| Arsenic is widely distributed in the environment. Many arsenic compounds are known to be toxic. Apart from the lethal effect of arsenic ingested at high doses, it is a known carcinogen when taken in trace amounts over a long period. The maximum threshold of arsenic in drinking water revised by WHO, the European Commission, Japan and USA, has been reduced from50to10μg/L. Drinking water health standards (GB5749-2006) also recommended a maximum concentration of10μg/L in China. Therefore, the research on improving the established processes for removing arsenic from contaminated groundwater is an emerging issue.Iron, manganese, and arsenic often coexist in groundwater. The removal of Iron, manganese, and arsenic simultaneously by biological process from contaminated groundwater is a new and promising method. This method is simple, reliable, economic and environment friendly. In this study, a long-term and stable operation of biofilter for the simultaneous removal of arsenic, iron and manganese from simulated groundwater has been discussed. The effects of microbial growth by arsenic contamination groundwater in the biofilter has been investigated. The different water quality parameters and process running condition influence on arsenic removal have been analyzed in detail. Furthermore, the physiology and biochemistry of biological iron and manganese oxides and community structure and genetics feature of the microorganisms in the biofilter system based have been investigated. This study combined the macroscopical process performance and micropscopical biological characteristics for mechanism study of arsenic, iron and manganese removal from simulated groundwater by the biofilter, aiming at guidance on project.The study of a long-term and stable operation of the biofilter shows that arsenic, iron and manganese has been simultaneously removed from simulated groundwater. Arsenic did not affect the growth of microorganisms in the biofilter, but effectively removed. With pH increases, the surface charge on biogenic iron manganese oxides gradually decreased. Arsenic adsorption ability of biogenic iron manganese oxides gradually decline, so arsenic removal rate would gradually decrease. The presence of phosphate in water would have great influence on arsenic removal, because phosphate had adsorption competition to arsenic, reducing the adsorption efficiency of arsenic on biogenic iron and manganese oxides. The presence of silicate in water would not have influence on arsenic removal, because the most silicate in water was non-ionic state, which was not easy to be adsorbed by biogenic iron manganese oxides. Silicate would have no adsorption competition with arsenic. Different concentrations of iron and manganese in water have great influence on arsenic removal. Increased iron concentration is conducive to the adsorption of arsenic, the adsorption effect of As(V) is superior to As(III). Increased manganese concentration is conducive to oxidize As(III) to As(V).Influent arsenic concentration affects effluent quality of the biofilter. The effect of As(V) removal in the biofilter was better than As(III). With filtration rate increasing, the efficiency of arsenic removal would gradually decrease. Under backwashing intensity8L/(s·m2), backwashing times3min, backwashing cycle72h conditions, the effluent iron and arsenic concentration would exceed standard in a short period after backwashing, but return to normal within30min.SEM analysis of the sand in the biofilter was performed, in order to ensure bacterial existence and growth. Micrograph illustrated the presence of the most common Iron oxidizing bacteria (Gallionella) and Manganese oxidizing bacteria (Leptothrix), which were indigenous in groundwater and very common microorganisms in groundwater treatment plants for the removal of iron and manganese. It can be shown from micrographs that these microorganisms present an increased tendency to deposit large amounts of iron oxides and manganese oxides on their surface. EDS spectrum analysis showed that the surface of the sand containing the main elements of Fe, Mn and As. XPS analysis showed the characteristics of biological oxidation for iron and manganese in biofilter. As(III) can be removed by oxidation and sorption onto the iron and manganese oxides.Under the condition of20oC±0.2, pH6.9, biogenic iron manganese oxides have strong adsorption ability on the As(III) and As(V). The maximum adsorption capacity of As(III) and As(V) were68.97mg/g and35.46mg/g, respectively. In the same conditions, the adsorption effect of As(III) is superior to As(V). Under the condition of20oC±0.2, pH6.9, As2mg/L, arsenic adsorption by biogenic iron manganese oxides conforms to pseudo second order kinetics equation. Equilibrium adsorption capacity of As(III) and As(V) were14.88mg/g and12.25mg/g, respectively. The initial adsorption rate were4.36mg/(g, min) and6.11mg/(g, min), respectively. In the same conditions, adsorption capacity of As(III) is bigger, but initial adsorption rate of As(V) oxides is faster.16s rDNA clone library of biofilter microbial system has been built. The composition of bacteria analysis showed that bacteria is very similar in1#and2#biofilter. Bacteria can be divided into9classes, which were α-Proteobacteria,β-Proteobacteria, γ-Proteobacteria, δ-Proteobacteria, Verrucomicrobia, Nitrospina, Acidobacterium, Firmicutes and Actinobacteria, respectively. Most of these bacteria belong to Proteobacteria. Some bacteria with known Manganese oxidizing bacteria and Arsenic oxidizing bacteria have close genetic relationship. Archaea species in1#and2# biofilter was very little, which belong to Crenarchaeota and have not close genetic relationship with known Crenarchaeota.As shown by T-RFLP analysis at different filter bed depths in the biofilter, there were more microorganisms at upper layer of filter bed than at the bottom of filter bed. Microbial species in2#biofilter were more than1#biofilter. Using principal component analysis and clustering analysis to explore the changes of microbial community structure in biofilter, microbial community structure was different in1#and2#biofilter and the changes of community structure at the different filter bed in biofilter was relatively small.PCR-DGGE analysis showed that Iron oxidizing bacteria, Manganese oxidizing bacteria and Arsenic oxidizing bacteria coexist at different filter bed depths in the biofilter. These bacteria have the corresponding relationship with Fe(II), Mn(II), and As removal at different filter bed depths. With XPS analysis of the backwashing matters, it was shown that Iron oxidizing bacteria and Manganese oxidizing bacteria involved in Fe(II) and Mn(II) oxidation and Arsenic oxidizing bacteria maybe participated in As(III) oxidation.Manganese oxides play an important role for As(III) oxidation and removal in the biofilter. Manganese oxides were produced by Manganese oxidizing bacteria, so Manganese oxidizing bacteria in the biofilter is related to manganese and arsenic removal. As a results, in this study, we successfully screened8strains of Manganese oxidizing bacteria from biofilter reactor by MSVP medium, and the eight strains were proved belong to Pseudomonas, Bacillus and Arthrobacter. |
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