Total Nitrogen Recovery Research Articles

Exploration of novel DPP-IV inhibitory peptides from discarded eggshell membrane: An integrated in silico and in vitro study

Dipeptidyl peptidase-IV (DPP-IV) inhibitory peptides exhibit great potential to alleviate type II diabetes. This study aimed to prepare, identify and explore the molecular mechanism of DPP-IV inhibitory peptides from eggshell membrane (ESM), an abundant byproduct of the food industry, based on combining in silico and in vitro assessments. In silico evaluation of ten major ESM proteins revealed the great potential of ESM as a novel source of DPP-IV inhibitors. Moreover, the optimal combined-enzymatic strategy (alkaline protease + papain) for the efficient production of DPP-IV inhibitory peptides from ESM was designed by simulated proteolysis. Corresponding with in silico predictions, in vitro assessments confirmed that sodium sulfite assisted the screened dual enzymes led to better production of DPP-IV inhibitory peptides from ESM, with preferable total nitrogen recovery (86.61% ± 4.83%), DPP-IV inhibitory activity (64.61% ± 2.75%) and total antioxidant activity (215.27 ± 1.75 μg/mL). Due to the correlation between diabetes and oxidative stress, the antioxidant and DPP-IV inhibitory activities of the hydrolysate facilitate its amelioration of diabetes. Three novel DPP-IV inhibitory peptides with high abundance in ESM hydrolysates (YPEPPPQ, HDGADVS and VTDGQPH) were identified by LC-MS/MS, in silico characterization and molecular docking. These peptides were synthesized and confirmed to possess DPP-IV inhibitory activity, potentially relating to their stable binding with key residues within the S1, S2 pockets and catalytic triad of DPP-IV via hydrogen bonding and hydrophobic interaction. Overall, this research suggests an effective and time-saving strategy to investigate novel DPP-IV inhibitory peptides from ESM, as well as opens new opportunities for ESM application in antidiabetic foods.

Effects of multi-plant harvesting on nitrogen removal and recovery in constructed wetlands

The harvesting of plants is considered an effective method for nutrient recovery in constructed wetlands (CWs). However, excessive plant harvesting can lead to a decrease in plant biomass. It remains unclear what harvesting frequency can optimize plant nutrient uptake and pollutant removal. In this study, CWs planted with Myriophyllum aquaticum were constructed, and three different frequencies of plant harvesting (high: 45 days/time; low: 90 days/time; none: CK) were set to investigate nitrogen removal and its influencing mechanism, as well as the capacity for plant nutrient recovery. The results showed that the average removal efficiencies of ammonia nitrogen (NH4+-N) at 45 days/time, 90 days/time, and CK were 90.3%, 90.8%, and 88.3% respectively, while the corresponding total nitrogen (TN) were 61.2%, 67.4%, and 67.4%. Dissolved oxygen (DO) concentration and water temperature were identified as the main environmental factors affecting nitrogen removal efficiency. Low harvest frequency (90 days/time) increased DO concentration and NH4+-N removal efficiency without impacting TN removal. Additionally, TN recovery from plants under high and low harvest was found to be approximately 9.21–9.32 times higher than that from no harvest conditions. The above studies indicated that a harvest frequency of every 90 days was one appropriate option for M. aquaticum, which not only increased NH4+-N removal efficiencies but also facilitated more efficient nitrogen recovery from the wetland system.

Different fates and retention of deposited NH4+ and NO3− in the alpine grasslands of the Qinghai-Tibet plateau

The fate and redistribution of nitrogen in terrestrial ecosystems are of paramount significance to nitrogen cycling. Nevertheless, the destiny and content of nitrate (NO3–) and ammonium (NH4+) in grassland ecosystems in the context of atmospheric nitrogen deposition remain unclear. This study utilized field-based 15N tracing experiments to investigate the fate of NO3– and NH4+ in a high alpine meadow on the Tibetan Plateau over three growing seasons, elucidating the patterns of nitrogen allocation in plant stems, roots, and soil. The results indicate that: 1) NO3– and NH4+ exhibit distinct behaviors and distribution patterns in the high alpine grassland ecosystem, with significantly higher plant uptake capacity for NH4+ than for NO3–; 2) Nitrogen was primarily concentrated in the 0–15 cm soil layer during the first two growing seasons, shifting to the 15–30 cm soil layer in the third growing season; 3) Total nitrogen recovery decreased progressively from the first to the third growing season. In summary, with the progression of the growing season, the majority of nitrogen is retained in the soil, and the retained nitrogen is redistributed to aboveground and root components of plants through the soil, with plants exhibiting a preference for NH4+ uptake.

Multi-objective optimized pre-collection of sewage resources by sustainable enhanced coagulation process

Methods by which resources and energy in wastewater are recycled have been gaining popularity as promising and sustainable ways of wastewater treatment. Enhanced coagulation process (ECP) can effectively achieve resources recovery through its synergistic separation of particles, colloids, and dissolved substances. In this study, four significant parameters of ECP influencing simultaneous C, N and P recovery were obtained through the Plackett-Burman method: polyaluminium chloride (PACl), polyferric sulphate (PFS), powder-activated carbon (PAC), and zeolite. Considering the demanding connection between multivariate input and output parameters in practical engineering, their multi-objective parametric interaction and performance were investigated and optimized through the response surface method (RSM) on the basis of the varied values of retention efficiency. Specifically, PFS and PACl exhibited negative synergistic effects on soluble chemical oxygen demand (SCOD) recovery, while their interaction on the retention of total phosphorous (TP) and total nitrogen (TN) were not significant. Meanwhile, PAC and zeolite had positive synergistic effects on the retention of SCOD and on the TN recovery at high dosages, and revealed negative synergistic effects on the retention of TN at low dosages. However, their interaction on TP recovery was insignificant. According to obtained RSM model, the optimized parametric conditions were 20 mg L−1 of PACl, 136 mg L−1 of PFS, 50 mg L−1 of zeolite, and 236 mg L−1 of PAC, achieving the actual maximum values of 69.4%, 92.9% and 45.0% for the collaborative retention of SCOD, TP, and TN respectively. This suggests the promising performance of ECP in multi-objective pre-collection for fast integrated sewage resource recovery.

Preparation and characterization of a decentralized modular yellow water nutrient recovery system

The peculiarity of human urine as a resource, in terms of ethical and cultural considerations, and human perception, demands the development of a highly decentralized modular system, to enable the feasibility of the emerging concept of nutrient recovery in this regard. Consequently, a modular reactor, with packed bed of granular Gastropod shell (GS) was constructed for nutrient recovery from yellow water and the operational parameters were derived in simulated and pilot studies. Prior to the design of the modular reactor, both the raw and calcined GS were screened in a batch reactor for nutrient fraction recovery (i.e. total phosphorus (TP) and total nitrogen (TN)). The TP recovery was more favorable than the TN, but the TN recovery was the rate determining step. Despite the low TN capture efficiency of the reactor, the hydrolysis of the urea fraction that promotes nitrogen loss from yellow water was greatly impeded. The system showed higher selectivity for the nutrient fraction than the other constituents of the human urine, as manifested in the high residual COD and creatinine values in the treated yellow water. The volume of yellow water treated and the nutrient recovery capacity of the modular reactor were HRT dependent. A diminution in the nutrient recovery efficiency was observed in the pilot study, when compared with the simulated studies of the same HRT.

Coupling of biochar-mediated absorption and algal-bacterial system to enhance nutrients recovery from swine wastewater

Algal-bacterial system (ABS) used in treatment of high-strength ammonium wastewaters receives more and more attentions. In this paper, biochar-mediated absorption (BMA) and ABS were applied to recover nutrients from swine wastewater (SW) with high-strength ammonium, respectively. The results showed that the BMA could recover ammonium from the SW, which mitigated ammonia toxicity to the ABS. The bacterial community diversity containing four phyla of bacteria was identified for the first time during nutrients recovery from the SW by the ABS. Proteobacteria and Firmicutes were the two most abundant phyla. A novel scheme for nutrients recovery from the SW by the coupled BMA-ABS method was proposed and evaluated. Nutrients recovery was obviously improved by the coupled BMA-ABS method with biomass concentration of 1.97 g L−1, and a NH4+-N recovery efficiency of 96%, a total nitrogen recovery efficiency of 95%, a total phosphorus recovery efficiency of 96%, and a chemical oxygen demand recovery efficiency of 99%. The coupled BMA-ABS method could enhance nutrients recovery from the SW.

Recovery behavior of high ammonium inhibition in a sequencing batch biofilm reactor

Few studies have investigated the process of microbial recovery in wastewater inhibited by high-strength ammonium but focused on the inhibition effects only. In this study, one contrast way and four efficient ways were developed and applied to five parallel sequencing batch biofilm reactors (SBBRs). The effluent ammonia nitrogen (NH4+-N), nitrite nitrogen (NO2−-N), nitrate nitrogen (NO3−-N), total nitrogen (TN), chemical oxygen demand (COD) and total phosphorus (TP) concentrations were investigated to assess the recovery effects of removal rates as well as the recovery mechanism. Results showed that all of these methods have a positive effect, among which introducing algae was optimum for the fast removal recovery of TN and TP within 4 days. And its recovery time is at least 8 days shorter than that in natural recovery. The recovery ways proposed in this study can be potentially applied in the emergency treatment in SBBR system.

Recovery efficiency and loss of 15N-labelled urea in a rice-soil system under water saving irrigation in the Songnen Plain of Northeast China

From 1980 to 2016, the increase in grain production in the Songnen Plain of Northeast China was significant, which was caused by a continuous increase in the use of nitrogen fertilizer; however, the partial fertilizer productivity (PFP) is gradually decreasing, and a large amount of nitrogen fertilizer is lost to the environment, leading to a series of environment problems. This test combining a field plot test with an in-situ micro-zone test, six treatments were set: two irrigation regimens (controlled irrigation and flooded irrigation) and three N rates (85, 110 and 135 kg ha−1). the recovery and loss of nitrogen fertilizer in a rice-soil system under controlled irrigation regimen were studied. Also, the basal, tillering, and panicle fertilizers were marked, and the absorption and accumulation of different periods 15N-labeled urea under different water and nitrogen control regimens as well as the residual rate of 15N-labeled urea in the soil layer of paddy soil in each period were quantitatively analyzed. Each period nitrogen fertilizer recovery and loss in the rice-soil system were quantitative analyzed. The results showed that the loss rate of the rice-soil system of controlled irrigation was reduced by 15.42–28.64% under the three nitrogen application levels compared to conventional flooded irrigation in the black soil region of the Songnen Plain. The system also maintained the original rice yield and greatly reduced the threat of nitrogen fertilizer loss to the environment in the black soil region. The total nitrogen recovery rate in the rice-soil system ranged from 69.18% to 74.19% with controlled irrigation regimen under the different nitrogen application levels, and the nitrogen recovery rate of the basal fertilizer was 41.46%–48.85%. The nitrogen recovery rate of the tillering fertilizer was 84.20–95.85%, whereas the nitrogen recovery rate of the panicle fertilizer was 89.57–93.80%. The total nitrogen recovery rate of the rice plant was 32.05–38.83%, the recovery rate of the basal fertilizer was 11.78–16.44%, and the recovery rate of the tillering fertilizer was 37.49–39.02%. The recovery rate of panicle fertilizer was 54.15–68.38%. Rice-planting under controlled irrigation can effectively reduce N fertilizer losses and also increase the dry matter accumulation of rice plants, which was found to be 13.27–27.52% more than that of the conventional flooded irrigation regimen. The controlled irrigation regimen not only meets the current requirements for grain yield, but also plays a role in saving water, increases production, and reduces environmental pollution.

Biochemical and Sensory Changes of Low-Salt Anchovy (Engraulis japonicus) Sauce Prepared by a Novel Technique

ABSTRACTUsing a process spanning less than 10 days, a novel technique combining two-stage autolysis and external koji fermentation for the rapid preparation of low-salt anchovy sauce is presented. High-efficiency autolysis of anchovy was achieved by two-stage autolysis, and the second-stage autolysis exhibited a significantly positive effect on the improvement of hydrolysis yield and recovery of anchovy proteins. During the external koji fermentation of hydrolysates from two-stage autolysis, the optimum amounts of added water and salt were determined to be 20% and 8.0%, respectively. By investigating the effect of fermentation time on the biochemical and sensory changes of anchovy sauce, the optimal fermentation time was 8 days. The prepared anchovy sauce had an umami taste, was rich in free glutamate, and contained taurine and γ-aminobutyric acid. In addition, it contained 0.890 g/100 mL amino acid nitrogen (AAN), 1.886 g/100 mL total nitrogen, and 9.76% NaCl. Meanwhile, the recovery of total nitrogen (RTN) was 63.89%.

Effective ammonia recovery from swine excreta through dry anaerobic digestion followed by ammonia stripping at high total solids content

This study investigated ammonia recovery from swine excreta through dry anaerobic digestion (AD) and ammonia stripping. The effects of different operation conditions including temperature (25–55 °C), total solids (TS, 20%–35%) and initial pH (7.0–12.0) were assessed on ammonia production in terms of organic nitrogen (organic-N) conversion and process kinetics through 8 days' trials. All experimental data fitted well to the pseudo first-order kinetic models during dry AD and the maximum organic-N mass conversion ratio was estimated to be 65.1% when dry AD was conducted at 55 °C, 20% TS, and initial pH 10.0. Changes in biogas production and its composition signaled the bioactivity of the bacteria involving in ammonia production from swine excreta during dry AD. Finally, greater than 90% of the total ammonia nitrogen (TAN) in swine excreta was efficiently recovered by air stripping at TS about 20%. Restated, the combination of short-term dry AD with ammonia stripping achieved about 60% of total nitrogen recovery efficiency.

Enhancement of nutrient recovery from microalgae in hydrothermal liquefaction using activated carbon

For large-scale biofuel production from microalgae, the following outcomes are needed to reduce environmental impact and biofuel production cost 1) extraction of a large amount of lipid (bio-oil), 2) recovery of almost all nutrients and their reuse for cultivation, and 3) effective utilization of solid residue (char). We investigated the possibility of using activated carbon to enhance recovery of bio-oil and nutrients in hydrothermal liquefaction (HTL). Microalga Fistulifera solaris was reacted at 573K and 10MPa for 0–60min with or without commercial activated carbon (0.5–5.0wt.% of microalgae) in an autoclave reactor. Results show 0.5wt.% activated carbon can substantially promote total nitrogen recovery in the aqueous phase, probably because of catalytic effect of activated carbon on the decarboxylation reactions to produce amines. Larger amount of activated carbon (>1.0wt.%) decreasd bio-oil recovery due to adsorption. A kinetic study of nitrogen recovery in the aqueous phase showed the apparent reaction order was 3 and the reaction rates were 1.1×10−7s−1 and 5.6×10−8s−1 with and without activated carbon, respectively. 0.5wt.% of activated carbon has an insignificant influence on bio-oil and phosphorus recovery.

Electrochemically driven extraction and recovery of ammonia from human urine

Human urine contains high concentrations of nitrogen, contributing about 75% of the nitrogen in municipal wastewaters yet only 1% of the volume. Source separation of urine produces an ideal waste stream for nitrogen and phosphorus recovery, reducing downstream costs of nutrient treatment at wastewater treatment facilities. We examined the efficiency and feasibility of ammonia extraction and recovery from synthetic and undiluted human urine using an electrochemical cell (EC). EC processing of synthetic urine produced an ammonium flux of 384 ± 8 g N m−2 d−1 with a 61 ± 1% current efficiency at an energy input of 12 kWh kg−1 N removed. EC processing of real urine displayed similar performance, with an average ammonium flux of 275 ± 5 g N m−2 d−1 sustained over 10 days with 55 ± 1% current efficiency for ammonia and at an energy input of 13 kWh kg−1 N removed. With the incorporation of an ammonia stripping and absorption unit into the real urine system, 57 ± 0.5% of the total nitrogen was recovered as ammonium sulfate. A system configuration additionally incorporating stripping of the influent headspace increased total nitrogen recovery to 79% but led to reduced performance of the EC as the urine ammonium concentration decrease. Direct stripping of ammonia (NH3) from urine with no chemical addition achieved only 12% total nitrogen recovery at hydraulic retention times comparable with the EC systems. Our results demonstrate that ammonia can be extracted via electrochemical means at reasonable energy inputs of approximately 12 kWh kg−1 N. Considering also that the hydrogen generated is worth 4.3 kWh kg−1 N, the net electrical input for extraction becomes approximately 8 kWh kg−1 N if the hydrogen can be used. Critical for further development will be the inclusion of a passive means for ammonia stripping to reduce additional energy inputs.

Microbial community and removal of nitrogen via the addition of a carrier in a pilot-scale duckweed-based wastewater treatment system

Carriers were added to a pilot-scale duckweed-based (Lemna japonica 0223) wastewater treatment system to immobilize and enhance microorganisms. This system and another parallel duckweed system without carriers were operated for 1.5years. The results indicated the addition of the carrier did not significantly affect the growth and composition of duckweed, the recovery of total nitrogen (TN), total phosphorus (TP) and CO2 or the removal of TP. However, it significantly improved the removal efficiency of TN and NH4+-N (by 19.97% and 15.02%, respectively). The use of 454 pyrosequencing revealed large differences of the microbial communities between the different components within a system and similarities within the same components between the two systems. The carrier biofilm had the highest bacterial diversity and relative abundance of nitrifying bacteria (3%) and denitrifying bacteria (24% of Rhodocyclaceae), which improved nitrogen removal of the system. An efficient N-removal duckweed system with enhanced microorganisms was established.

Optimization and evaluation of a novel technique for hydrolyzing Antarctic krill (Euphausia superba) proteins

The endogenous proteases of Antarctic krill (Euphausia superba) were efficiently utilized to hydrolyze proteins by a novel technique called two-stage autolysis. The hydrolysates contained 0.951g/100ml amino acid nitrogen, which was sufficient to meet the special grade requirements of soy sauce according to the Chinese National Standard. Meanwhile, the recovery of total nitrogen was 75.33%. The second-stage autolysis exhibited a great positive effect on the improvement of nitrogen recovery. The optimal conditions for obtaining the highest amino acid nitrogen level during the second-stage autolysis were hydrolysate/krill ratio of 2.23, initial pH of 7.23, 40.94°C, which were determined by response surface methodology. The dominant compositions of krill protein hydrolysates were peptides. Proline, glutamate, aspartate, glycine and valine were the dominant amino acids in the hydrolysates, accounting for 15.18, 14.03, 12.14, 10.78 and 10.16mg/ml, respectively. The hydrolysates prepared by two-stage autolysis had high amino acid nitrogen level and high recovery of total nitrogen, which could be used in the preparation of high quality krill sauce.

Autolysis of Anchovy (Engraulis japonicus) Protein: Evaluation of Antioxidant Properties and Nutritional Quality of Resulting Hydrolysates

Protein hydrolysis of underutilized marine organisms is a method to improve added value. In the present article, anchovy protein was autolyzed at pH 8.5 and 50°C. After autolysis processing for 24 h, total nitrogen recovery reached 92.1%. The protein hydrolysates had antioxidant properties on reducing power, 1, 1-diphenyl-2-picryhydrazyl radical, and hydroxyl radical scavenging activity. More than 79% of the peptides in the hydrolysates had a molecular weight of lower than 3 kDa. Protein hydrolysates had balanced amino acid compositions, and total content of essential amino acids had no significant difference after hydrolysis. Protein hydrolysates from anchovy autolysis have potential in nutritional and functional foods.

Separation of total nitrogen from sediments into organic and inorganic forms for isotopic analysis

Nitrogen isotopic (δ 15N) analysis is widely applied in palaeoceanography, palaeolimnology and soil science. Most measurements are performed on untreated bulk samples containing both organic and inorganic nitrogen (ON and IN), which could sometimes confound isotopic signals. Here we propose a sample pretreatment method to physically and quantitatively separate the total nitrogen into ON and IN for subsequent concentration and isotopic measurements. In the method, the sediment is first demineralized with HCl and HCl–HF, adsorbed and bound IN being liberated into the acid solutions during the demineralization. Then, organic matter (OM) dissolved in the acid solutions is recovered by a sorbent of styrene divinyl benzene polymer (Bond Elut PPL, Varian) and is then combined with the acid-resistant OM to obtain the total OM. Finally, IN in the acid solutions passing through the PPL sorbent is recovered by way of steam distillation. Total nitrogen recovery with the method is 92.4% and isotopic mass balance calculations show that the δ 15N value of total nitrogen can be quantitatively partitioned into the organic and inorganic forms. The δ 13C values of the separated OM show a small positive excursion of 0.2‰ relative to those of bulk OM, ruling out significant loss of ON during the pretreatment. By comparison, the KOBr method, widely used to remove ON from bulk samples for isotope measurements, gives lower IN concentration and higher δ 15N IN values, attributed to the partial loss of IN and escape of some oxidation-resistant ON.

Moisture and vegetation controls on decadal‐scale accrual of soil organic carbon and total nitrogen in restored grasslands

AbstractRevitalization of degraded landscapes may provide sinks for rising atmospheric CO2, especially in reconstructed prairies where substantial belowground productivity is coupled with large soil organic carbon (SOC) deficits after many decades of cultivation. The restoration process also provides opportunities to study the often‐elusive factors that regulate soil processes. Although the precise mechanisms that govern the rate of SOC accrual are unclear, factors such as soil moisture or vegetation type may influence the net accrual rate by affecting the balance between organic matter inputs and decomposition. A resampling approach was used to assess the control that soil moisture and plant community type each exert on SOC and total nitrogen (TN) accumulation in restored grasslands. Five plots that varied in drainage were sampled at least four times over two decades to assess SOC, TN, and C4‐ and C3‐derived C. We found that higher long‐term soil moisture, characterized by low soil magnetic susceptibility, promoted SOC and TN accrual, with twice the SOC and three times the TN gain in seasonally saturated prairies compared with mesic prairies. Vegetation also influenced SOC and TN recovery, as accrual was faster in the prairies compared with C3‐only grassland, and C4‐derived C accrual correlated strongly to total SOC accrual but C3‐C did not. High SOC accumulation at the surface (0–10 cm) combined with losses at depth (10–20 cm) suggested these soils are recovering the highly stratified profiles typical of remnant prairies. Our results suggest that local hydrology and plant community are critical drivers of SOC and TN recovery in restored grasslands. Because these factors and the way they affect SOC are susceptible to modification by climate change, we contend that predictions of the C‐sequestration performance of restored grasslands must account for projected climatic changes on both soil moisture and the seasonal productivity of C4 and C3 plants.

Degradation Kinetics and Mechanism of RDX and HMX in TiO2 Photocatalysis

This study was undertaken to examine the photocatalytic degradation of explosives hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) with a circular photocatalytic reactor, using a UV lamp as a light source and TiO2 as a photocatalyst. The effects of various parameters, such as the RDX or HMX concentration, the amount of TiO2, and the initial pH, on the photocatalytic degradation rates of explosives were examined. In the presence of both UV light and TiO2, RDX and HMX were more effectively degraded than with either UV or TiO2 alone. The degradation rates were found to obey pseudo-first-order kinetics represented by the Langmuir-Hinshelwood model. Increases in the RDX and HMX degradation rates were obtained with decreasing initial concentrations of the explosives. The RDX and HMX degradation rates were higher at pH 7 than at either pH 3 or pH 11. A dose of approximately 0.7 g l−1 of TiO2 degraded HMX more rapidly than did higher or lower TiO2 doses. RDX (20 mg l−1) photocatalysis resulted in an approximately 20% decrease in TOC, and HMX (5 mg l−1) photocatalysis resulted in a 60% decrease in TOC within 150 minutes. A trace amount of formate was produced as an intermediate that was further mineralized by RDX or HMX photocatalysis. The nitrogen byproducts from the photocatalysis of RDX and HMX were mainly NO3 − with NO2 − and NH4 +. The total nitrogen recovery was about 60% from RDX (20 mg l−1), and 70% from HMX (5 mg l−1), respectively. Finally, a mechanism for RDX/HMX photocatalysis was proposed, along with supporting qualitative and quantitative evidence.

Fluidized zero valent iron bed reactor for nitrate removal

A fluidized zero valent iron (ZVI) reactor is examined for nitrate reduction. Using the system, the pH of solution can be maintained at optimal conditions for rapid nitrate reduction. For hydraulic retention times of 15 min, the nitrate reduction efficiency increases with increasing ZVI dosage. At ZVI loadings of 33 g l −1, results indicate that the nitrate removal efficiency increases from less than 13% for systems without pH control to more than 92% for systems operated at pH of 4.0. By maintaining pH at 4.0, we are able to decrease the hydraulic retention time to 3 min and still achieve more than 87% nitrate reduction. The recovery of total nitrogen added as nitrate, ammonium, and nitrite was less than 50% for the system operated at pH 4.0, and was close to 100% for a system without pH control. The possibility of nitrate and ammonium adsorption onto iron corrosion products was ruled out by studying the behavior of their adsorption onto freshly hydrous ferric oxide at variable pH. Results indicate the probable formation of nitrogen gas species during reaction in pH 4.0.

SIMULTANEOUS DETERMINATION OF NITRITE AND NITRATE IN DRINKING WATER AND HUMAN SERUM BY HIGH PERFORMANCE ANION-EXCHANGE CHROMATOGRAPHY AND UV DETECTION

A rapid, accurate, and sensitive method has been developed for the simultaneous determination of nitrite and nitrate. A low capacity strong anion exchange PRP-X100 Hamilton, 150 × 4.1 mm, 10 μm, with exchange capacity 0.19 ± 0.02 meq/g, analytical column was used, with a mixture of 0.1 M NaCl-CH3OH, at a volume ratio 45:55. The flow rate of 2 mL/min, lead to a pressure of 150 kg/cm2. Detection was performed with a variable wavelength UV-visible detector, at 230 nm, resulting in detection limits of 0.1 ng and 0.2 ng for nitrite and nitrate, respectively, per 20 μL injection. For the quantitative determination, iodide was used as internal standard, at a concentration of 7.0 ng/μL. A rectilinear relationship was observed up to 12 ng/μL for nitrite and up to 25 ng/μL for nitrate. Analysis time was less than 6 min. The statistical evaluation of the method was examined performing intra–day (n = 8) and inter-day calibration (n = 8) and was found to be satisfactory, with high accuracy and precision results. The method was applied to the direct determination of nitrite and nitrate in drinking water (tap, table, and ground water) and in biological fluids (human serum). Solid phase extraction was tested for sample clean-up and analyte retention using SAX Lichrolut Merck cartridges. Low recovery of nitrite ca. 70% and high recovery of nitrate, approximately 130%, indicated the partial oxidation of nitrite during sample preparation. Solid phase extraction of solutions containing solely nitrite proved that a 40% oxidation is taking place, while the solid phase extraction of spiked serum samples yielded a 30% oxidation of nitrite anions. Therefore, a sample pre-treatment assay avoiding the solid phase extraction technique was applied, leading to approximately 80% recovery of nitrite and 120% recovery of nitrate. The pre-treatment involves protein precipitation, using organic solvent and centrifugation of the sample. Endogenous compounds of human serum did not interfere in both cases, after SPE or without SPE. Total nitrogen recovery appeared to be quite the same, 98.6% and 98.0% respectively. Ascorbic acid used as anti-oxidant did not prevent the oxidation of nitrite.