Volatile fatty acids carbon source for biological denitrification

In the suspended sludge system, when pH and temperature were suitable and carbon source was not limited, the denitrification and C-uptake rate followed a zero-order reaction. Mixed VFA C-source had higher denitrification rate than single VFA C-source consisting of them. When VFA were used as carbon source, consumed carbon quantity for denitrification was closed to 1. 07 mg VFA-C/mg NO-N. About 20% of the applied C/N was used for assimilative purposes. As rising up influent C/N by increasing the C load ,the sludge yield YN increased. The part of carbon consumption increased and the effluent oxidized nitrogen decreased. At pH 7. 5, 25℃ and sludge yield 0.72 mg VSS/mg NO-N, the calculated influent VFA-C /NO critical value was 1. 43 for complete denitrification.

Impacts of the unsaturation degree of long-chain fatty acids on the volatile fatty acid profiles of rumen microbial fermentation in goats in vitro

This study investigated the impacts of the degree of unsaturation （unsaturity） of long-chain fatty acids on volatile fatty acid （VFA） profiles of rumen fermentation in vitro. Six types of long-chain fatty acids, including stearic acid （C18：0, control group）, oleic acid （C18：1, n-9）, linoleic acid （C18：2, n-6）, a-linolenic acid （C18：3, n-3）, arachidonic acid （C20：4, n-6） and eicosapentaenoic acid （C20：5, n-3）, were tested. Rumen fluid from three goats fitted with ruminal fistulae was used as inoculum and the inclusion rate of long-chain fatty acid was at 3% （w/w） of substrate. Samples were taken for VFA analysis at 0, 3, 6, 9, 12, 18 and 24 h of incubation, respectively. The analysis showed that there were significant differences in the total VFA among treatments, sampling time points, and treatment×time point interactions （P〈0.01）. a-Linolenic acid had the highest total VFA （P〈0.01） among different long-chain fatty acids tested. The molar proportion of acetate in total VFA significantly differed among treatments （P〈0.01） and sampling time points （P〈0.01）, but not treatment×time point interactions （P〉0.05）. In contrast, the molar proportion of propionate did not differ among treatments during the whole incubation （P〉0.05）. However, for butyrate molar proportions, significant differences were found not only among sampling time points but also among treatments and treatment×time point interactions （P〈0.01）, with eicosapentaenoic acid having the highest value （P〈0.01）. Additionally, no statistically significant differences were found in the acetate to propionate ratios among treatments groups （P〉0.05）, even the treatments stearic acid and a-linolenic acid were numerically higher than the others. The inclusion of 3% long-chain unsaturated fatty acids differing in the degree of unsaturation brought out a significant quadratic regression relation between the total VFA concentration and the double bond number of fatty acid. In conclusion, the a-linolenic acid with 3 double bonds appeared better for improving rumen microbial fermentation and the total VFA concentration.

Acid Volatile Sulfide and Simultaneously Extracted Metals in Tidal Flat Sediments of Jiaozhou Bay, China

It is well known that acid-volatile sulfide (AVS) plays an important role in influencing the toxicity of divalent cationic metals within anoxic sediments. In studying sediment core samples collected from tidal flats within the Jiaozhou Bay, China, we found that the AVS concentration gradually increases with depth and decreases from high tidal flat to low tidal flat areas. We evaluated the chemical activity and bioavailability of heavy metals in the tidal flat based on the molar ratio of simultaneously ex- tracted metals (SEM) and AVS. The value of SEM/AVS is generally less than 1 in this area except for the surface layer, which suggests that the heavy metals only have chemical activity in the surface layer. SEM is most highly concentrated at the boundary of the redox layer. SEM have similar depth distributions throughout the tidal flat. The aeration of low tidal flat sediment indicates that SEM gradually move to deeper sites via interstitial water.

BAIMUXIFURANIC ACID,A NEW SESQUITERPENE FROM THE VOLATILE OIL OF AQUILARIA SINENSIS(LOUR.)GILG.

A new sesquiterpenoid,named baimuxifuranic acid,has been isolated from the volatile oil of Aquilaria sinensis(Lour.)Gilg.belonging to the family Thymelaceae by silica gel column chromatography.Based on spectral analysis its structure was deduced as(1).Reduction of baimuxifuranic acid by LiAlH_4 gave a

Acid volatile sulfide and simultaneously extracted metals in superficial sediments from Baihua Lake, China

The bioavailability of five divalent cationic heavy metals （Pb, Cd, Cu, Zn and Ni） in 10 superficial sediment samples from Baihua Lake was assessed based on the molar ratio of simultaneously extracted metals （SEMs） to acid volatile sulfide （AVS）. Atomic absorption spectrometry （AAS） and X-ray powder diffraction （XRD） were used to determine the heavy metal concentrations and examine the mineralogy of the crystalline phases, respectively. The AVS loadings in sediments from Baihua Lake ranged from 64.30 to 350.08 ~rnol/g （dry weight）. The corresponding SEM levels for the sampling sites varied from 1.770 to 14.660 vrnol/g. The molar ratio of SEMs to AVS ranged from 0.014 to 0.084 with a mean value of 0.034. The XRD analysis also confirmed the presence of some metal sulfides in sediments from Baihua Lake. The SEMs/AVS ratios for all sampling sites were significantly lower than 1.0, indicating that AVS in the sediments was sufficient to bind the five heavy metals; thus, these heavy metals are currently not significantly bioavailable to benthic organisms. Comparing the SEMs results to published guideline values for metal toxicity to benthic organisms in sediments, however, suggests that Zn and Ni pose a risk at some sampling locations in Baihua Lake.

A combined pig model to determine the net absorption of volatile fatty acids in the large intestine under different levels of crude fiber

Background:This study aimed to develop a combined model to quantify the net absorption of volatile fatty acids(VFA)in the large intestine(LI)of pigs.Methods:Fifteen female growing pigs(Duroc×Large White×Landrace)were ranked by body weight(30±2.1 kg)on day 0 and assigned to one of three treatments,namely the basal diet containing different crude fiber(CF)levels(LCF:3.0%CF,MCF:4.5%CF,and HCF:6.0%CF).The pigs were implanted with the terminal ileum fistula and the cannulation of the ileal mesenteric vein(IMV),portal vein(PV),and left femoral artery(LFA)from days 6 to 7.[13 C]-Labeled VFA and P-aminohippuric acid were constantly perfused into the terminal ileum fistula and the cannulation of the IMV(day 15),respectively.Blood samples were collected from the PV and the LFA during perfusion(5 h),and LI samples were collected.Results:The net flux of[12 C]-acetic acid in the PV was greater for LCF versus MCF(p=0.045),but no difference was observed in the net flux of[12 C]-propionic acid(p=0.505)and[12 C]-butyric acid(p=0.35)in the PV among treatments.The deposition of[12 C]-acetic acid in the LI was greater for LCF versus MCF(p=0.014),whereas the deposition of[12 C]-propionic acid(p=0.007)and[12 C]-butyric acid(p=0.037)in the LI was greater for LCF versus HCF.Conclusions:In conclusion,this pig model was found conducive to study the net absorption of VFAs in the LI,and LCF had more net absorption of VFAs in the LI than MCF and HCF.

Microbial Community Profiles Related to Volatile Fatty Acids Production in Mesophilic and Thermophilic Fermentation of Waste Activated Sludge Pretreated by Enzymolysis

Mesophilic and thermophilic anaerobic fermentation performance of waste activated sludge(WAS)pretreated by enzymes catalysis associated with microbial community shifts were investigated.WAS disintegration was boosted considerably by enzymolysis with 8750 mg/L of soluble COD release within 180 min.Mesophilic anaerobic fermentation(MAF)produced nearly equal VFA accumulation with over 3200 mg COD/L compared with that of thermophilic fermentation(TAF).Bacterial community consortia showed great shifting differences in dynamics of main T⁃RFs between MAF and TAF.Moreover,MAF was conducive to form intermediate bacterial community evenness compared to TAF,which preserved a robust function of VFA production.The enzymes catalysis prompted bio⁃energy(electricity)recovery potential of WAS organics via anaerobic fermentation(MAF/TAF)with evaluating electricity conversion efficiency of 0.75-0.82 kW·h/kg VSS(3.9 times higher than control test).Finally,this study proposed some novel thinking on future WAS treatment/management towards energy recovery coupled with energy⁃sufficient wastewater treatment by co⁃locating WAS anaerobic fermentation,MFC plant with wastewater treatment plant(s).

Effects of Ultrasonic and Acid Pretreatment on Food Waste Disintegration and Volatile Fatty Acid Production

This study aims at investigating the effects of ultrasonic and acid pretreatment on food waste( FW)disintegration and volatile fatty acid( VFA) production. Single-factor experiments are carried out to obtain optimal conditions of individual ultrasonic and acid pretreatment,and response surface method( RSM) is applied to optimize the conditions of the combination of ultrasonic and acid( UA) pretreatment. Results show that the optimal acid,ultrasonic and UA pretreatments conditions are individual pH 2,individual ultrasonic energy density of 1. 0 W / mL and the combination of ultrasonic energy density1. 11 W / mL and pH 1. 43,respectively. Correspondingly,the maximum disintegration degrees( DD) of 46. 90%,57. 38% and68. 83%are obtained by acid,ultrasonic and UA pretreatments,respectively. After optimizing pretreatment conditions,batch experiments are operated to produce VFA from raw and pretreated FW under anaerobic fermentation process. Both the maximum VFA production( 976. 17 mg COD / gV S) and VFA / SCOD( 72. 89%) are obtained with ultrasonic pretreatment, followed by UA pretreatment, non-pretreatment and acid pretreatment,respectively. This observation demonstrates that a higher acidity on acid and UA pretreatments inhibits the generation of VFA. Results suggest that ultrasonic pretreatment is preferable to promote the disintegration degree of FW and VFA production.

Processing Effect on the Physicochemical and Volatile Fatty Acid Profile of African Breadfruit (Treculia africana) Seed Oil

African breadfruit seeds were subjected to three processing methods—parboiling, cooking and toasting, and the raw was used as control. The purpose of this research was to extract the oil from the seed and to determine the effect of processing on the oil for physicochemical properties and volatile fatty acid profile. Physicochemical properties showed that the colour of the oil varied from golden yellow to brownish yellow with specific gravity varying between 0.802 g/cm3 and 0.813 g/cm3. Percentage yield of oil was 6.14% for raw extract, 6.62% for parboiled extract, 7.56% for toasted extract, and 5.01% for cooked extract. Acid, peroxide and saponification value for oil extracted from the raw seed varied with the processed samples value. The Volatile Fatty Acid (VFA), also known as Short Chain Fatty Acid (SCFA) found inherent in varying concentration, were formic, acetic, propionic, isobutyric, butyric, isovaleric, valeric, isocarproic, hexanoic and heptanoic acid. Overall results prove that heat results in increases in the VFA concentration of the processed oil.

Study on the Stability of Volatile Acid in Lycium barbarum Wine

[ Objective] The study aimed to discuss the suitable storage conditions of fermented Lycium barbarum wine. [ Methodl In view of the fact that volatile acid in Lycium barbarum wine is unstable during storage process, we analyzed the stability changes of volatile acid in Lycium barba- rum wine with storage temperature, pH and SO2 content. [Result] When temperature was 5℃ pH was 3.7 or 3. 8, and residuary SO2 content was 60 mg/L, volatile acid in Lycium barbarum wine was relatively stable. [ Conclusion] The research could provide theoretical foundations for controlling the stability of volatile acid in Lycium barbarum wine.

CHARACTERISTICS OF THERMOPHILIC ANAEROBIC ACIDOGENESIS OF STARCH WASTEWATER

Acidogenic dissimilation of synthetic starch wastewater (1 000～10 000 mg COD·L -1 ) was studied in a thermophilic (55 ℃) upflow anaerobic sludge blanket (UASB) reactor.The production of volatile fatty acids (VFA) was proportional to the chemical oxygen demand (COD) loading rate.The yield of VFA was around 0.28 g VFA/g COD over the COD loading rate from 1.25 to 30 g COD·L -1 ·d -1 and the hydraulic retention time from 8.8 h to 24 h.Distribution of organic acids,the contents of propionic and butyric acids in the effluent in particular were also dependent on the COD loading rate.The thermophilic UASB reactor showed a stable performance on hydrolysis and acidogenesis of starch as well as suspended solid removal at short hydraulic retention times and high influent pH(10～11),during the operation of 110 d.

Releasing characteristics of phosphorus and other substances during thermal treatment of excess sludge

The releasing characteristics of phosphorus, nitrogen compounds, organics, and some metal cations during thermal treatment of excess sludge were investigated. It was found that during heating not only phosphorus, but also nitrogen compounds, organics, and some metal cations could be released in abundance. The maximum orthophosphate （ortho-P） release of about 90 mg/L in concentration was observed at 50℃ in 1 h. Except for volatile fatty acids （VFAs）, comparatively little total nitrogen （TN）, total organic carbon （TOC）, and metal cations were released at the same time. Such results might favor further process of phosphorus recovery. VFAs were considerably released only at 50℃. Acetic, butyric, and propionic acid were the most abundant components in turn and their releasing profiles exhibited good linear relationship with time （R2 = 0.9977, 0.9624, and 0.8908, respectively）. The concentrations of Mg^2＋ and K^＋ increased with time and temperature during thermal treatment, but Ca^2＋ decreased. The release of Mg^2＋ and K^＋ agreed well with TP release （R^2 = 0.9892 and 0.9476, respectively）. Temperature in the experimental range had very little impact on the linear relationships, especially of Mg^2＋. Moreover, the parameter of mixed liquor suspended solids （MLSS） was found to be an important factor for thermal sludge treatment as the released ortho-P and total phosphorus （TP） at 50℃ increased more than one-fold when MLSS was increased from 4000 to 8000 mg/L.

Effect of Sodium Ion Concentration on Hydrogen Production from Sucrose by Anaerobic Hydrogen-producing Granular Sludge

This work evaluated the effects of sodium ion concentration, ranging from 0 to 16000mg·L^-1（Na^＋）, on the conversion of sucrose to hydrogen by a high-activity anaerobic hydrogen-producing granular sludge. At the optimum sodium ion concentration [1000-2000mg·L^-1（Na^＋）] for hydrogen production at 37℃, the maximum sucrose degradation rate, the specific hydrogen production yield and the specific hydrogen production rate were 393.6-413.1mg·L^-1.h^-1, 28.04-28.97ml·g^-1, 7.52-7.83ml·g^-1.h^-1, respectively. The specific production yields of propionate, butyrate and valerate decreased with increasing sodium ion concentration, whereas the specific acetate production yield increased, meanwhile the specific production yields of ethanol and caproate were less than 55.3 and 12.6mg·g^-1, respectively. The hybrid fermentation composition gradually developed from acetate, propionate and butyrate to acetate with the increase in sodium ion concentration.

Application of Equilibrium Partitioning Approach to the Derivation of Sediment Quality Guidelines for Metals in Dianchi Lake

For the past 20 years, numerous studies have been carried out on the application of equilibrium partitioning approach （EqPA） for the derivation of sediment quality guidelines （SQGs）. However, for metals, few Equilibrium-partitioning- based numerical SQGs have been developed or are currently available because of the confounding factors mediating the bioavailability of metals. A study was conducted at Dianchi Lake, which is a heavily eutrophicated lake on the Yunnan- Guizhou Plateau, China with the focus on the measurement of partitioning coefficient （Kp） and SQGs derivation and normalization to acid volatile sulfide （AVS）, fine material, and organic carbon. Using new normalization methods, SQGs were formulated for seven metals including copper, zinc, lead, cadmium, chromium, mercury, and arsenic in Dianchi Lake. In Dianchi Lake sediments, the fine material contributed 25.4%-36.0% to the SQG values, with the largest contribution to the SQG value of mercury; AVS contributed 2.9%-75.0% to the SQG values, with the largest contribution to the SQG value of cadmium. This indicated that the fine material and the AVS were the most important controlling factors to the bioavailability of mercury and caximium, respectively. The contribution of total organic carbon （TOC） to the SQG values of copper and leaxi was 3.8% and 7.1%, respectively, indicating that at relatively lower concentrations, the contribution of TOC was not significant. In addition to normalization methods, appropriate procedures for the application of EqPA including sample collection, storage, and analysis are also essential to improve the reliability of SQGs. The normalized Dianchi Lake SQGs were higher than most of the empirically based SQGs developed in North America, but lower than Hong Kong interim SQGs except for cadmium and arsenic. The differences could be attributed to the approaches used for derivation of SQGs and the water quality criteria adopted and the differences in the physical and chemical characteristics of the sediments.

Transformation of organic matters in fresh leachate during anaerobic degradation under long hydraulic retention time

This study investigated the biodegradability of fresh leachate from a young municipal solid waste landfill at the inoculum to substrate ratios （V/V） of 10/100, 25/100, 50/100 and 100/100, as well as the transformation of organic matters in leachate under prolonged hydraulic retention time （HRT） of 50 d. Fresh leachate showed a good biodegradability, with chemical oxygen demand （COD） removal efficiency as high as 87%-92% and cumulative methane yield close to theoretical value （0.35 L CH4/g CODdeg）. Methane production and COD depletion presented biphasic characteristics associated with the successive utilization of two major intermediates, acetate and propionate, The biphasic degradation of fresh leachate was resulted from the different hydrolysis rates of diverse substrates and the changes in microorganism community structure. After 50 d, the effluents were dominated by high-MW organic compounds （MW 〉 10 kDa） at each inoculum ratio, which might be refractory compounds released from cell lyses.

Reduced inorganic sulfur in the sediments of the Yellow Sea and East China Sea

Cold diffusion methods are used to separate and quantify the three reduced inorganic sulfur species into acid volatile sulfide （AVS）, pyrite-S and element sulfur （ES） in the sediments of the Yellow and East China Seas. The results show that up to 25.02 pmol/g of AVS, 113.1 pmol/g of pyrite-S and 44.4 pmol/g of ES are observed in the sediments of the Yellow Sea and East China Sea. Pyrite-S is the predominant sulfide mineral in the sediments, while the concentration of AVS is quite low at most stations in the study area. The amounts and reactivity of organic matter are the primary limited factor for the sulfide formation, while an iron limitation and a sulfate limitation are not observed in the sediments of the Yellow Sea and East China Sea. The irregular profiles of the three reduced inorganic sulfur sediment composition and sedimentation rates. species also reflected the comprehensive influence of

Effects of Different Irrigation Modes on Internal Quality of Flue-cured Tobacco Leaves

[Objective] This study aimed to investigate the effects of different irrigation modes on chemical constituent and aroma substance contents and sensory quality in flue-cured tobacco leaves. [Method] Field experiments were conduced with two treatments, including watering irrigation and micro-spraying irrigation. In watering irrigation treatment, flue-cured tobacco seedlings were respectively watered by artificial root irrigation once at rosette stage（May 26） and vigorous growth stage（June 18）, 1kg/plant each time; in micro-spraying treatment, flue-cured tobacco seedlings were watered by artificial root irrigation once at rosette stage（May 26）, 1 L/plant. [Result]Compared to watering irrigation mode, micro-spraying irrigation could extremely significantly increase the content of main aromatic substance in middle and upper leaves.Contents of main aromatic substance non-volatile organic acids, volatile organic acids, petroleum ether extracts, neophytadiene and neutral aromatic substances in C3F tobacco leaves were improved by 24.4%, 32.6%, 20.7%, 33.8% and 26.3%, respectively; contents of main aromatic substance non-volatile organic acids, volatile organic acids, petroleum ether extracts, eophytadiene and neutral aromatic substances in B2F tobacco leaves were improved by 16.3%, 6.0%, 6.1%, 10.8% and8.1%, respectively. In addition, micro-spraying irrigation could extremely significantly reduce the nicotine content in middle and upper leaves and improve the aromatic coordination, mellow taste and smoldering property. [Conclusion] In flue-cured tobacco production, micro-spraying technology is an important means to improve the internal quality of flue-cured tobacco leaves, which can be adopted to effectively solve the problem of declining quality of flue-cured tobacco leaves resulted from the dry climate at maturation stage in seasonal frequent-drought areas.

Effects of Urechis unicinctus Juveniles on Chemical Characteristics of Organically Contaminated Coastal Sediment

Biological activities of marine benthos such as burrowing and feeding may change sediment characteristics.We conducted three experiments to examine the potential of using juveniles of a spoon worm Urechis unicinctus to improve the quality of organically contaminated coastal sediment.Sediment samples were collected from a site that was heavily contaminated with organic matter (Seonso) and two sites that were clean (Myo-do,Dolsan-do).Urechis juveniles,obtained by artificial fertilization and cultured in the laboratory,were introduced to the sediment (weight 3 kg,depth 10 cm) at a density of 500 individuals per aquarium (length 50 cm,width 35 cm,height 30 cm) (Experiment 1),or at densities ranging from 100 to 900 individuals per beaker (Experiment 2).To examine how sediment contamination can be modified by the effects of Urechis,500 individuals (per aquarium) were exposed to the Seonso contaminated sediment that had been mixed with 0-100% clean sand (Experiment 3).Each experiment lasted two months and sediment samples were collected every 15 d to determine the several indexes of sediment quality,which included acid volatile sulfide (AVS),chemical oxygen demand (COD) and total ignition loss (TIL).In Experiment 1,the existence of Urechis did not result in significant changes in quality indexes in the sediments collected from Myo-do,Dolsan-do.However,AVS,COD and TIL of the Seonso sediment all decreased significantly after co-incubation with Urechis juveniles for 30 to 45 d.Experiment 2 showed that a density of at least 300 juveniles per beaker was necessary to significantly reduce all three quality indexes,and the magnitude of reduction was positively correlated with juvenile density.Experiment 3 revealed that Urechis juveniles were effective in reducing the AVS,COD and TIL of the Seonso sediment that had been mixed with 60%,80%,and 80% of clean sand,respectively.The results of the present study therefore indicated that juveniles of this spoon worm have the potential to be used to improve the quality of organically contaminated sediment in coastal waters.

Evaluation of in vitro digestibility of Aspergillus oryzae fungal biomass grown on organic residue derived‑VFAs as a promising ruminant feed supplement

Background As demand for high quality animal feed continues to raise,it becomes increasingly important to mini-mize the environmental impact of feed production.An appealing sustainable approach to provide feed fractions is to use organic residues from agro-food industry.In this regard,volatile fatty acids(VFAs)such as acetic,propionic and butyric acids,derived from bioconversion of organic residues can be used as precursors for production of micro-bial protein with ruminant feed inclusion potential.This study aims to investigate the in vitro digestibility of the Asper-gillus oryzae edible fungal biomass cultivated on VFAs-derived from anaerobic digestion of residues.The produced fungal protein biomass,along with hay clover silage and rapeseed meal were subjected to various in vitro assays using two-stage Tilley and Terry(TT),gas,and bag methods to evaluate and compare its digestibility for application in ruminant feed.Results The produced fungal biomass contained a higher crude protein(CP)(41%–49%)and rather similar neutral detergent fiber(NDF)(41%–56%)compared to rapeseed meal.The rumen in vitro dry matter digestibility(IVDMD)of the fungal biomass in the TT method ranged from 82%to 88%(statistically similar to that of the gas method(72%to 85%)).The IVDMD of fungal biomass were up to 26%and 40%greater than that of hay clover silage and rapeseed meal,respectively.The type of substrate and bag method had pronounced effect on the fermentation products(ammonium-N(NH4+-N),total gas and VFAs).Fungal biomass digestion resulted in the highest release of NH4+-N(340–540 mg/L)and the ratio of acetate to propionate ratio(3.5)among subjected substrates.Conclusion The results indicate that gas method can be used as a reliable predictor for IVDMD as well as fermenta-tion products.Furthermore,the high IVDMD and fermentation product observed for Aspergillus oryzae fungal biomass digestion,suggest that the supplementation of fungal biomass will contribute to improving the rumen digestion by providing necessary nitrogen and energy to the ruminant and microbiota.

Effect of Sodium Ion Concentration on Hydrogen Production from Sucrose by Anaerobic Hydrogen-producing Granular Sludge

This work evaluated the effects of sodium ion concentration, ranging from 0 to 16000mg·L-1(Na+), on the conversion of sucrose to hydrogen by a high-activity anaerobic hydrogen-producing granular sludge. At the opti- mum sodium ion concentration [1000—2000mg·L-1(Na+)] for hydrogen production at 37℃, the maximum sucrose degradation rate, the specific hydrogen production yield and the specific hydrogen production rate were 393.6— 413.1mg·L-1·h-1, 28.04—28.97ml·g-1, 7.52—7.83ml·g-1·h-1, respectively. The specific production yields of propionate, butyrate and valerate decreased with increasing sodium ion concentration, whereas the specific acetate production yield increased, meanwhile the specific production yields of ethanol and caproate were less than 55.3 and 12.6mg·g-1, respectively. The hybrid fermentation composition gradually developed from acetate, propionate and butyrate to acetate with the increase in sodium ion concentration.