Preparation Of Nanoscale Iron And Palladiumized Iron Particles And Their Application In Groundwater Remediation

Nitrate and halogenated organic compounds (such as trichloroethylene) are the main pollutants of the groundwater. And Halogenated organic compounds are environmental contaminants which are of strong toxicity and difficult to be biodegraded. Concern for possible health consequences has led to a desire for remediation of the contaminated groundwater. But all the current technologies to remove nitrate and TCE from groundwater have disadvantages, especially when used for the purpose of in situ remediation. Various technologies have been explored for treatment of nitrate and TCE, including bioremediation and permeable reactive barriers. Among many technologies tested so far, abiotic remeidation using nanoscale zerovalent iron or palladiumized iron particles appears to be one of the most promising technologies. Nanoscale materials such as nanoscale iron and bimetallic iron which have small diameter (1¹00nm), high specific surface area to mass ratios, and very great surface reactivity have been approved to remove many pollutants in water effectively such as organic contaminants, dangerous heavy metals and nitrate. And due to its small diameter, nanosacle Fe and Fe/Pd can be injected directly under pressure and transported by the flow of groundwater to the contaminated plume. So it has many potential advantages for in situ remediation applications and attracted great attentions.But there is little research has focused on the production and mechanisms of chemical reduction of nitrate by nanosacle iron particles and to this day, there is no unified understanding of the main product and mechanism of reaction which nanoscale iron denitrification with nitrate.However, due to the extremely high reactivity, nanoparticles prepared using current methods tend to either react with surrounding media [e.g., oxygen in air, dissolved oxygen (DO) or water] or agglomerate, resulting in the formation of much larger flocs and significant loss in reactivity. Due to these disadvances in nanotechnology, its application is highly limited because of its no air-stable and high cost.To overcome these drawbacks, our research on preparing nanoscale iron and palladiumized iron particles by microemulsification technology (MET) in lab and ultilizing them to remove the nitrate and TCE in water were carried out. At the same time, the research work is a part of the cooperation project named"The Basic Research on Ecological Remediation of the Water Circumstance in Coastal Cities", which is supported by ministry of education, and the project named"The In-situ Remediation of Chlorinated Hydrocarbons in Groundwater with Supported Nanoscale Iron Emulsion", which is supported by the national fund. There were three main parts in our study:1. The formation of the microemulsion employed biodegradable environmentally friendly nonionic surfactants span80 and Tween60 as mixed emulsifier was studied. And then the microemulsion was prepared by using this mixed emulsifier, n butanol as cosurfactant and iso octane as oil and water. The effect of different emulsifier, oils and cosurfactants on microemulsion region was studied. The results showed that the maximum content of water in the W/O microemulsion was found when the HLB values of emulsifier is 14, the weight ratio of emulsifier to cosurfactant is 0.5, and the weight ratio of oil to emulsifier is 2. And then the microemulsion system consisting of Fe²⁺ solution was used to synthesizeα-Fe ultrafine particles by reduction reaction. The nanoparticles were characterized by using the powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and Energy dispersive X-ray microanalysis (EDX). Results show that the average diameter of the particle was about 80nm.2. The nanoscale iron particles was used for nitrate removal from aqueous solution and got a satisfying result. The effects of some factors were discussed, such as initial pH, dispersion methods and intensity, nanoscale iron content, initial concentration of nitrate, DO exist or not and so on. The kinetics and the products of denitrification by nanoscale iron were investigated, too. The main results are as follows:(1) The batch studies showed that the nanoscale iron particles with a diameter of 80nm have extremely high reactivity, which removed mostly nitrate within a period of 30 min in anaerobic (sealed) system under ambient conditions with no pH control. The denitrification of nitrate with nanoscale iron is much fater than iron powders.(2) The nitrate removal rate was strongly dependent on intensity of dispersion. Great intensity would accelerate the reaction when a rotary shaker was used. But both excessively powerful and weak ultrasonic would inhibit the reaction when ultrasonic was used. Satisfied results have obtained when the ultrasonic power was 70w/cm2.(3) The reaction rate increased with nanoscale iron content and the initial concentration of nitrate. DO would inhibit nitrate reaction to some extent especially when the content of nanoscale iron was low.(4) The results of the batch experiments have shown that nitrate transformed to [NH₄⁺] and [NO2-] undergo the process of from increase to decrease, note the maximum in the intermediate species [NO2-], and quantity of the total nitrogen (sum of nitrate, nitrite and ammonia) displayed a minor dip in the initial stage (from 0 to 30 min) of nitrate reduction, and considerd that the reaction of nitrate reduction by nanoscale iron with a character of consecutive reaction.(5) Assuming that the possible reactions pathway for nitrate reduction by nanoscale iron in anaerobic (sealed) system under ambient conditions is NO₃^-→NO₂^-→NH₄⁺. The role of nanoscale iron in the nitrate reduction should be take part in two major processes: (a) directly participate in the redox reaction; and (b) adsorption of nitrate was proposed. The ammonia was the main products of the denitrification, and all the factors had discussed above appeared to have no obvious impact on the final products of the denitrification with nanoscale iron.(6) Almost 99% of the nitrate containt in the contaminated groundwater was removed by the nanoscale iron particles which synthesized with microemulsion in our lab within a period of 15 min.3. The formation of the microemulsion employed biodegradable surfactant Cetyltrimethyl Ammonium Bromide (CTAB) as emulsifier, n butanol as cosurfactant and iso octane as oil and water was studied. And the best microemulsion region was prepared when the weight ratio of CTAB to cosurfactant is 1.2, and the weight ratio of oil to emulsifier (CTAB and cosurfactant) is 1.5. The nanoscale palladimized iron with 80nm average diameter was synthesized by this CTAB microemulsion. These nanoparticles show some air-stable characteristic. The nanoscale Fe/Pd particles exhibited high reactivity. Dechlorination efficiency of trichloroethylene (TCE concentration is 15mg/L) with 1.6g/L the nanosacle Fe/Pd (mass fractions of Pd is 0.5%) is above 99% in 30 min.