Environ. Sci. Technol. 2009, 43, 3681–3687
Effect of Suspended-Sediment Concentration on Nitrification in River Water: Importance of Suspended Sediment-Water Interface XINGHUI XIA,* ZHIFENG YANG, AND XUEQING ZHANG School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing, 100875, China
Received December 25, 2008. Revised manuscript received March 14, 2009. Accepted March 19, 2009.
High suspended sediment (SPS) concentrations exist in many Asian river systems. In this research, the effects of SPS concentration on nitrification in river water systems were studied. With or without introducing ammonium-oxidizing bacteria isolated from the water and sediment samples of the Yellow River, the microbially mediated nitrification rate increased with SPS concentration as described by the power function y ) a · xb, where y is the nitrification rate, x is the SPS concentration, and a and b are constants. With an indigenous ammoniumoxidizing bacteria, nitrification rate constants, i.e., K4 (µmax/KS) values obtained from the Monod model, were 0.0016, 0.0036, 0.0040, 0.0063, 0.0066, 0.0071, and 0.0077 day-1 µM-1 for the systems with SPS concentrations of 0, 0.2 1.0, 5.0, 10, 20, and 40 g/L, respectively. The sorption percentage of NH4+-N increased with SPS concentration as a power function. Bacteria tend to attach onto SPS, and the maximum specific growth rate at the SPS-water interface was about twice that in the water phase. An increase of bacterial population and nitrification rate constant with SPS as a power function resulted in an increase of nitrification rate with SPS as a power function. Therefore, the high SPS concentration caused by erosion and bottom sediment resuspension and other factors will accelerate ammonium oxidation in many turbid river systems. This has useful implications for nitrogen removal from river systems.
1. Introduction Nitrogen is one of the most important macro nutrients for aquatic organisms. However, discharge of nitrogen to receiving waters can lead to significant impacts on water quality which, with high levels of phosphorus, can cause excessive growth of phytoplankton and eutrophication of water bodies. The rates of nitrogen uptake (by plants) and the effects on aquatic organisms are different from one nitrogen species to another. Nitrification is a fundamental process of the nitrogen cycle in aquatic systems in which ammonia is converted to nitrite, then to nitrate (1, 2). Nitrate produced by nitrification is available for denitrification process and thus has an impact on nitrogen removal from * Corresponding author tel: +86-10-58805314; fax: +86-1058805314; e-mail:
[email protected]. 10.1021/es8036675 CCC: $40.75
Published on Web 04/08/2009
2009 American Chemical Society
water systems (3, 4). Consequently, many studies of nitrification in freshwater systems have been conducted in recent decades (5-8). Nitrification was considered to be a surface-based process in water and wastewater systems (9-11). Previous studies indicate that nitrifying bacteria tend to grow by attaching to the surface of sediment particles (5, 6, 12). This is especially true in shallow streams (13-15). Increasing evidence has demonstrated the important roles of sediment-based nitrification in rivers and lakes (16-18). For example, Pauer and Auer (19) noted that the nitrification rate was rapid in the sediments; whereas lack of nitrification was observed in the water column of a hyper-eutrophic lake and the adjoining river system. Gribsholt et al. (20) measured total system and water column nitrification of a tidal freshwater marsh and showed the dominance of particle/sediment related nitrification. In addition, many researchers have studied the nitrification rates and the effects of fluff deposition, organic carbon, nitrogen availability, and pH on nitrification in sediments (21-24). Several research results have shown that nitrate production takes place in an aerobic surface zone that has a maximum thickness of a few millimeters in most shallowwater sediments (25, 26). However, most of the previous studies of nitrification in fresh water systems considered only nitrification in water and/or the bed-sediment phases. There are few studies on nitrification processes in fresh water bodies with a high suspended sediment (SPS) concentration as commonly exists in many Asian river systems (27, 28). For example, the SPS concentration is so high in the Yellow River of China that concentration is reported in g/L rather than the usual mg/L, with an average SPS concentration of 28 g/L (29) that can reach as high as several hundred grams per liter. Since SPS is one of the major factors influencing the water quality of turbid rivers (29), we believe that the effect of SPS on nitrification may be important and merits attention. Similar to riverbed sediment, SPS can also provide a large surface area for nitrification processes. Our previous research indicated that the presence of SPS could accelerate the nitrification process (30). Nitrogen pollution is one of the most critical problems for surface water quality in China, due mainly to high concentrations of ammonium nitrogen (NH4+-N). Some reaches of some rivers cannot be used as drinking water sources because their NH4+-N concentrations are higher than 1.0 mg/L (71 µmol/L) with the water quality worse than grade III of the Chinese water quality grade scale in which grade I is best and grade V is worst (31). For China, nitrification is particularly important as a natural process that converts ammonia from wastewater and leads eventually to removal of nitrogen from surface waters. Therefore, the goal of the present research was to study the effect of suspendedsediment concentration on nitrification rate. Experimental studies were conducted with natural waters from the Yellow River under laboratory conditions. The mechanisms regarding the effects of SPS were analyzed through investigation of the features of nitrification kinetics, the sorption of ammonium nitrogen, and the process of bacterial growth. Furthermore, a semipermeable membrane experiment was carried out to compare the nitrification rates at the SPS-water interface and in the water phase.
2. Experimental Section 2.1. Introduction of the Study Site. As the second-longest river in China, the Yellow river provides critical water VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Semipermeable Membrane Experiment Design and Zero-Order Kinetic Rate Constants of Nitrification in the Beginning of Cultivation (the First Two Days) treatment
inside of the membrane
outside of the membrane
k1(NH4+-N), µmol N/(L · day)
k1(NO3--N), µmol N/(L · day)
treatment 1 treatment 2
sediment, bacteria sediment
NH4+-N, water NH4+-N, bacteria, water
157 75
49 21
resources to northern and northwestern China. The Yellow River watershed contains 15% of China’s arable land and 12% of the country’s population. However, these regions are facing increasingly severe water shortage due to increasing water demand for economic development. The situation is becoming more serious due to deterioration of water quality (32, 33), and NH4+-N is the principal pollutant (34). The yearly average NH4+-N concentrations ranged from 7 to 2214 µmol/L in the mainstream. Higher NH4+-N concentrations were observed at locations near wastewater discharge outlets (often higher than 714 µmol/L) (30). The concentrations of NO2--N and NO3--N ranged from 0.7 and 73 µmol/L to 18 and 298 µmol/L in the mainstream, respectively (34). 2.2. Materials. In this study, the sampling site was located at the Huayuankou Hydrological Station near Zhengzhou which is in the middle reach of the Yellow River. Water and SPS samples were collected with a TC-Y water sampler (produced by TECH Instrument of Shenyang, China) at a depth of 0.2 m in the midstream on July 21, 2003; this is within the wet season with high SPS concentration and high runoff. All samples were placed in a cooler, and then transported to the laboratory for experimental analyses within 24 h. The SPS were separated from the water phase through sedimentation for 48 h. All the chemical reagents used in the experiments were of analytical grade. 2.3. Media and Cultivation. 2.3.1. Isolation and Cultivation of Ammonia- and Nitrite-Oxidizing Bacteria from the Water and SPS Samples. According to the method provided by Wang (35), ammonia- and nitrite-oxidizing bacteria were isolated from water and SPS samples from the Yellow River. For ammonia-oxidizing bacteria, the cultivation media contained 2.0 g of (NH4)2SO4, 0.25 g of KH2PO4, 0.01 g of MnSO4 · 4H2O, 0.75 g of K2HPO4, 0.03 g of MgSO4 · 7H2O, and 5.0 g of CaCO3 in 1 L of distilled water. For nitrite-oxidizing bacteria, the cultivation media contained 1 g of NaNO2, 0.75 g of K2HPO4, 0.25 g of NaH2PO4, 1 g of Na2CO3, 0.03 g of MgSO4 · 7H2O, 0.01 g of MnSO4 · 4H2O, and 1 g of CaCO3 in 1 L of distilled water. All media were sterilized by autoclaving prior to use. 2.3.2. Effect of SPS Concentration on Nitrification. The collected water and SPS were used as media for nitrification experiments; the indigenous bacteria in the samples remained active. A series of flasks that contained 400-mL water samples were spiked with 0, 0.08, 0.4, 2, 4, 8, and 16 g of SPS, obtaining SPS concentrations of 0, 0.2, 1.0, 5.0, 10, 20, and 40 g/L, respectively. The flasks were placed in the dark at 20 °C for 24 h. A known amount of NH4+-N was then added as (NH4)2SO4 to the flasks to obtain an initial NH4+-N concentration of 11.0 mg/L (786 µmol/L). The flasks were then covered by eight layers of gauze to exclude external bacteria, then incubated in the dark at 20 °C with a magnetic stirrer to ensure oxygen saturation and to promote the growth of indigenous nitrifying microorganisms as well as to encourage the interactions between water and SPS. Aliquots were withdrawn for the determination of concentrations of nitrogen species. Nitrification rate was calculated based on NO3--N concentrations in the water phase and/or total NH4+-N concentrations in both water and SPS phases. Each flask and the gauze were sterilized before the experiment. Each experimental set was conducted in triplicate with a set of controls. The control experiments were carried out with 3682
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sterilized water and SPS samples; microbial activity was inhibited by adding 0.5% mercuric chloride. To study the effect of SPS-water interface on nitrification processes, the collected water and SPS were sterilized by autoclaving for 30 min at 120 °C, and used as media for cultivation; the ammonia and nitrite oxidizing bacteria isolated from the water and SPS samples were introduced into the system, with both the ammonia-oxidizing and nitriteoxidizing bacteria populations being 105 cell/mL and the initial NH4+-N concentration being 786 µmol/L in each water system. The cultivation experiments were then carried out as described in the above section. 2.3.3. Effect of the State of Sediment on Nitrification. The collected water and SPS were used as media for nitrification experiments; the indigenous bacteria in the samples remained active. Three stirring regimes, i.e., no stirring, intermittent stirring (stirring for 12 h and without stirring for 12 h), and continuous stirring, were carried out to study the state of sediment on nitrification rate. The initial NH4+-N concentration was 5.0 mg/L (357 µmol/L) and the SPS concentration was 5.0 g/L in each water-sediment system, and other experimental processes were the same as mentioned in Section 2.3.2. 2.3.4. Semipermeable Membrane Experiment. Hydrophilic polytetrafluoroethylene (PTFE) membrane (0.22 µm) (Jiuding Filter Company of China) is immiscible with organic solvents such as cyclohexane, and the membrane can be sterilized at 121 °C for 30 min without destroying its stability/integrity. As water and nitrogen can freely pass across the PTFE membrane while bacteria and sediment cannot, two treatments (Table 1 and Figure S1) were designed to compare the nitrification rates in the water phase and at the SPS-water interface. The semipermeable membrane was placed in the flasks containing 400 mL of presterilized water sample. The sedimentconcentrationwas3g/Linthewholewater-sediment systems; both of the inoculated ammonia-oxidizing and nitrite-oxidizing bacteria populations were 105cell/mL and the initial NH4+-N concentration was 357 µmol/L. Nitrification experiments were the same as mentioned in Section 2.3.2. 2.4. Chemical and Microbiological Analysis. Concentrations of NH4+-N, NO2--N, and NO3--N in water-sediment systems were examined daily. Aliquots were filtered through 0.45 µm filters before the analysis was conducted. The Nesslerization colorimetric method was used for NH4+-N analysis (36). NO2--N was determined by colorimetry through the formation of a reddish-purple azo dye produced at pH 2.0-2.5 by coupling diazotized sulfanilamide with N-(1naphthyl)-ethylenediamine (36). NO3--N was determined by the phenol disulfonic acid ultraviolet spectrophotometric method (36). Detection limits were 1.4, 2.7, and 5.1 µmol/L, respectively. Total nitrogen was measured with ultraspectrophotometry after digestion of the sample with basic potassium peroxydisulfate (36). NH4+-N concentration on SPS was calculated based on the adsorption isotherm equation (see Table S1) and its concentration in the water phase. The sorption experiments were conducted as described in the previous study (30). The SPS was mechanically agitated, and the size characteristics of SPS were then measured through the gravimetric method (37). The organic content of the SPS was determined
through the high-temperature K2Cr2O7 method (37). The densities of ammonifying, nitrosifying, and nitrifying bacteria in water were determined by using the most probable number (MPN) techniques (38), which were described previously (30). The quality assurance and quality control (QA/QC) included the following. (a) The ranges of recoveries for NH4+N, NO2--N, NO3--N, and total nitrogen in the samples were 95-104%, 96-102%, 95-103%, and 95-105%, respectively; the relative standard deviations were less than 4.4%, 2.8%, 5.4%, and 5.0%, respectively. (b) The variations of NH4+-N, NO2--N, and NO3--N were less than 5% in the control experiments with the sterilized water and SPS samples. (c)As there was no substantial ammonification of organic nitrogen and denitrification in the water and sediment samples, TIN (including NO2--N, NO3--N, and NH4+-N) should remain constant during nitrification. However, when NH4+-N concentration was calculated based on its sorption data, sometimes the variations of TIN through the incubation could be up to 10%. These variations were probably caused by the variations of NH4+-N sorption on SPS during incubation. Therefore, to avoid the sorption effect of NH4+-N, the reaction kinetics were derived from NH4+-N concentration variations which were calculated by subtracting NO2--N and NO3--N from the initial TIN concentration.
3. Results and Discussion 3.1. Characteristics of the Water and SPS Samples. The concentration of the total dissolved solids in the water sample was 351.3 mg/L (Table S2). The pH value was 7.58, indicating that the water was slightly alkaline. According to Table S3, 23.8% of the SPS was particles smaller than 0.002 mm, with a large specific surface area. The contents of organic matter and organic nitrogen in SPS were low: 9.164 and 0.88 g/kg, respectively. The concentrations of NH4+-N, NO2--N, and NO3--N were 41, 8.6, and 300 µmol/L in the water sample, respectively. 3.2. Effect of SPS Concentration on Nitrification Rates. When the collected water and SPS were used as media for nitrification experiments, and the indigenous bacteria in the samples remained active, the nitrification rate increased with the SPS concentration (Figure 1). When the SPS concentrations were 5, 10, 20, and 40 g/L, the NH4+-N concentration decreased rapidly at the beginning of the incubation, and decreased virtually to zero after three days, while NH4+-N concentration decreased virtually to zero after nine, five, and four days when SPS concentrations were 0, 0.2, and 1.0 g/L, respectively. When the SPS concentration was higher than 5.0 g/L, nitrite was almost completely transformed to nitrate after three days. When the SPS concentrations were 0, 0.2, and 1.0 g/L, NO2--N was almost completely transformed to nitrate after nine, five, and four days. As shown in Figure 2A, the average nitrate formation rate for the first three days increased with the SPS concentration as described by the power function y ) 150x 0.103
-
dS µmaxS ) (S + X 0 - S) dt Ks + S 0
(3)
where S is the substrate concentration (mol/L); B is the bacterial density (cell/L); q is the bacteria quota or inverse yield (cell/mol); X is the bacterial population quota (mol/L), which corresponds to the amount of substrate required to produce a population density equal to B; S0 is the substrate concentration at time zero (mol/L); X0 is the bacterial population quota at time zero (mol/L); µmax is the maximum specific growth rate (day-1); and KS is the half-saturation constant for growth (mol/L). According to our previous research (30), for incubations with the indigenous bacteria, the substrate (NH4+-N) was initially present at much less than saturating level (S0 , Ks). In this case, a simplified form of the Monod kinetics, the Logistic model describing mineralization rate of a test substrate, can be used to analyze the nitrification kinetics (42): -
dS µmax · S · (S0 + X0 - S) ) dt Ks
(4)
Since (S0 + X0 - S) is equal to the bacterial population quota, eq 4 can be expressed as -
dS µmax ·S·X ) dt Ks
(5)
(1)
where y is nitrate formation rate in µmolN/(L · day), and x is the SPS concentration in g/L. Several previous researchers also obtained similar results (39-41), for example, in situ Tamar Estuary nitrate and nitrite concentrations were strongly correlated with suspended particulate material, ascribed to midestuarine inorganic N inputs derived from bacterially mediated nitrification. 3.3. Effect of SPS on Nitrification Kinetics. The following Monod kinetics was used to study the nitrification process (42): -
dS 1 dB dX ) · ) dt q dt dt
(2)
FIGURE 1. Variations of NH4+-N, NO2--N, and NO3--N during nitrification experiments with different SPS concentrations. VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effects of SPS concentration on nitrification rate with indigenous and introduced bacteria. where X is the bacterial population quota at time t, mol/L. Therefore, µmax/KS, named as K4, can be regarded as the reaction rate constant. The integrated form of the eq 4 is S)
S0 + X 0 1 + (X 0 ⁄ S0) · eK 4(S0+X 0)t
(6)
Based on the variations of NH4+-N concentrations during the nitrification process, kinetic parameters were derived from the Logistic model through nonlinear regression analysis (see Table S4, with R2 higher than 0.985). As shown in Figure 2B, K4 increased with the SPS concentration as a power function, this was consistent with the increasing trend of average nitrate formation rate as shown in Figure 2A. In addition, when SPS concentration was within 5.0 g/L, K4 value increased with SPS concentration with a higher rate. For example, when SPS concentration increased from 0 to 5.0 g/L, the resulted increment of K4 was 0.0047 day-1µM-1; when SPS concentration increased from 5.0 to 40 g/L, the resulted increment of K4 was only 0.0014 day-1 µM-1. 3.4. Mechanism Regarding the Effect of SPS ConcentrationonNitrificationRates.3.4.1. ImportanceofSPS-Water Interface. With the introduction of the cultivated bacteria, the average nitrate formation rate for the first three days increased with SPS concentration as described by the power function (Figure 2A)
times those in the water phase (30). As shown in Table S1, NH4+ adsorption on suspended sediment could be described as a linear equation, and its linear adsorption coefficient on SPS was comparable to the results obtained by Mackin and Aller (43). Therefore, the following equation can be obtained for NH4+-N sorption:
y ) 107x 0.066
q)c+k·c·s
(7)
where y is nitrate formation rate, µmolN/(L · day), and x is the SPS concentration, g/L.When the SPS concentrations were 0, 0.2, 1.0, 5.0, 10, 20, and 40 g/L, the average nitrate formation rates were 62, 91, 94, 119, 124, 137, and 148 µmolN/(L · day) for the first three days, respectively. This was consistent with the increasing trend of nitrification rate for the indigenous bacteria. With introduced bacteria, the increasing of exposed surface area of SPS was a major reason for the increasing trend of nitrification rate with the SPS concentration. In addition, the nitrification rate was higher with the presence of suspended-sediment than deposited-sediment. As shown in Figure S2, the sequence of nitrification rate was “no stirring” < “intermittent stirring” < “continuous stirring”; the zero-order kinetic rate constants of nitrification (based on NO3--N variations) were 12, 33, and 60 µmolN/(L · day) during the first two days, respectively. Since stirring suspended the sediment and led to an increase in the exposed surface area of the SPS, the above results support the view that the increase of exposed surface area of the SPS would stimulate the nitrification processes. This was consistent with the results obtained by Abril et al. (40) that sediment and fluid mud resuspension/settling resulted in high nitrification and denitrification rates. Nitrification in water systems would not occur until both bacteria and nitrogen are available. Our previous research found that the bacteria in the SPS phase were about ten 3684
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FIGURE 3. Percentage of NH4+-N sorbed on SPS in the whole water-sediment system with variable SPS concentrations.
(8)
where q is the total amount of NH4+-N in the water-sediment system, µmol/L; c is the equilibrium concentration of NH4+-N in the water phase, µmol/L; k is the sorption coefficient, L/g; and s is the SPS concentration, g/L. Based on the above equation, the sorption quantity of NH4+-N on SPS can be obtained with the following equation: Q)k·c·s)
k·s·q 1+k·s
(9)
where Q is the sorption quantity of NH4+-N on SPS, µmol/L. Therefore, the percentage of NH4+-N on SPS in the whole water-sediment system is equal to ks/(1 + ks). Based on the sorption equations of NH4+-N on SPS as shown in Table S1, the percentage of NH4+-N on SPS to the whole water-sediment system increased with SPS concentration as described by a power function (see Figure 3). Therefore, bacteria and NH4+-N are concentrated at the SPS-water interface; the chances for them to react will be increased. This was manifested by the increase of K4 in eq 6 with SPS concentration as a power function. 3.4.2. Comparison of Nitrification Rates at the SPS-Water Interface and in the Water Phase. As bacteria and sediment cannot cross the membrane and there was very little water in the membrane bag, bacteria were basically confined to the sediment phase for Treatment 1 and confined to the water phase for Treatment 2 (Table 1 and Figure S1). In this
FIGURE 4. Variations of NH4+-N and NO3--N during nitrification experiments under different semipermeable membrane conditions. way nitrification basically occurred at the SPS-water interface for Treatment 1 and in the water phase for Treatment 2. As shown in Figure 4, NH4+-N concentration decreased with zero-order kinetics during the first two days. The following zero-order kinetics, a simplified form of the Monod kinetics shown as follows, was used to calculate the biodegradation rate constant (42): -
d[S] ) k1 ) µmax · X0 dt
(10)
where [S] is the concentration of the substrate (mol/L); k1 is the zero-order nitrification rate constant (mol/(L · day)); µmax is the maximum specific growth rate of bacteria (day-1); and X0 is the initial bacterial population quota (mol/L). For treatment 1, the nitrification rate at the SPS-water interface could be expressed as -
bT 0 d[S]s ⁄ w ) µmax(s ⁄ w) · dt q · VS
(11)
where [S]s/w is the concentration of NH4+-N at the SPS-water interface (µmol/L); t is cultivation time (d); µmax(s/w) is the maximum specific growth rate of bacteria at the SPS-water interface (day-1); bT0 is the initial bacterial population (cell); q is the bacteria quota or inverse yield (cell/mol); Vs is the volume of sediment phase (L). Then, the nitrification rate in the whole water-sediment system could be expressed as -
bT 0 V s d[S]T · ) µmax(s ⁄ w) · dt q · VS VT
(12)
where [S]T is the total concentration of NH4+-N in the whole system (µmol/L); VT is the total volume of water-sediment system (L). The eq 12 could be rearranged as -
bT 0 d[S]T ) µmax(s ⁄ w) · dt q · VT
(13)
As the sediment concentration was only 3 g/L in the systems, the sediment/water volume ratio was very low. According to eq 10, for treatment 2, the nitrification rate in the whole water-sediment system could be expressed as -
bT 0 d[S]T ) µmax(w) · dt q · VT
(14)
where µmax(w) is the maximum specific growth rate of bacteria in the water phase (day-1). As shown in Table 1, the zeroorder nitrification rate constants in treatment 1 were about twice that in treatment 2 for the first 2 days. Furthermore, as the initial bacteria population was the same for treatments 1 and 2, according to eqs 13 and 14, the µmax (s/w) value in treatment 1 was about twice µmax(w) in treatment 2. This means that, the maximum specific growth rate of bacteria at the SPS-water interface was about twice that in the water phase, implying that the nitrification rate at the SPS-water interface was about twice that in the water phase.
FIGURE 5. Effects of SPS concentration on ammonium-oxidizing bacterial population in the water-sediment system after incubation for one day. 3.4.3. Effect of SPS Concentration on Bacterial Population. Ammonia and nitrite oxidizing bacteria tend to grow at the SPS-water interface (30), and as mentioned in Section 3.4.2, the maximum specific growth rate of bacteria at the SPS-water interface was about twice that in the water phase. Since the increase of SPS concentration would provide more interfaces for bacteria to attach, the increase of SPS will stimulate the growth of bacteria. As shown in Figure 5, when the indigenous bacteria in the samples remained active, the ammonia-oxidizing bacteria population increased with SPS concentration in the water systems as described by a power function. We infer that the mechanisms between SPS concentration and nitrification rates are as follows: (1) SPS will provide surface area for the attachment of bacteria, and ammonium can also adsorb to the SPS, with the percentage of NH4+-N on SPS to the whole water-sediment system increasing with SPS concentration as a power function. This will result in the enhancement of bacterial access to the ammonium, and it was manifested by the increasing of nitrification rate constant (µmax/KS in eq 5) with SPS concentration as a power function. (2) Bacteria tend to sorb onto the SPS and the maximum specific growth rate of bacteria at the SPS-water interface was about twice that in the water phase, which will stimulate the growth of bacteria. The indigenous ammonia-oxidizing bacteria population increased with SPS concentration as a power function. Therefore, according to eq 5, an increase of bacterial population and µmax/KS with SPS as a power function would result in an increase of nitrification rate with SPS as a power function. Several researchers have proposed new processes in nitrogen cycling, including heterotrophic nitrification, anaerobic ammonium oxidation, and archaeal ammonia oxidation (44-46). In this research, as the experiments were conducted under aerobic conditions, anaerobic ammonium oxidation was impossible, and the ammonia-oxidizing bacteria may include heterotrophic and archaea bacteria in addition to VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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autotrophic bacteria. Therefore, an increase in organic substrate concentration with SPS concentration will lead to the increase of the heterotrophic nitrification rate. This suggests that, for incubations with indigenous bacteria, nitrification may be enhanced due to the elevated organic substrate availability in addition to the physical effects of SPS concentrations. For the incubation with introduced bacteria, there were no heterotrophic nitrifiers; the increasing of nitrification rate with SPS was caused by the increased surface area for bacteria. For the incubation with indigenous bacteria, heterotrophic nitrifiers might exist; the increasing of nitrification rate with SPS was caused by the increased autotrophic and heterotrophic nitrifiers besides surface area for bacteria. According to the results shown in Figure 2A, the increasing extent of nitrification rate with SPS for the introduced bacteria was about half of that for the indigenous bacteria, thus it can be deduced that, for the increasing of nitrification rate with SPS, the increased surface for bacteria was more important than the greater availability of organic substrate for heterotrophic nitrifiers. Of course, this needs to be studied further. Also, SPS will provide nutrients for the autotrophic nitrification bacteria, resulting in accelerated nitrification. Turbid rivers have a poor light climate which severely limits phytoplanktonic uptake of ammonium. Therefore, nitrification is a key process in the fate of ammonium in turbid rivers. According to this research, nitrification rate increases with SPS concentration as a power function. Especially, when SPS concentration is less than 5 g/L, nitrification rate will increase with SPS concentration at a higher rate. Therefore, the increase of SPS concentration caused by erosion and sediment transport as well as by inriver sediment resuspension will promote the nitrification in aquatic systems. For the Yellow River, the SPS concentration often ranges from 0 to 100 g/L. When SPS concentration increases from 0.001 g/L to 1, 5, 50, and 100 g/L, the nitrification rate will increase about 1.1, 1.4, 2.1, and 2.3 times, respectively. According to the water quality data of the Huayuankou Station of the Yellow River in 2000, the ratios of nitrite and nitrate to ammonium (adjusted by the sorption on SPS) were positively correlated with the SPS concentration, with a correlation coefficient of 0.89 (p < 0.01). This was consistent with the results that nitrification rate increases with SPS concentration in water system. According to the nitrification rates in this study, the half-life of NH4+-N with nitrification increased from 1 to 6 days with the SPS decreasing from 40 to 0 g/L, and it was about 2 days with SPS being 0.2 g/L. As SPS concentration normally will be higher than 0.2 g/L in the middle and lower reaches of the Yellow River, the half-life of NH4+-N with nitrification is about 2 days in these reaches. Therefore, the presence of SPS promotes the removal of ammonium from river water. Because of the high ammonia content in many Chinese rivers from partially treated wastewater, the environmental significance of particlecontrolled nitrification is likely a significant component in river ammonium processing capacity. In addition, nitrification will provide nitrate for denitrification process (3, 4). An increased nitrification rate arising from the presence of SPS may thus eventually stimulate the denitrification process, promoting nitrogen removal from the river system. However, this needs to be studied further.
Acknowledgments The study was supported by the National Natural Science Foundation of China (40871228 and 40571138). We thank Dr. Edwin Ongley for improving the English of the manuscript and his helpful comments. We also thank the editors and anonymous reviewers for their comments that improved this paper. 3686
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Supporting Information Available
Experimental results: sorption of NH4+-N on SPS; chemical composition of the water sample and distribution of SPS size collected from the Yellow River; schematic diagram of the semipermeable membrane experiment; and mechanical stirring conditions on nitrification rate with indigenous bacteria. This information is available free of charge via the Internet at http://pubs.acs.org.
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