZUL-33 plays a significant role in bio-remediation of organophosphorus-heavy metal compound contamination.

Introduction

Lead is a toxin, accumulating in the body through the food chain, which can harm the liver, kidney, nervous system, and blood vessels (Bai et al., 2014, Beier et al., 2013, Yang et al., 2013). Especially for children, excess levels of lead in the body can cause movement disorders, reduction of attention, hyperactivity, barriers to cognitive ability and other issues, seriously affecting the growth of children and threatening human health (Adriano, 2001, Rainbow, 2002). Nowadays, people can become easily exposed to lead pollution due to its wide sources, including air, water, soil, dust and food (Gloag, 1981).

Many chemical methods have been used to reduce the bioavailability and mobility of Pb2 + to avoid its entry into the food chain. However, most of the chemical methods have the disadvantages of high operating cost, incomplete precipitation (Kobya et al., 2005) and inhibition of soil fertility (Lasat, 2002). Recently a number of studies on removal of lead by plants, macro-algae, fungi, and bacteria (Fein et al., 1997, Lin and Rayson, 1998, Klimmek et al., 2001, Vasconcelos and Leal, 2001, Zang et al., 2014) have been conducted. Several studies have reported that a variety of bacteria are considered to have the ability to affect heavy-metal speciation, bioavailability and mobility by biosorption, biomineralization and redox reactions (Rizlan et al., 2003, Gadd, 2004, Huang et al., 2002). Muthusamy et al. (2013) reported the biomineralization of lead in mine tailings by Bacillus sp. KK1 and indicated the versatility of autochthonous Bacillus sp.KK1 for bioremediation of mine tailings. Bai et al. (2014) have studied the mechanism of soil Pb2 + immobilization by Bacillus subtilis DBM and reported the ability of the strain to convert Pb2 + into more stable forms, which showed the value of biomineralization for phytostabilization of multi-heavy metals in soil. Huang and Liu (2013b) reported that the biosorption capacity of the bacterium Pseudomonas sp. LKS06 for Pb2 + was found to be 77.9 mg/g under optimum conditions, and that the biomass of Pseudomonas sp. LKS06 can be evaluated as an alternative biosorbent. Guo et al. (2010) reported the excellent adaptation ability and remediation of entophytic Bacillus sp. L14 at the initial Pb2 + concentration of 10 mg/L, which proved the superiority of this endophyte in heavy metal bio-remediation at low concentrations.

On the other hand, organophosphorus pesticides (OPs) have become common pollutants due to their extensive use in agriculture, which could be seriously harmful to humans and lead to extremely serious environment issues. Acephate, O,S-dimethyl acetylphosphoramidothioate, as one of the most widely used organophosphorus pesticides, has gained public attention. In recent years, it has been reported frequently that OPs could be degraded by various bacteria under different conditions. Bacillus cereus, B. subtilis, Brucella melitensis, Pseudomonas aeruginosa, Pseudomonas fluorescens, and Serratia marcescens are all found to have the characteristic of being able to utilize Ops as carbon sources in aqueous media (Lakshmi et al., 2008). Pseudomonas sp. S-2 separated from a methamidophos workroom was found to possess an enzyme that is able to degrade methamidophos (Wu et al., 2005). Ramu and Seetharaman (2014) reported that P. aeruginosa Is-6 has the ability to degrade acephate, methamidophos, methyl parathion, dimethoate, and malathion. Xie et al. (2008) reported that Chryseobacterium sp. can use acephate as the sole carbon and nitrogen sources and that the biodegradation rate of acephate by this strain was up to 87.76% at the concentration of 500 mg/L acephate.

Although studies on removal of Pb2 + and biodegradation of organophosphorus have been conducted respectively, bioremediation of Pb2 +-organophosphorus remains largely unexplored. Human activities such as misuse of pesticides and fertilizers, and mining, industrial, and vehicle emissions often lead to complex environmental problems of Pb2 +–acephate (Cao et al., 2010, Liu et al., 2013). Some studies on bio-remediation of multiple contaminants have been reported in recent years. Zang et al. (2014) studied the removal of acephate and Hg2 + with an immobilized microorganism. Wang et al. (2014) revealed that B. subtilis 38, a mutant species acquired by UV irradiation, could be an ideal bio-adsorbent for adsorption of multiple heavy metals. However, there are few reports on bio-remediation of Pb2 +–acephate so far. The objectives of this article were: (1) to investigate the process of removal of Pb2 + and the characteristics of biodegradation of acephate by B. subtilis FZUL-33 under different conditions; (2) to reveal the influence of acephate on biomineralization of Pb2 + and the influence of Pb2 + on biodegradation of acephate by B. subtilis FZUL-33; and (3) to prove that biodegradation of acephate by B. subtilis FZUL-33 releases PO43 +, which promotes mineralization of Pb2 +.

Section snippets

Preparation of bacterial suspensions

The lead-resistant bacterium was isolated from sediment in a lake located in Fuzhou University, China. Identification of the strain was conducted by 16S rDNA sequence homology analysis. The process includes extraction of bacterial DNA according to the standard procedure of Sambrook et al. (1982), amplification of bacterial 16S rDNA via PCR with universal primers (27f: AGA GTT TGA TCM TGG CTC AG and 1492r: TAC GGY TAC CTT GTT ACG ACT T) and conducting sequence homology alignment in GenBank using

Removal of Pb2 + by B. subtilis FZUL-33

The isolated strain FZUL-33 was identified as B. subtilis based on its 16S rDNA sequence homology analysis. The removal of Pb2 +, as affected by pH, interaction time and Pb2 + concentration, was investigated in this section. The results are shown in Fig. 1. The pH played a significant role in the process of removal of Pb2 +. The biosorption of Pb2 + rose as the pH value increased, due to the metal ions competing with protons for available binding sites on the cell surfaces at lower pH values (Fig. 1

Conclusions

This study investigated the removal of Pb2 + and acephate by B. subtilis FZUL-33 respectively and focused on the removal of acephate–Pb2 + mixture by the strain. The conclusions we made were: (1) The maximum removal of Pb2 + of B. subtilis FZUL-33 was 381.31 ± 11.46 mg/g under optimal conditions. (2) This strain has the ability for biomineralization of Pb2 +. The product biomineralization is Pb5(PO4)3OH, and the crystals become more apparent as the time increases. (3) In the mixed acephate–Pb2 + system

Acknowledgments

This work was supported by the National Basic Research Program (973) of China (No. 2014CB846003) and the National Natural Science Foundation of China (Nos. 41372346; 21577018).

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