Li Hai-ming ^1,2, Zheng Xi-lai¹, Liu Zhan-guang ², Liu Xian-bin²

(1) Institute of Environmental Science and Engineering, Ocean University of China, Qingdao 266071 ,China;

(2) Institute of Marine Science & Engineering , Tianjin University of Science &Technology, Tianjin 300457 ,China)

 

Abstract：On the basis of the field investigation of ammonia-nitrogen contaminated by landfills in the Tianjin shallow groundwater, two representative water-bearing media are sampled in the Tianjin coastal plain, and their major physico-chemical properties are analyzed. In addition, dynamic curves and isotherms of ammonia-nitrogen adsorption on water-bearing media have been measured. At last, retard coefficients(R) of ammonia-nitrogen are calculated by means of columns experimental data. The studies show the equilibrium period of the adsorption is 20-24 hours, and all the adsorption isotherms are Langmuir curves. The saturated capabilityunit weight decontaminated to ammonia-nitrogen of the silty sand and the silty soil are 0.771mg/g and 0.755mg/g respectively. The retard coefficient of the silty sand and the silty soil are 5.00 and 4.85 respectively, which means ammonia- nitrogen in landfill leachate can be adsorbed strongly by the water-bearing media when leachate penetrates into the subsurface of the coastal plain. As a result, the ammonia-nitrogen can difficultly migrate into the water-bearing in the Tianjin coastal plain.

Keywords: Littoral area; landfill leachate; water-bearing media; ammonia-nitrogen; adsorption

 

1 Introduction

          Sanitary landfills have been, and continue to be, an economical method for solid waste disposal. In China, until very recently, few municipal landfills were lined. The older landfills typically have no liner and a great reliance is placed upon the natural attenuation of contaminants in the underlying unsaturated soils and the groundwater aquifer by biological and physico-chemical processes. Leakage of inorganic and organic pollutants from unlined landfills over time can influence the groundwater quality and may pose a threat to drinking water resources. The transport of landfill leachate in soils is subject to various physical, chemical and biological processes that affect the eventual concentration of pollutants in soils and groundwater. Understanding the movement of leachate in soil is essential to predicting the potential for groundwater pollution from landfills(Islam,2002).

High concentrated ammonia-nitrogen is the principal water quality character of landfill leachate (Zhang,2000;Liu,2001;Jiang,2001;Guo,2002;Zhao,2002;Wen et al.,2004). (Wang, 1992;Xia et al.,2001; Wen et al.,2004) evaluated by an analysis of characteristics of landfill leachate and an assessment of the potential effects of nitrogen contamination on groundwater, which results showed that there were large amounts of ammonia-nitrogen in landfill leachate and the contamination of ammonia-nitrogen was the major form of pollution in groundwater of landfill. Kimmel and braids (1980) suggested that NH_4, when migrating into aerobic zones, would be oxidized resulting in elevated NO₃^- concentrations. However, no clear filed evidence was presented. Dissociation of NH₄ to free ammonia is not very likely in aquifers(Stumm and Morgan,1996). The disappearance of  NH₄ coincided with increased Mn concentrations in the plume, but the processes involved are not understood(Ludvigsen et al.,1998).On the basis of water and pollutants movement, transformation simulation test under the condition of different soil columns，a dynamic model for the soil-groundwater system was described.The model includes 4 pollutents(organic N, NH₄, NO₂, NO₃)transport and transformation in the unsaturated and saturated zone.The integrated simulation model was used to simulate wastewater and pollutants movement under the condition of landfill in Laogang town,Shanghai City (Wang et al.,1998). Wen et al. (2004) studied the effects of waste leachate on the ammonia-nitrogen adsorption ability in the soil nearby municipal waste landfill. The results showed that when large amounts of organic matter and metal ions coexisted with ammonia-nitrogen in the soil, it could decrease the adsorption ability of soil to ammonia-nitrogen. In addition, the high concentration of ammonia-nitrogen in the landfill the soil could inhibit the nitrifying microorganisms in the soil and decreased the transformation speed from ammonia-nitrogen to nitrate, resulting in that the nitration of soil has been restrained. 

The ammonia-nitrogen in leachate is known to penetrate into the water-bearing media of subsurface associated movement of leachate. In this process, ammonia-nitrogen is adsorbed by solid particles. Thus, it migrates in the water-bearing media retardingly comparing with leachate moves, when the more adsorption quantity is, more retarded action is. The adsorption isotherms of ammonia-nitrogen have been set up through tests, but very few people studied the adsorption of high concentrated ammonia-nitrogen in leachate on the water-bearing media, particularly strand plain of Tianjin.In this paper, adsorption kinetics equation and isotherms of ammonia-nitrogen are obtained by means of the tests.Retard coefficients(R) of ammonia-nitrogen in water-bearing media through column tests have combined convection-dispersion press of contaminants in soil-water systems with adsorption process, which is very important to predicting the potential for groundwater pollution from landfills.

2 The Background of the Research Areas

Tianjin is situated in northeast of North China plain, southern of Yanshan ’s mountain chain and west of Bohai Bay. The study area is a topography high in north and low in south, whose topographical features are mountain, hills, plain, littoral tidal flat from north to south. The study area is typically characteristic of a climate with warm and half wettish continental monsoon with an annual average temperature of 11 degrees centigrade , average evaporation of 1683 mm and average rainfall of 590.1 mm, over 80 percent of which comes between June and September.

Shallow groundwater is mainly Quaternary pore water, which the phreatic water changes gradually the weak-confined water downward from surface. Generally, from north to south, the aquiferous lithology changes from rough to fine, while its layer varies gradually from mono-layer to multi-layer. Groundwater generally follows the same course from north to south. Sand layers, well penetrated, distribute in the foot of the mountain with water replacing intensively. In this district, phreatic water is mainly extracted.

From north to south, the hydrochemistry distribution characteristic of shallow groundwater is horizontal zoning, which hydrochemistry type changes from HCO₃-Ca to HCO₃-Na, HCO₃.Cl-Na, and total dissolved solids (TDS) changes from 1g/L to 10 g/L, but exceeds to 30 g/L in coast local areas.

3 Adsorption Equilibrium of Ammonia-Nitrogen

3.1 Characteristic of landfill leachate

Landfill leachate were collected from a landfill in Tianjin, which pre-precipitated 24 hours before experimenting. Physical chemistry properties of the landfill leachate see table 1 which shows that the principal water quality characteristic of landfill leachate is high concentrated ammonia-nitrogen, Cl^-,TDS, total hardness (TH), CODcr、BOD₅

Tab.1   Physical chemistry properties of the landfill leachate

3.2 Physico-chemical properties of the water-bearing media

All the water-bearing media samples were collected from the strand plain of Tianjin. Considering the structure and compositions of the strand plain, two samples were selected respectively in depths of 10 and 20 m. The mechanical composition of the water-bearing media was analyzed with eyedropper method. At the same time, organic content, bulk density, porosity of the soils was analyzed. The mechanical composition and main characteristics for the water-bearing media samples are presented in Table 2.

Table 2 Mechanical composition and major characteristics of selected water-bearing media

3.3 Determination of adsorptive dynamic curves of ammonia-nitrogen

In order to determine the adsorption equilibrium time of ammonia-nitrogen, an adsorption test was conducted with silt (FT) and silty sand (FS).

Eleven samples of 150ml landfill leachate were added into 250ml cleaning Erlenmeyer flasks respectively. After the temperature becomes constant, 5g soil samples FS were added into each Erlenmeyer flasks. In the process of the test, oscillation frequency was controlled at 60rpm, and oscillation periods were 1min, 3min, 8min, 20min, 1 hour, 2 hours, 6 hours, 12 hours, 16 hours, 20 hours and 24 hours respectively. And then, solid and liquid phases were segregated using a centrifuger, and the ammonia-nitrogen contents were measured respectively in order to determine the adsorptive dynamic curves of water-bearing media FS (see Fig.1). The determination of adsorptive dynamic curves for FT is the same as FS. The adsorptive dynamic curves are also presented in Fig.1.

According to the Fig.1, it is concluded the adsorptive dynamic curves of ammonia-nitrogen are logarithmic. In other words, the adsorption rate decrease with the increase of time. The adsorption for FT and FS reach equilibrium after 20-24 hours, so 24 hours may be used as equilibrium time for the following isotherm measurements.

 

 

 Fig.1 Dynamic curves of ammonia-nitrogen adsorbed by silty soil (FT) and silty sand (FS).

3.4 Determination of ammonia-nitrogen adsorption isotherms

The landfill leachate mentioned above was diluted to seven different concentrations. Then, 150ml-diluted leachates were added into 250ml Erlenmeyer flasks, and 5g soil (FS) were added. After being oscillated for 24 hours in homothermal oscillator (20℃) at 60rpm, the supernatants were centrifuged for 10min, and ammonia-nitrogen contents were measured. In this case, the equilibrium adsorption quantity can be attained with their mass balance. The adsorption isotherm for FS is presented in Fig.2. The adsorption tests for soil(FT) is the same as the FS. 

 

Fig.2  Adsorption isotherms of ammonia-nitrogen for the silty soil (FT) and silty sand (FS).

In the Fig.2, all the adsorption isotherms are Langmuir model despite the different water-bearing media compositions (organic matter and clay contents).

4 Retard Coefficients(R) of Ammonia-Nitrogen

4.1 Determination of retard coefficients(R) of ammonia-nitrogen

In order to simulate the retard action of ammonia-nitrogen adsorbed by the water-bearing media, the media were installed into soil columns in accordance with the same bulk density. The adsorption test was conducted with the landfill leachate under the constant water head. The control conditions are listed in table 3.

Table 3 Conditions of adsorption test for the water-bearing media

Sample code	Column diameter (cm)	Column length (cm)	water head difference (cm)	Hydraulic gradient(-)
FS	10	5.5	108	10.8
FT	4	5.5	108	27

Under the condition of constant water head, the leaching tests last for 50 h and 200 h for FT and FS respectively. The effusive volumes and theirammonia-nitrogen concentrations were measured periodically. In this case, the curves of the ammonia-nitrogen concentration related to time can be calculated (Figure 3).

 

Figure3 Ammonia-nitrogen concentration profile with time

Tab.4 The saturated capability of ammonia-nitrogen adsorbed by the water-bearing media

Soil types	Cin （mg）	Cout （mg）	Saturatedadsorptivecapability, Sm (mg/g)
Silty sand	482.19	210.89	0.771
Silty soil	293.47	201.33	0.755

4.2 Calculation of retard coefficients(R)

4.2.1 Saturated adsorptive capability of ammonia-nitrogen ()

In order to obtain the saturated capability of ammonia-nitrogen adsorbed by the water-bearing media, according to law of conservation of mass, considering the inlet minus the effluent is equal to capability of ammonia-nitrogen adsorbed by soil columns at sample interval. The saturated adsorptive capability of ammonia-nitrogen is calculated as

                          (1)

thus，

where C0_i is the concentration of ammonia-nitrogen food into soil columns , mg/L; C_i is the concentration of ammonia-nitrogen outflow from soil columns ,mg/L; Q_i is the flux outflow from soil columns, L/d；△t_i is the sample interval ,d; N is the number of sampling period. C_in is the total mass of ammonia-nitrogen food into soil columns, mg; C_out is the total mass of ammonia-nitrogen outflow from soil columns, mg/L;W is the total mass of the water-bearing media in soil columns, g.

The saturated capability of ammonia-nitrogen adsorbed by two soil columns is calculated from the Eq.(1). The results see table 4.

4.2.2 Calculation of retard coefficients(R)

Transport of ammonia-nitrogen in the soil columns can be approximated as a one-dimensional advection dispersion equation, here extended with source-sink terms to account for the sorption process:

      （2）

where C is the concentration of the ammonia-nitrogen (mg/L); C₀ is the initial concentration of the ammonia-nitrogen (mg/L); x is the coordinate in the column(cm); t is the time(h);  is the coefficient of hydrodynamic dispersion (cm²); is actual average velocity of landfill leachate flow in the column (cm/h).is retard coefficient(-)；Due to the adsorption isotherms of ammonia-nitrogen are Langmuir model, so

  （3）

here is partition coefficient of ammonia-nitrogen between solid phase and liquid phase (cm³/g)； is relative concentration(-)；denotes the bulk density（g/cm³）and  the porosity(-).      

Numerical method is used to solve Eq.(2). The length of the column are divided into segments, , taking time step is . denotes the relative concentration of where and when , the following result of discretization to Eq.(1) used implicit difference method:

      （4）

where,

When of Eq.(3) is replaced by , it leads to the  corresponding expression of Eq.(2) .

Due to ,,,,,, and  of the equations are known, selecting the varied value of、 used Langmuir model lead to series of adsorption curve. By means of comparing the adsorption curves with the measured breakthrough curves of the ammonia-nitrogen concentration, a goodness of fit curve is gained (Fig 4). It shows that it is good fitting to useLangmuir model, corresponding parameters see table 5.

 

Fig.4 The fitting curve of ammonia-nitrogen concentration profile with time

Table 5 Retard coefficient and partition coefficient of ammonia-nitrogen at different water-bearing media

Sample	Flux (cm3/h)	(cm/h)	D (cm2)	K (cm3/g)	R (－)
FS	18.71	0.79	7.70	0.10	5.00
FT	18.45	0.73	2.28	0.12	4.85

From the table 5, adsorption of the silty sand to ammonia-nitrogen is greater than silt and retard coefficient are 5.00 and 4.85 respectively, which means ammonia-nitrogen of landfill leachate can be adsorbed strongly by the water-bearing media when landfill leachate penetrates into the water-bearing media; Moreover, the value of coefficient obtains from the column experiment is less from absorption isotherms. In theory, the result from column experiment is close to the fact, and is also more dependable.

5 Conclusions

(1) The adsorption of ammonia-nitrogen on the water-bearing media is conformity with the logarithmic dynamic curves, which means the adsorption rates decrease with the increase of experimental time. And the adsorption equilibrium time is 20-24 hours.

(2)All the adsorption isotherms of ammonia-nitrogen on the water-bearing media are Langmuir model although the media have different adsorptive capacities.

(3) Retard coefficient is from 4.85 to 5.00, which means ammonia-nitrogen of landfill leachate can be adsorbed strongly by the water-bearing media when leachate penetrates into the media; That is to say,ammonia-nitrogen of landfill leachate not easily move in the water-bearing media.

Acknowledgement:

This research work was funded by the Science and Technology Development Foundation of Tianjin（20040709）,and the Startup Scientific Research Foundation of Tianjin University of Science and Technology.

 

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