The pinyon pine is stih regarded as a distinctively Indian resource to be conserved and respected . At present the main threat to continued plant-food collecting derives from land-development activities such as large-scale open-pit mining operations at prime habitat locations. Conflicts over hunting and fishing rights have been more confrontational, leading, in some instances, to open defiance of state laws requiring hcenses and fishing rights on the Truckee River . Though some special concessions have been made in the state laws, it appears that these problems whl continue to be a source of aggravation into the future. Heavy metal pollution, especially cadmium , is a major environmental issue in China and many other parts of the world . The Guangzhou Food and Drug Administration recently reported that the content of Cd in 44% of sampled rice and rice products exceeded national standards of 0.2 and 0.1 mg/kg respectively , which underlined the severity of Cd pollution in China’s main grain producing areas. Some of these grain producing areas have large mines . Poor management of wastes from mining activities have resulted in severe water contamination at levels of up to 3000e5000 mg Cd/L. Due to water shortages and lack of treatment facilities, water contaminated with nitrate and metals is used for irrigating vegetables and grains , which could lead to high nitrate and heavy metal concentrations in vegetables and grains . While Cd damages lungs, kidneys, liver and reproductive organs , nitrate can cause methemoglobinemia in infants . Hence, the importance of treating this kind of contamination cannot be overemphasized. Nanoscale zerovalent iron , with Fe core and iron oxide shell, has been proposed for the treatment of Cd contamination . However, there is a large discrepancy in Cd removal capacity of nZVI reported , 66.9 mg/g , 769.2 mg/g.

The discrepancies may arise from different initial Cd concentrations, temperature, and water chemistry such as pH ,drainage pot and concentrations of dissolved oxygen , phosphate , and nitrate . According to previous studies , X-ray photoelectron spectroscopy analysis indicated that Cd immobilization by nZVI was mainly through adsorption. However, reduction of Cd to Cd may also occur due to its slightly more positive standard electrode potential than Fe . As demonstrated in this study, X-ray diffraction may help to better understand Cd reduction on the nZVI particles, since it can show characteristic peaks of Cd if present. With regard to nitrate pollution, it has been widely observed that ammonium is the main reduction product in the presence of nZVI, with only a small fraction of nitrite detected; nitrite is regarded as an intermediate . Although ammonium is a toxic pollutant to some organisms , it serves as major nitrogen source for plants . Furthermore, nZVI is mainly transformed into magnetite after reaction with nitrate , which avoids significant increase of Fe2þ or exchangeable Fe concentration. This vastly reduces the potential environmental impact of nZVI . Hence, it is possible to employ nZVI to treat contaminated groundwater after it has been pumped out of the ground. Given the prevalence of nitrate and metal contamination in many regions, it is important to understand the influence of one pollutant on the removal of the other using nZVI. In this study we focused on the interplay between nitrate and Cd. Nitratemay affect Cd removal in two ways: in terms of Cd adsorption, while nitrate may not affect adsorption significantly through changing ionic strength , it may enhance Cd removal by driving solution pH above 9 ; and in terms of Cd reduction, nitrate reduction will consume a large part of Fe and produce iron oxided reducing electron supply and restricting electron flow for Cd reduction. Likewise, the presence ofCd may also have two important implications on nitrate reduction: First, similar to Cu islands , Cd islands compounds or Cd) may be formed on nZVI, which may enhance electron transport to nitrate.

Enhanced electron transport may occur given the lower electrical resistivity of Cd than Fe . Second, if Cd is reduced to Cd, it can reduce nitrate to nitrite , which may lead to an increased nitrite yield ratio.However, nitrite accumulation is undesirable in natural environment as nitrite is highly toxic to several organisms, including humans . Several previous studies have shown that catalysts, such as Cu , Ag , or Au , can facilitate nitrite reduction. In this study, Cd removal performance of nZVI in the presence or absence of nitrate was investigated. In addition, the effect ofCd on nitrate reduction was systematically examined.We also evaluated the potential of nZVI with 1 wt.% Cu, Ag, or Au to treat Cd and nitrate co-pollution with minimal nitrite yield. XRD was employed to detect Cd and characterize the transformation of the nanoparticles under different conditions.Batch Cd adsorption experiments with and without nitrate addition were carried out in a 300 ml 3-neck flask. A 200 mg/L Cd2 stock solution was prepared and used for all experiments. Final Cdconcentration was 10e40 mg/L and the load of iron nanoparticles was 500 mg/L. The flask was agitated by an electromagnetic stirrer at 25  C under Ar atmosphere. In the series of tests with nitrate, the concentration of nitrate was 15 mg-N/L. After reacting for 90 min, solution pH was measured, and aqueous samples were collected for Cd and total Fe analyses. Batch tests for determining Cd removal capacity under different nitrate loads were carried out in a series of 100 ml conical flasks sealed with screw caps. The concentrations of Cd were 50, 100, and 150 mg/L while the concentrations of nitrate were 8, 12, 16 mg-N/L. The load of iron nanoparticles was 500 mg/L. The suspensions were mixed at 200 rpm for 90 min. At the end of the experiment pH was measured , and aqueous samples were removed to determine concentrations of nitrogen compounds and metals , total Fe).

To further investigate the influence of nitrate on Cd removal, two different series of tests were performed. In Series 1, nitrate was not present in the reaction system. 125 mg nZVI were added into 250 ml of a 40 mg-Cd/L solution, the solution pH was adjusted and maintained at 9.0, stirring for 30 min. After stirring, an aqueous sample was collected and analyzed for total Fe and Cd. The remaining mixture was then divided into 5 parts, which were adjusted to and maintained at pH 8.5, 8.0, 7.5, 7.0, or 6.5, stirring for another 30 min. Aqueous samples were then collected from the 5 sub-samples for Cd and Fe analyses. In Series 2, 125 mg nZVI was added into 250 ml of 40 mg/L Cd and 15 mg-N/L nitrate. After 20 min, solution pH increased to 9, and remained stable for ~10 min, so it was not adjusted as in Series 1. As before, an aqueous sample was collected and analyzed. The suspension was divided into 5 parts whose pH was adjusted and kept at 8.5, 8.0, 7.5, 7.0, or 6.5. After stirring for another 30 min, aqueous samples were collected and analyzed. Additionally, we performed a control test to see the effect of pH on Cd removal, by adjusting the pH of 15 mg Cd/L solutions to 7, 7.5, 8.0, 8.3, 8.5, or 9.0, in separate vials. After 5 min mixing and another 5 min standing, 1 ml aqueous media was collected for Cd measurement. Sodium hydroxide and hydrochloric acid were used to adjust and maintain solution pH in all cases. To examine the effect of different Cd loads on nitrate reduction,frambueso maceta nZVI was added into a series of 15 mgN/L nitrate solutions with 10, 20, 30, 40, 50 or 100 mg/L Cd in 100 ml conical flasks sealed with screw caps. After 120 min shaking , samples were collected and nitrogen compounds in solution were analyzed. To investigate the effect of Cd addition on nZVI oxidation, 250 ml of a 500 mg/L nZVI suspension with or without 10 mg/L Cd load was agitated vigorously by an electromagnetic stirrer for 45 min in a 500 ml beaker without seal or Ar protection. Throughout the experiment, oxidation-reduction potential was carefully monitored. In batch tests for determining the effect of pH on nitrate reduction in the presence of Cd, the pH of five 250 ml nitrate solutions with 500 mg/L nZVI and 20 mg/L Cd was maintained for 60 min at pH 7.0, 7.5, 8.0, 8.5, or 9.0, using sodium hydroxide and hydrochloric acid to adjust pH. Aqueous samples were then collected for nitrate, nitrite, and ammonium analyses.

Tests for effect of Me catalysts on nitrate reduction and nitrite yield were carried out in a series of 100 ml conical flasks sealed with screw caps. Freshly-made nZVIeCu, nZVIeAg, or nZVIeAu was added into a 45 mg-N/L nitrate solution with or without 30 mg/L Cd load. The nZVI load was 1500 mg/L. After 90 min, aqueous samples were collected for nitrate, nitrite, and ammonium analyses. Based on the effect of Me catalysts on nitrate reduction, further investigation was done to see the effect of Au on nitrate reduction under different Cd load conditions. nZVIeAu was introduced to a set of 15 mg-N/L nitrate solutions with 10, 20, 30, 40, 50, and 100 mg/L Cd. nZVIeAu concentration was 500 mg/L. Aqueous samples were isolated and analyzed , total Fe after 90 min of shaking at 200 rpm. To simulate Cd and nitrate contaminated groundwater, sodium nitrate and cadmium acetate were added into real groundwater to achieve 15 mg-N/L nitrate and 20 mg/L Cd load. nZVIeAu concentration was still 500 mg/L. After 90 min shaking, aqueous samples were collected and analyzed. Characteristics of the groundwater are shown in Table S1. Nitrite reduction by nZVI or nZVIeAu in the presence of Cd at different pH was performed to better understand nitrite accumulation during nitrate reduction. Aqueous samples were collected for 2 h every 30 min and NO2 , NH4 þ were analyzed. All tests were performed in triplicate.ORP and pH were monitored throughout using a HACH Sc200 . Cadmium and total iron in the collected samples were determined by inductively coupled plasma after filtering with 0.22 mm filter and acidifying with 4% ultrahigh purity HNO3. All nitrogen containing compounds in filtered samples were assessed spectrophotometrically based on a previous study that used a colorimetry technique . A Zetasizer Nano-ZS90 was used to determine the zeta potential of particles. XRD was carried out on a Bruker D8 Advance X-ray diffraction instrument , and the diffraction angle from 10 to 90 was scanned. For preparing samples, the collected solid samples were transferred into a vacuum freeze dryer immediately. After 24 h, the dried samples were analyzed via XRD.As shown in Fig. 1, in the absence of nitrate, 500 mg/L nZVI only completely removed Cd when the initial concentration was 10 mg Cd/L or less. However, with 15 mg-N/L nitrate in solution, complete Cd removal was observed even when the initial Cd concentration was increased to 40 mg/L. Cd removal capacity of nZVI was 40 mg/g in the absence of nitrate, while it reached 80 mg/g when nitrate was present at 15 mg/L. Several studies showed that nZVI reduces nitrate to ammonium, accompanied by increased solution pH ) , which could enhance Cd removal by nZVI through precipitation 2) . Under these conditions we observed that solution pH exceeded 9; while without nitrate the pH was below 8 . Increased pH may therefore be responsible for increased Cd removal capacity of nZVI, due to the presence of nitrate. Additionally, final total Fe concentrations detected in these two reaction systems were also very different. As seen in Fig. 1, total Fe in the supernatant at the end of the experiments exceeded 9 mg/L in all the conditions without nitrate while Fe was not detected in the supernatant of reaction systems with nitrate. The absence of Fe in suspension was probably due to the oxidation to Fe and precipitation stimulated by nitrate.In the series of tests with pH control, Cd removal efficiency decreased as pH decreased both in the presence and absence of nitrate . At pH 9, Cd removal efficiency of both reaction systems were very close.