Simulation Groundwater Nitrate Reduction By Zero-Valent Iron In The Loess Plateau Area

China is one of the shortest countries for water resource in the world, especially in the arid and semiarid Loess Plateau Area, groundwater is a more precious resource. But as the development of industry and agriculture, a mass of waste water, waste contained nitrogen and nitrogenous fertilizer come into environment, which has disrupted the nature original nitrogen equilibrium and caused serious environmental nitrogen pollution. Among them, groundwater nitrate pollution is one of the main nitrogen pollution. The traditional groundwater pump-treat technology is not obviously harmony with the great area groundwater nitrate pollution. It must adopt in-situ remediation technology. At present, chemical denitrification by Fe⁰ is considered a potential technology for nitrate contamination groundwater remediation. And zero-valent iron-based permeable reactive barrier (Permeable Reactive Barrier, PRB) technology has been used to remove lots of contaminations in the groundwater, such as chlorinated organics, nitrobenzene, heavy metals(As, Cr(â…¥)) and oxo-anion(for example nitrate, phosphate and sulfate) and obtained very good effect. But the application of it is not reported in China. Batch and column experiments were conducted to investigate the effect and major effect factors for zero-valent iron removal simulated nitrate contaminated groundwater in the Loess Plateau and confirm the feasibility and filling of PRB used in the Loess Plateau Area. The results were as follow:(1) Batch tests with loess indicated that it was effective for nitrate reduction by Fe⁰ in the alkaline condition of Loess Plateau. Nitrate removal percentage could be increased by enhancing the dosage of Fe⁰, acid-pretreated Fe⁰, added Cu/Fe, Ni/Fe or active carbon. A more appropriate iron-to-nitrate-N ratio of mass is 100:1;(2) Batch tests with loess indicated that nitrate removal was enhanced by augmenting the system with Fe³⁺, Fe²⁺ and Cu²⁺. It decreased in the order Fe³⁺>Fe²⁺>Cu²⁺ under the same concentration. It also increased nitrate removal percentage in the presence of organic and inorganic anions with a decreased order of C₆H₅O₇^3->CH₃COO^->SO₄^2->HCO₃^->C₂O₄^2->Cl^->PO₄^3-, only PO₄^3- inhibited the reaction.(3) Column experiments indicated that the influent nitrate solution with different pH made no obvious difference for nitrate removal, as a result of soil's cushioning effect. It was very necessary for mixing some sand to avoid plugging in the packed column. And the appropriate ratio of sand-to-iron in volume is 6-8. Porosity and reactivity decreased during long-term performance of column. Adding active carbon not only enhanced nitrate reduction, but also cut down the speed of porosity and reactivity losses, and increased the longevity of PRB. Nitrate removal was also enhanced by increasing hydraulic retention time (HRT), but was inhibited by adding fly ash, though helpful for decreasing ammonium in the effluent. It was the most effective method for nitrate removal when iron and sawdust were added in the system. The concentrations of nitrite and ammonium in the effluent were below the limit of drinking water standard of China.(4) In the system of Fe⁰-H₂O-ion, nitrate adsorption on the surface of iron and iron corrosion product preceded the removal. Fe²⁺ was very important for nitrate reduction, for it not only promoted electron transfer, but also participated in nitrate reduction, and then transformed to magnetite. As the reaction processing, a black film formed on the surface of iron and nitrate reduction halted quickly. After adding Fe²⁺, Fe³⁺or Cu²⁺ in the system, lots of Fe²⁺ was produced in a short time, and greatly enhanced nitrate reduction. At the same concentration of them, the promotion decreased in the order of Fe³⁺ > Fe²⁺ > Cu²⁺. The order was the same as the loess was present. Except for PO₄^3- blocking the reaction, the augment of C₂O₄^2-,C₆H₅O₇^3-,CH₃COO^-,HCO₃^-,SO₄^2- or Cl^- all increased the nitrate reduction with a different degree. The presence of inorganic ions(Cl^-,SO₄^2- or HCO₃^-) not only promoted iron corrosion and released Fe²⁺, but also formed green rust with Fe(â…¡-â…¢) hydroxid, which reacted with nitrate. But the strong adsorption between PO₄^3- and iron or corrosion product of iron greatly inhibited nitrate reduction. The primary effect of organic ions was chelation, which not only enhanced the dissolution of iron but also reduced the formation of iron oxide and hydroxid in some degree. The effect of different anion without loess led to a pseudo-first-order rate for nitrate reduction, but it rather accorded with Logistic model in the presence of cation.(5) In the whole batch tests, the sum of nitrogen in the solution, including nitrate, nitrite and ammonium determined was lower than that of theoretics value of nitrogen. There were three possible reasons:â‘ the transgression of ammonia under the high pH;â‘¡the adsorption of nitrogen on the surface of iron and iron corrosion products;â‘¢the product of other nitrogen contained gas. The first and second reasons may be mostly responsible for the nitrogen loss.(6) In the batch and column experiments with loess, the dissoluble iron was not obvious in most systems, but due to the chelation between organic anion and Fe(â…¡) and Fe(â…¢) promoted the dissoluble iron .