Characterization of Leachate from Contaminated Sediment After Treatment by Stabilization/Solidification

Nataša Slijepčević1, Dragana Tomašević Pilipović1, Đurđa Kerkez1, Milena Bečelić-Tomin1, Dejan Krčmar1, Srđan Rončević1, Božo Dalmacija1

 

1 University of Novi Sad, Faculty of Sciences, Department for Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

There has been a rapid disturbing of the sediment quality as an inseparable part of the aquatic system due to exploitation of surface waters. The sediment contaminated by heavy metals represents potential risk for the environment. Certain remediation measures are required, but prior to applying any technique it is necessary to perform pilot studies. This paper presents the ex situ remediation of sediment applying stabilization/solidification treatment with cement as an immobilization agent after which a pilot-scale landfill was formed. The monitored parameters in leachate from the pilot-landfill, demonstrated the efficiency of the remediation technique, in addition to the safe disposal of such treated sediment into controlled landfills.

Keywords: leachate, cement, stabilization/solidification, metals.

Introduction

 

When sediment is contaminated due to inadequate management of aquatic ecosystems, the basic solutions for reducing the environmental risk are in situ and ex situ remediation. It is of great importance to emphasize that in selecting the techniques/methods to treat contaminated sediment, it is not advisable to use only facts from written sources but also those from completed pilot and laboratory research describing other possible technical solutions (Dalmacija and Agbaba, 2008). Ex situ remediation, which was implemented during this research, implies extraction of contaminated sediment (dredging/excavation), processing (dewatering, solidification/stabilization), transport and further disposal. The objective of solidification processes is to reduce solubility or chemical reactivity of the waste by changing its chemical condition or its physical properties (by microencapsulating). Processes of solidification try to convert waste into solid mass by which it can easily be handled with reduced hazard of evaporation, leaching or rinsing. Both processes are observed and implemented together because they have the same purpose of improving and maintaining potential components in treated waste. The solidification and stabilization (S/S) technique is considered the most suitable solution in solving the problem of toxic metal pollution in sediment. This pollution represents the most severe form of sediment pollution because toxic metals, unlike organic pollutants, cannot be degraded over time.

The implementation of Portland cement as an agent for solidification and stabilization of waste has been in practice for decades. This method is an excellent choice for the treatment of many types of waste because of its numerous advantages: low temperature during cement hydration; no waste gases are produced, aquatic waste is being chemically bound to the matrix; relatively low costs. Disposal of treated, contaminated sediment in controlled landfills is the most frequently applied option of sediment disposal as the last phase of ex situ remediation. It is considered to be economically profitable and it is very convenient because of reduced opportunities for pollution migration.

Leachate is the liquid being decanted through the landfill body. It consists of filtered water of atmospheric origin, constituent water, as well as water which is formed in the landfill body, polluted with organic and inorganic contaminants that exert a negative influence on the quality of landfill leachate. It is very hard to predict the real content of leachate because it depends on several factors such as: waste content, temperature and moisture content in waste, exterior parameters, migration course of liquid, landfill body thickness, phase of waste decomposition, possible self-cleaning processes, the capability of intermediate layers to adsorb/absorb and minimize the pollution and quality of the waters which are being infiltrated into the landfill.

Consequently, prior to establishing remediation technologies and assessing methods for treatment of contaminated sediment (waste) on the polluted locality, it is necessary to perform pilot studies. Only pilot studies can determine the operational and design parameters along with the required materials and investments with a required level of certainty and that can guarantee reliable results. Therefore, pilot studies are the most appropriate approach to selecting the best technology, taking into account investment and operating costs, the attained results and ultimate goals in managing the plant and the waste.

This paper deals with the problem of Grand Backi Canal where the sediment is contaminated with metals. The proposed sediment treatment methods are solidification/stabilization processes using cement as an immobilization agent. The results obtained by simulating these conditions can be of great assistance in modeling the behavior of treated mixtures after their disposal. In order to assess the applicability of such disposal methods for treated sediments and cement mixtures, the following parameters were investigated: the leaching of heavy metals, chlorides and sulfates in leachate; hydraulic conductivity of leachate; as well as parameters such as pH and electro-conductivity in leachate. The results should provide the information needed to assess whether it is possible to deposit treated material into the landfills as inert waste. This may also be of great importance in evaluating possible soil and groundwater contamination with heavy metals from the treated materials if these were to be used as construction material or if they are disposed into the landfill.

 

Materials and Methods

Investigations were carried out in several stages:

  1. Dredging (3-4 m3) and dewatering of the sediment which was used in subsequent investigations for determining optimal disposal techniques.
  2. Treatment of the sediment on the pilot-scale (solidification/stabilization)
    1. Forming solidified/stabilized mixtures of sediment with 5% and 10% of added cement mass (Capital Letters: C for cement and S for sediment).
    2. Curing of immobilized sediment for 28 days, followed by air drying of the mixtures.

 

After the mixtures had been formed, a certain period of time for diffusion and adsorption processes of polluted matters was necessary.

A location for sediment dredging was selected based on previous research, i.e., a location that exhibited the poorest sediment quality with concentrations of contaminated matter that significantly endangered the quality of this watercourse. The sediment was then dried to obtain the desired total water content. After drying, the sediment was mixed with an immobilization agent (cement) in order to obtain two mixtures containing 5% and 10% cement content relative to the total sediment mass. After the homogenization of these compounds, the optimal amount of water was added to each compound and the compounds were compacted all in accordance with the ASTMD1557-00 method, providing the pressure of 2700 kNm/m3 (56,000 ft1bf/ft3). The compounds were then placed into inert plastic bags and left for 28 days at a temperature of 20oC to initiate both the curing process of the immobilized sediment and the diffusion and adsorption processes of polluted matter.

The pilot–experiment lasted 6 months, from January to June. Each compound was treated in the same manner. All samples were exposed to humidification, adding certain quantities of water followed by observation of leachate. It took some time for added water to start leaking and this is followed by the time required for infiltrated water to pass as leachate through the sediment. The time required for water to flow through a porous medium, or in this case, as a mixture of the sediment and immobilization agent, was measured. The collected leachate with dissolved polluted matter was filtrated through a membrane filter (0.45 µm) and was subjected to analyses of various paremeters (hydraulic conductivity, pH, electro-conductivity, the content of chlorides and sulfates, and content of toxic metals).

The pseudo-total content of metals was determined in leachate samples which were analyzed using the flame technique AAS (Perkin Elmer AAnalyst™ 700) for the content of total metals Ni, Cu, Cd, and Zn in accordance with the EPA7000B procedure; and content of As was analyzed using the graffito technique AAS (Perkin Elmer AAnalyst™ 700), in accordance with the EPA 7010 procedure. Hydraulic permeability of materials was calculated from the testing results using a formula which is based on Darcy's law (Papadakis et al. 1991; Krishnan and Sotirchos 1994; Garrabrants et al. 2004). Potentiometric determination of pH was performed according to the SRPS H.Z1.111:1987 method and determination of electro-conductivity was performed according to the SRPS EN27888:2009. Determination of chlorides was performed by titration with silver nitrate according to the SRPS ISO9297:1997, completely from point 1 to point 8 of the standard, with the change of the SRPS ISO9297/1:2007. Determination of sulfates in water was performed using iodometric titration according to the P-V-44A (A2) method from the handbook Drinking water.

 

Results and Discussion

As shown in Table 1 it can be concluded that the mixture of sediment containing 10% of added immobilization agent proved to be a material with very low permeability, whereas the 5% cement mixture was of low permeability. Therefore, the transport of polluted materials through the mixtures was reduced due to the reduced mobility of toxic matter. Most of the water would not infiltrate the solidified/stabilized mixture of sediment and cement due to its increased impermeability, which from the standpoint of this parameter means that the treatment is efficient.

 

Table 1: Results of hydraulic permeability
tab01

 

fig01b     fig01b
Figure 1: pH values of leachate: a) mixture of 95% sediment and 5% cement and b) mixture of 90% sediment and 10% cement.

 

As shown in figure 1 it can be concluded that the pH value of leachate ranges between 6.2 to 9.5. At the beginning of the experiment the pH value was slightly above 7, during the experiment the value was stabilized and the value achieved a stable value of pH 8 at the end of the experiment. Stabilization of pH values from leachate of S/S mixtures was possible due to buffer characteristics and water reactions with mixtures. In the case where cement was used as an agent, water binds to and calcium silicate hydrated gel appears, in which no larger pH variations occurred and the media remained mostly neutral. These pH values of leachate compared with those prescribed by the Regulations of pollutants in surface waters ("Official Gazette of the Republic of Serbia" number 50/2012), satisfied the criteria and were classified into the Class 1 of surface waters.

The measured electro-conductivity of leachate in the analyzed mixtures (Figure 2) exhibited higher initial values for both mixtures which, over time became stable with little variation. Comparing these values with those prescribed in the Regulations of pollutants in surface waters ("Official Gazette of the Republic of Serbia" number 50/2012), it can be seen that the leachate from the mixture where cement was used as an immobilization agent belongs to Class 1. The presence of inorganic dissolved solid matter affects conductivity in water and therefore it can be concluded that if the values are high, the increased concentrations of sulfates and chlorides are present in leachate.

 

fig02a     fig02b
Figure 2: Eh values of leachate: a) mixture of 95% sediment and 5% cement and b) mixture of 90% sediment and 10% cement.

 

In Figure 3 an overall decreasing trend in sulfate concentration can be observed, while the content of chlorides remained largely unchanged throughout the experiment, and there were no high variations of content for both mixtures of sediment and cement. Comparing the measured values of leachate from mixtures with prescribed values from the Regulation on categories, researching and classifying of waste ("Official Gazette of the Republic of Serbia" number 56/2010), it can be concluded that values are within the limit values for disposal of non reactive hazardous waste. Based on the content of these salts in leachate, the treated sediment can be disposed of or used for other purposes.

Comparing the obtained values for chlorides and sulfates from the observed mixtures with the Statute on limit values of polluted matter in surface water, groundwater and sediment ("Official Gazette of the Republic of Serbia" number 50/2012), it can be concluded that the content of chlorides and sulfates in the leachate of both mixtures can be classified as Class 2 water, which can be used as drinking water, for recreation, for irrigation or in industry (as cooling or process water) after conventional treatment methods have been applied.

 

fig03a     fig03b
Figure 3: The content of sulfates and chlorides in leachate from the mixture of the sediment and: a) 5% cement, b) 10% cement.

 

Comparing cumulative values of leached metals (Figure 4) with the prescribed limit values in leachate from monolithic waste ("Official Gazette of the Republic of Serbia" number 56/2010), it is obvious that none of investigated metals exceeded limit values of concentration in leachate for non reactive waste. At the same time it meets the limit values for disposal of hazardous waste. Therefore both mixtures of sediment and cement are considered non-hazardous waste.

If values of leached metals are compared with the values of limit concentrations of metals in surface water which are prescribed by the Statute on limit values of polluted matter in surface water, groundwater and sediment ("Official Gazette of the Republic of Serbia" number 50/2012), it can be observed that the concentration of leached copper (Cu) in leachate from the 5% cement mixture is classified as Class 5, which corresponds with a bad ecological status according to the mentioned classification. The concentration of leached zinc (Zn) in the leachate from the 10% cement mixture classifies this leachate as Class 2, which represents a good ecological status. Contents of Cd and Ni in the leachate from the mixture of sediment and 10% cement do not exceed the maximum permitted concentrations according to the Statute on limit values of preferred substances and of preferred hazardous substances which pollute surface waters, and time limits for their achievement ("Official Gazette of the Republic of Serbia" number 24/2014) according to this it can be concluded that the leachate from the mixture belongs to Class 1 waters, while regarding the content of metals where a 5% cement mixture was used as an immobilization agent, the leachate exceeds the value for Class 5.

 

fig04a     fig04b
Figure 4: The cumulative percentages of leached metals from the mixture of the sediment and a) 5% cement, b) 10% cement.

 

By comparing values of leached metals with the limit values for leachate from surface disposal, as per the Statute on limit values of emission of polluted matter in waters and time limits for their achievement ("Official Gazette of the Republic of Serbia" number 48/12) it can be concluded that only copper (Cu) concentration in leachate from the 5% mixture exceeds prescribed limit value, while the rest of the metal concentrations meet the prescribed criteria from the Statute for both mixtures.

 

Conclusion

Based on the presented results of parameters in leachate: pH values, electro-conductivity and salt content, these low-permeability mixtures of sediment and cement after treatment can be deposited without harmful effects to the environment because they satisfy the prescribed criteria according to Serbian regulations. If cumulative percentages of leached metals are taken into consideration, the 10% cement mixture proved to be more efficient in retaining pollutants, especially concerning the metals in leachate. It is evident that additional research is needed aimed at implementing certain toxicity tests on bacteria and microorganisms, which can potentially exist in leachate from treated sediment.

 

Acknowledgments

The research results presented in this papaer have been produced with the financial assistance from The Ministry of Education, Science and Technological Development of the Republic of Serbia (Project Numbers III43005 and TR37004).

 

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