Exponential Collapse of Benthic Depurative Capacity in A Eutrophic Tropical Marine Ecosystem: A Threshold Response to Organic Nitrogen Loading

Microbial decomposition of particulate organic matter in sediments can substantially modulate eutrophication of nearshore marine environments. Flux budgets for nitrogen compounds and quantification of rates of microbial transformation can provide important information on the process of eutrophication. This survey documents sediment nitrogen budgets for a eutrophic nearshore marine environment at La Parguera on southwest coast of Puerto Rico, including consideration of the organic fraction in addition to microbial transformations.

Sediments of the inshore channel at La Parguera denote the input of heavy organic loads with low redox potentials and high ammonium contents. Benthic fluxes of dissolved organic nitrogen are below those recorded for ammonium. These observations suggest that nearshore tropical sediments subject to heavy nitrogen loads act as a sink for organic nitrogen and that ammonification is a key process in the release of nitrogen from sediments to the water column. Nitrification and denitrification rates are low overall and inversely related to the redox potential. Depuration of excess nitrogen through denitrification is ineffective in these sediments. in contrast to more robust temperate environments, our work demonstrates that tropical marine systems are particularly susceptible to eutrophication given their limited capacity for depuration of excess nitrogen.     

Nitrogen cycling in microbial mats: rates and patterns of denitrification and nitrogen fixation

Spatial and temporal variations in nitrogen fixation and denitrification rates were examined between July 1991 and September 1992 in the intertidal regions of Tomales Bay (California, USA). Microbial mat communities inhabited exposed mudflat and vegetated marsh surface sediments. Mudflat and marsh sediments exhibited comparable rates of nitrogen fixation. Denitrification rates were higher in marsh sediments. Nitrogen fixation rates were lowest during January at both sites, whereas highest rates occurred during summer and fall. Denitrification rates were highest during fall and winter months in marsh sediments, while rates in mudflat sediments were highest during summer and fall. In mudflat sediments, nitrogen fixation and denitrification rates, integrated over 24 h, ranged from 6 to 79 mg N m^-1 d^-1 and 1 to 10 mg N m^-2 d^-1, respectively. Rates of denitrification represented between 6 and 20% of nitrogen fixation rates during the day, but exceeded or were equivalent to nitrogen fixation rates at night. The highest integrated rates of both nitrogen fixation and denitrification occurred during July, whereas, the highest percent loss occurred during spring when denitrification rates amounted to 20% of nitrogen fixation rates during the day. Over an annual cycle, inputs of fixed N to mudflat communities occurred exclusively during daylight. These results underscore the importance of determining integrated diel rates of both nitrogen fixation and denitrification when constructing N budgets. Using this approach, it was shown that microbial denitrification can represent a significant loss of combined nitrogen from mats on daily as well as monthly time scales.     

The enigma of progress in denitrification research.

Humans have dramatically increased the amount of reactive nitrogen (primarily ammonium, nitrogen oxides, and organically bound N) circulating in the biosphere and atmosphere, creating a wide array of desirable products (e.g., food production) and undesirable consequences (e.g., eutrophication of aquatic ecosystems and air pollution). Only when this reactive N is converted back to the chemically unreactive dinitrogen (N2) form, do these cascading effects of elevated reactive N cease to be of concern. Among the quantitatively most important processes for converting reactive N to N2 gas is the biological process of classical denitrification, in which oxides of nitrogen are used as terminal electron acceptors in anaerobic respiration. This Invited Feature on denitrification includes a series of papers that integrate our current state of knowledge across terrestrial, freshwater, and marine systems on denitrification rates, controlling factors, and methodologies for measuring and modeling denitrification. In this paper, we present an overview of the role of denitrification within the broader N cycle, the environmental and health concerns that have resulted from human alteration of the N cycle, and a brief historical perspective on why denitrification has been so difficult to study. Despite over a century of research on denitrification and numerous recent technological advances, we still lack a comprehensive, quantitative understanding of denitrification rates and controlling factors across ecosystems. Inherent problems of measuring spatially and temporally heterogeneous N2 production under an N2-rich atmosphere account for much of this slow progress, but lack of interdisciplinary communication of research results and methodological developments has also impeded denitrification research. An integrated multidisciplinary approach to denitrification research, from upland terrestrial ecosystems, to small streams, river systems, estuaries, and continental shelf ecosystems, and to the open ocean, may yield new insights into denitrification across landscapes and waterscapes.     

Heavy metal and nutrient contamination of marine sediments in Hong Kong

A monitoring programme was performed to use marine sediments as an indicator of marine contamination in Hong Kong. A total of 51 samples were collected from eight marine sites and analysed for pH, redox potential, salinity, total‐N, total‐P, total organic carbon, and total Cu, Zn Pb, Cr and Cd. Sites with high industrial or aquacultural activities contained high total organic carbon contents, total‐N and total‐P contents. Sediments from sites with high industrial activities also enriched with heavy metals. Metal speciation results indicate that Cu and Cr existed mainly in the organic fraction, Pb and Zn were distributed approximately equally in both the organic and residual fractions while Cd mainly occurred in the residual fraction. The Cu, Cr and Zn contents were highly correlated with total organic carbon contents. All samples were classified as non‐acid forming according to the net acid generation test and only samples from the industrial site released a high concentration of heavy metals under a complete oxidising condition. The present study reveals that organic and heavy metal pollution is serious in the marine sediments of Hong Kong, and industrial and aquacultural activities are probably the major sources of contamination.     

Assessment of a model of pollution disaster in near-shore coastal waters based on catastrophe theory

In this study, a model for assessing of environmental disasters in near-shore areas was developed using a multi-criteria evaluation method of catastrophe theory. The assessment model involved scenarios of eutrophication, pollution with heavy metals and organic compounds. An evaluation system of the model was composed of seven mesosphere indicators and twenty underlying indicators including water chemistry, water physics, water biology, heavy metals and organic pollutants in water and surface sediments. The model was applied to possibility assessment of environmental disasters in different functional regions of the Dalian Bay in 2001 and 2006. Results showed that the environmental disaster indicators in 2001 were equivalent to the Level 4 standard values of marine functional areas, but the eutrophication disaster indicators were lower than the Level 4 standard values. It is consistent with the occurrence of a large-scale red tide in Dalian Bay in 2001. In 2006, eutrophication remained the dominant problem of the region but organic pollutants, such as oil, were reduced remarkably. This coincided with ongoing local environmental-friendly practices for industries.     

Effects of mussel aquaculture on the nitrogen cycle and benthic communities in Kenepuru Sound,Marlborough Sounds,New Zealand

Nitrogen pools and transformations and benthic communities at a Perna canaliculus farm and a nearby reference site without direct influence of marine farming in Kenepuru Sound, New Zealand, were compared on four dates between September 1982 and May 1983. The organic nitrogen pool in the top 12 cm sediment was 7.4 to 10.8 mol m^-2 at the mussel farm and 6.1 to 8.9 mol m^-2 at the reference site. The nitrate and nitrite pools were similar in both sediments, but the ammonium pool in the mussel farm sediment was about twice as high as in the reference sediment. In January, the sediment ammonium concentrations ranged from 418 nmol cm^-3 (surface) to 149 nmol cm^-3 (12 cm depth) at the mussel farm and from 86 to 112 nmol cm^-3 at the reference site. The molar C:N ratio of the sediment organic matter was 6.2 to 7.2 at the mussel farm and 7.9 to 10.0 at the reference site. The molar N:P ratio of the sediment organic matter was 4.3 to 7.2 and 3.3 to 6.1 at mussel farm and reference site, respectively. The total nitrogen mineralisation rate in the top 12 cm sediment ranged from 21.7 to 37.1 mmol m^-2 d^-1 at the mussel farm and from 8.5 to 25.0 mmol m^-2 d^-1 at the reference site. Ammonium excretion by mussels was about 4.7% (January) and 7.4% (May) of the combined nitrogen mineralisation by mussels and sediment. The sediment-denitrification rate was 0.7 to 6.1 mmol m^-2 d^-1 at the mussel farm and 0.1 to 0.9 mmol m^-2 d^-1 at the reference site. In January, 76 and 93% of the nitrate reduced in the sediments were denitrified at the mussel farm and reference site, respectively. The denitrification rate on the mussel lines (determined on detritus-covered mussels) was twice the mussel farm sediment-denitrification rate and 10 times the reference sediment-denitrification rate. Total denitrification at the mussel farm was 21% higher than at the reference site. The loss of nitrogen through mussel harvest and denitrification was 68% higher at the mussel farm. The surface layers of both sediments contained about 75 mg m^-2 chlorophyll a. Sediment phaeophytin levels were 52 mg m^-2 at the reference site and 137 mg m^-2 at the mussel farm. While the benthic infauna of the mussel-farm sediment consisted only of polychaete worms, the reference sediment contained also bivalve molluscs, brittle stars and crustaceans.     

Phosphate enrichment in the northern Gulf of Aqaba: Regulation by carbonate sediments and impact on nitrogen elevation

The effect of carbonate sediment in regulating phosphate concentrations in sea water was investigated by laboratory incubation experiments using different sediment types. Incubation experiments were made with two types of sediments: uncontaminated sediment from a marine reserve and contaminated sediment with deposited phosphate powders. Fluxes of inorganic nitrogen and phosphate were estimated from linear regressions of solute concentrations over incubation periods. Ammonium and phosphate fluxes were about twofold higher in the uncontaminated sediment that had significantly lower organic carbon and total phosphate concentrations than in the phosphate-contaminated sediment. To test the effect of dissolved phosphate on increasing nitrogen fixation, additional incubation experiments were carried out using treated carbonate and silicate sediments with added dissolved phosphate (20 μM). Incubations were made under sterile conditions with HgCl₂ added to distinguish between biologically enhanced processes and pure physicochemical processes. The adsorption rate of phosphate onto carbonate sediment was about twice that onto silicate sediments. No nitrogen elevation either as ammonium or as nitrate was observed in the soluble phosphate enrichment incubations. In conclusion, this study demonstrates the importance of the regulation of soluble phosphate concentrations in carbonate sediment environments where the carbonate sediment acts as a buffering system keeping soluble phosphate concentrations at certain steady-state levels. The study also demonstrates the lack of evidence on enhancement of nitrogen concentrations due to the increase phosphate concentrations.     

影响厌氧氨氧化与甲烷化反硝化耦合的因素

氨氮、氮氧化物对产甲烷菌有一定的抑制作用,但可以通过驯化去除毒性.亚硝酸盐在厌氧氨氧化菌作用下与氨发生厌氧氨氧化反应.虽然厌氧氨氧化菌是自养菌,但具有异养代谢能力,并且NO2可提高厌氧氨氧化菌的活性.因此,通过特殊的反应器技术,将厌氧氨氧化菌与甲烷菌、反硝化菌复合在一个有利的微生态环境中,充分发挥它们之间的协同耦合作用,把有机物转化为清洁能源又同时脱氮,是极有前景的废水厌氧(缺氧)处理研究新方向.表1参31     

Comparative measurement of the redox potential of marine sediments as a rapid means of assessing the effect of organic pollution

A rapid method for measuring the redox potential (Eh) values of marine sediments is described. This method has been used to relate changes in the areas of highly reduced sediment in a sea loch system to the continuously fluctuating inputs of organic waste from a pulp and paper mill. Subsequent faunal changes in these areas can be related to changes in the measured sedimentary redox level. The method is suggested as a rapid means of assessing the potential impact of an additional organic input to a marine sediment.     

沉积物不同天然有机组分对氨氮吸附特征的影响

为估算沉积物不同天然有机组分吸附态氨氮携载量．采用平衡吸附法研究西辽河沉积物不同天然有机组分对氨氮吸附特征的影响。结果表明,去除有机质后的沉积物对氨氮的吸附能力大大降低,其碳标化饱和吸附量（Гmoc）和吸附分配系数（Koc）分别为重组的55．30％和69．49％,说明有机质是影响氨氮在沉积物上吸附特征的主要因素。氨氮在轻组有机组分上的吸附以分配作用为主（Koc=85．57）;稳结态和紧结态腐殖质是形成沉积物疏松多孔团聚体结构的重要胶结物质,氨氮在重组有机组分上的吸附除分配作用外,还存在孔隙填充方式的吸附;重组有机组分中的紧结态腐殖质（胡敏素）对氨氮吸附起关键作用（Гmoc,=5857．78mg·kg-1）。轻组有机质、稳结态腐殖质和紧结态腐殖质携载的吸附态氨氮可分别按重组（Гmoc=3477．81mg·kg-1）的0．32、1．21和1．68倍估算。关键词：沉积物;天然有机组分;氨氮;吸附;碳标化吸附分配系数;碳标化饱和吸附量     

Denitrification across landscapes and waterscapes: a synthesis.

Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here, we compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater, and marine systems in which denitrification occurs can be organized along a continuum ranging from (1) those in which nitrification and denitrification are tightly coupled in space and time to (2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs influence denitrification rates whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves. Spatially distributed global models of denitrification suggest that continental shelf sediments account for the largest portion (44%) of total global denitrification, followed by terrestrial soils (22%) and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification. Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per-area denitrification rates in soils and groundwater (kg N x km(-2) x yr(-1)) are, on average, approximately one-tenth the per-area rates of denitrification in lakes, rivers, estuaries, continental shelves, or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.     

Floodplain restoration enhances denitrification and reach-scale nitrogen removal in an agricultural stream

Streams of the agricultural Midwest, USA, export large quantities of nitrogen, which impairs downstream water quality, most notably in the Gulf of Mexico. The two-stage ditch is a novel restoration practice, in which floodplains are constructed alongside channelized ditches. During high flows, water flows across the floodplains, increasing benthic surface area and stream water residence time, as well as the potential for nitrogen removal via denitrification. To determine two-stage ditch nitrogen removal efficacy, we measured denitrification rates in the channel and on the floodplains of a two-stage ditch in north-central Indiana for one year before and two years after restoration. We found that instream rates were similar before and after the restoration, and they were influenced by surface water NO3- concentration and sediment organic matter content. Denitrification rates were lower on the constructed floodplains and were predicted by soil exchangeable NO3- concentration. Using storm flow simulations, we found that two-stage ditch restoration contributed significantly to NO3- removal during storm events, but because of the high NO3- loads at our study site, < 10% of the NO3- load was removed under all storm flow scenarios. The highest percentage of NO3- removal occurred at the lowest loads; therefore, the two-stage ditch's effectiveness at reducing downstream N loading will be maximized when the practice is coupled with efforts to reduce N inputs from adjacent fields.     

Nitrate metabolism in sediments from seagrass (Zostera capricorni) beds of Moreton Bay,Australia

The fate of nitrate in sediments from seagrass (Zostera capricorni Aschers.) beds of Moreton Bay on the subtropical eastern coast of Queensland, Australia, was investigated. Added nitrate was metabolised at rates of 0.4 to 3.4 g N cm^-3 d^-1 when sediments were incubated under anaerobic conditions with a large excess of nitrate. The potential rate of nitrate utilization was as rapid in sediments from subtidal bare areas as from adjacent seagrass beds. Ammonium was produced rapidly from¹⁵N-nitrate by microbial action in all the subtidal sediments examined. After 12 h of incubation, 13 to 28% of the¹⁵N initially added as labelled nitrate was detected as labelled ammonium in the sediments. Denitrification, although not measured directly, appeared to be a relatively minor fate of nitrate. Benthic microbes took up large amounts of¹⁵N but only after a delay of 6 h; this pattern could have been due to induction and synthesis of the enzymes necessary for nitrate uptake, and the assimilation of labelled ammonium. Under field conditions, assimilation by seagrasses and denitrification by bacteria were probably not significant sinks for nitrate in comparison with uptake by benthic microbes and dissimilatory reduction to ammonium.     

Exploring the ‘Birch effect’ in reservoir sediments: influence of inundation history on aerobic nutrient release

The release of nutrients on wetting (‘the Birch effect’) is well documented for terrestrial soils. The effect for aquatic sediments has received much less attention. An extreme drawdown event in a large reservoir during a period of drought presented an opportunity to test whether or not reflooding dried sediments would be a source of nutrients to the overlying water column. Wetting and drying history influences the release of ammonium, but not oxides of nitrogen, organic nitrogen, filterable reactive phosphorus or total phosphorus, from dried sediments following re-wetting. Dried reservoir sediments that have never, or have only rarely, been desiccated since the 1920s produced a pulse of ammonium on re-inundation. Conversely, dried sediments from parts of the reservoir that undergo frequent wetting and drying cycles do not release substantial amounts of ammonium on rewetting.     

Nitrogen versus phosphorus limitation in a subtropical coastal embayment (Moreton Bay; Australia): Implications for management

An approach combining nutrient budgets, dynamic modelling, and field observations of phytoplankton and nitrogen (N₂)-fixing Lyngbya majuscula following changes in wastewater N loads, was used to demonstrate that Moreton Bay is potentially phosphorus (P) limited. Modelling and nutrient budgeting shows that benthic N-fixation loads are high, allowing the system to overcome any potential N-limitation. Phytoplankton biomass has shown little change from 1991 to 2006 in the sections of Moreton Bay most impacted by wastewater effluents, despite a large reduction in wastewater N loads from 2000 to 2002. This is consistent with modelling that also showed no reduction in primary productivity associated with reduced N loads. Most importantly, there have been rapid increases in the occurrence of N-fixing L. majuscula in Moreton Bay as wastewater P loads have increased relative to wastewater N loads. This is also consistent with modelling. This work supports the premise that there may be fundamental differences in nutrient limitation of primary production between subtropical and temperate coastal systems due to differences in the importance of internal nitrogen sources and sinks (N-fixation and denitrification). These differences need to be recognised for optimum management of coastal systems.     

Geochemical index of trace metals in the surficial sediments from the western continental shelf of India, Arabian Sea

The present study focuses on the determination and abundance of trace metals (viz. Cu, Ni, Zn, Cr, Co, Cd, Mn and Fe) in the surficial sediments of west coast of Arabian Sea along the Indian subcontinent. Sediment samples were collected from three transects along the western continental shelf of Arabian Sea. The enrichment of Fe and Mn in coastal oxic-sediments indicates the precipitation of these redox sensitive elements as Fe- and Mn-hydroxides and oxides, whereas the low Fe and Mn concentrations in the oxygen deficient sediments of deeper stations reflects the dissolution of their hydroxides and oxides. Concentrations of fairly redox insensitive trace metals like Cu, Ni, Zn, Cr and Cd (with the exceptions of Cr) showed higher values at nearshore sediments, then it decreased towards seaward and again showed a slight increase at oxygen minimum stations in all the three transects. This geochemical variability in their distributional characteristics is mainly associated with the extent to which the precipitation or dissolution of Fe- and Mn-oxides/hydroxides occur since the scavenging or releasing effects of Fe- and Mn-oxides/hydroxides act as significant ‘sinks’ or ‘sources’ of heavy metals. The change in wind pattern, coastal upwelling and increased productivity are also the reported factors which influence the biogeochemical cycling of trace metals in the surface sediments of west coast of India. Enrichment factor generally showed a high gradient accumulation from nearshore to shelf.     

Characterization of arsenic content in marine organisms from temperate, tropical, and polar environments

Arsenic is a widely distributed element which occurs in several chemical forms in the marine environment. Inorganic arsenic mediates the most toxic effects and predominates in sea water and sediments, while organisms generally accumulate non-toxic organic forms to concentrations probably reflecting species-specific characteristics in arsenic metabolism. This work represents an additional contribution to our knowledge on natural levels and chemical speciation of arsenic in marine organisms; basal concentrations were characterized in several species (bivalves, crustaceans, and fishes) from different environments (polar, temperate, and tropical latitudes), and results revealed an elevated variability with values ranging from less than 5 to about 200 μg g^-1. No significant effects were observed as a function of the geographical area, with the only exception of crustaceans always showing more elevated arsenic concentrations in Mediterranean species (about 45-110 μg g^-1) compared with tropical species (lower than 30 μg g^-1). Chemical speciation of arsenic was investigated in representative species from the three studied taxa; the predominance of organic forms confirmed the general tendency of marine organisms to bioaccumulate non-toxic arsenic compounds, probably resulting from a detoxification pathway.     

珠江三角洲第四纪各地层生成与存储天然铵能力的探讨与对比

珠江三角洲第四纪底部含水层当中的天异常高然铵,来自于第四纪沉积当中的有机质在厌氧条件下的矿化。珠江三角洲第四纪晚期的沉积序列,主要包括两层海相沉积层（M1和 M2）,以及两层陆相沉积层（T1和 T2）。然而,截至目前,上述地层对天然铵的生成与存储作用,尚不明确。从三角洲的内陆到近岸,选取重点钻孔BJ8、SD1、SD14和MZ4,利用准确的定年数据,以及钻孔剖面各类铵以及总有机碳等数据,探讨与比较了珠江三角洲第四纪各地层生成与储存天然铵的能力。结果表明：全新世时期的海相沉积层（M1）,具有高达17.4 g·kg-1的沉积有机质质量分数。对 SD14钻孔剖面上高精度的有机碳分析表明,M1、M2层平均有机质质量分数分别为11.7和10.1 g·kg-1。因此,相对于晚更新世时期的海相沉积层（M2）来说,全新世海相沉积层M1具备生成更多铵的能力。对SD14钻孔剖面上各类铵的分析表明,M1和M2总铵质量分数的平均值分别为0.41和0.31 g·kg-1;M1、M2和T2各地层单位面积所储存铵的平均值分别为28.6、11.25和0.34 kg·m-2。而不同地层铵含量的差异,在该研究关注的其他钻孔 BJ8,SD1和 MZ4也非常明显。因此得出结论,全新世海相沉积层M1,是主要的储铵层,而M2则是次要的储铵层。两个陆相沉积层T1和T2,不论在生铵和储铵的功能上,远远小于两个海相沉积层。在 M1层中,铵的量呈现随深度增加而升高的趋势,原因在于铵不断生成累积,并通过扩散作用向下运移。而M2层中铵的含量呈现由上至下递减的趋势,说明M2层中的铵主要来自于上部M1层的扩散,其本身生成铵的量比M1少。M1层在珠江三角洲广泛发育,M2层经过长期的风化和剥蚀,在珠江三角洲许多地方已缺失。珠江三角洲底部含水层中天然高铵的浓度,主要由M1层生成与存储的铵的总量所决定。     

Trophic state,ecosystem efficiency and biodiversity of transitional aquatic ecosystems: analysis of environmental quality based on different benthic indicators

Estuaries and coastal lagoons are characterized by a strong spatial and temporal variability of physicochemical characteristics and productivity patterns. In these environments, the magnitude and direction of the ecological responses to inorganic nutrient increase (i.e. eutrophication) are difficult to predict. In the framework of the project, New Indicators of Trophic state and environmental quality of marine coastal ecosystems and transitional environments (NITIDA), we analysed benthic indicators of trophic state, ecosystem efficiency, and environmental quality in four different transitional environments. The trophic state of the sediments was assessed in terms of quantity and bioavailability of sediment organic C pools; ecosystem efficiency was determined in terms of the prokaryote efficiency in exploiting enzymatycally degraded organic C; environmental quality was determined in terms of meiofaunal diversity. Here, we provide a synopsis of the results obtained and a meta-analysis of the scores assessments obtained using the different ecological indicators of environmental quality and demonstrate that trophic state, ecosystem efficiency, and biodiversity in transitional ecosystems are closely linked. We conclude that the assessment of the environmental quality of transitional ecosystems should be based upon a battery of trophic state indicators and ‘sensors’ of ecosystem functioning, efficiency, and quality.     

Improved nitrogen removal in dual-contaminated surface water by photocatalysis

River waters in China have dual contamination by nutrients and recalcitrant organic compounds. In principle, the organic compounds could be used to drive denitrification of nitrate, thus arresting eutrophication potential, if the recalcitrant organics could be made bioavailable. This study investigated the potential to make the recalcitrant organics bioavailable through photocatalysis. Batch denitrification tests in a biofilm reactor demonstrated that dual-contaminated river water was short of available electron donor, which resulted in low total nitrogen (TN) removal by denitrification. However, the denitrification rate was increased significantly by adding glucose or by making the organic matters of the river water more bioavailable through photocatalysis. Photocatalysis for 15 min increased the Chemical Oxygen Demand (COD) of the river water from 53 to 84 mg·L^-1 and led to a 4-fold increase in TN removal. The increase in TN removal gave the same effect as adding 92 mg·L^-1 of glucose. During the photocatalysis experiments, the COD increased because photocatalysis transformed organic molecules from those that are resistant to dichromate oxidation in the COD test to those that can be oxidized by dichromate. This phenomenon was verified by testing photocatalysis of pyridine added to the river water. These findings point to the potential for N removal via denitrification after photocatalysis, and they also suggest that the rivers in China may be far more polluted than indicated by COD assays.