The effect of bentonite/cement mortar for the stabilization/solidification of sewage sludge containing heavy metals

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Abstract

This study examines stabilization/solidification (S/S) techniques using a bentonite/cement mortar. These techniques are usually applied for the stabilization of sewage sludge, containing heavy metals such as Cu, Zn, Pb. Due to the high organic content of sludge, bentonite had been added in order to stabilize the system.

For this purpose, 4 × 4 × 16 cm mortar prism samples were prepared. Their composition was: 50% w/w sewage sludge (primary sewage sludge from Psyttalia Athens and secondary biological sewage sludge from Metamorphosis Athens), 30% w/w cement CEMI 42.5 and finally 20% w/w bentonite. The samples were cured for 28 days at 25 °C and compressive strength was tested. Highest strength and lowest leachability were the criteria for selection of the optimum product by the S/S technique.

An extensive study using several characterization techniques focusing on hydration reactions was carried out. The instrumental analysis included: X-ray diffraction analysis, thermal analysis (TGA, DTA), electron scanning microscopy (SEM), infrared analysis (FT-IR) as well as tests for the toxicity characteristic leaching procedure (TCLP). The S/S products had been proven throughout the study as a viable solution for stabilizing heavy metals. They can be used in many applications such as in landfill liners, slurry walls and building blocks.

Introduction

Pollution caused by heavy metals is one of the most serious environmental problems, endangering fauna equilibrium. Heavy metals are stable and persistent environmental contaminants, as they cannot be degraded or physically neutralized.

Sludge contamination by heavy metals emerged as one of the most serious drawbacks encountered in the use of sewage sludge as a natural fertilizer [1]. Several researchers have studied the possibility of using natural clays for heavy metal ions immobilizing, due to their low cost and their large availability. Bentonite is one among these clays and it can be found as ingredient in the most cement blends around the world [2].

Bentonites are argillaceous materials that can be effectively employed as adsorbents for many wastewater pollutants, namely heavy metal ions and organic compounds due to the fact that they exhibit an enormous surface area when they are hydrated. This outstanding capability is due to the presence of the mineral montmorillonite and its structure, which is a determinant factor for the clay’s properties. Bentonite presents strong colloidal properties and its volume increases several times when coming into contact with water, forming a gelatinus and viscous liquid. The special properties of bentonite (hydration, swelling, water absorption, viscosity, thixotropy) render it as a valuable material for a wide range of uses and applications [3].

Depending on the nature of their original formation, bentonites contain – in addition to montmorillonite - a variety of accompanied minerals that may include kaolinite, quartz, feldspar, calcite and gypsum. Their presence affects the industrial bentonite value, reducing or increasing its price according to the relevant application.

Depending on the amount and type of clay minerals present in the bentonite, the availability of alumina differentiates as follows: in kaolinite clays, the large amount of available alumina is released into the high pH solution at a high rate forming large amounts of ettringite which lead to large expansions (19% in the vertical direction). In contrast, montmorillonite clay’s smaller alumina content, is released into the high pH solution at a five times slower rate, forming ettringite with no swelling development [4].

Stabilization/Solidification (S/S) is known as one of the most popular techniques for the treatment of hazardous waste and it has been used for decades an encapsulation material, prior to the disposal of radioactive and hazardous wastes [5], [6].

Cement-based S/S is effective for inorganic wastes, but this is not appropriate for the treatment of waste with high organic content. This is because the interactions occur between the organics contaminants and the cementitious matrix, affecting the process (e.g. setting time) and the properties of the stabilized product. Many organic compounds are widely known to have a retarding effect on the cement hydration reaction and adversely affect on the microstructure, influencing the mechanical and the leaching properties of the cementitious materials [7]. One of the possible solutions to this problem engages the use of bentonite in conjunction with conventional cement-based techniques.

Clay is added in order to adsorb the organic materials and chemically bind them into the waste. In that way, when the cement is added, the organic materials do not interfere anymore, following the usual hydration reactions. Thus, the organic materials are held within the organophilic clay, while the clay itself is physically entrapped within the cementitious matrix [8].

The aim of the present study is to illustrate that the cement based S/S process techniques using bentonite can be effectively applied in order to sufficiently treat sewage sludge containing heavy metals and also to assess the properties of the S/S products in order to apply them in landfill liners, building blocks and slurry walls.

Section snippets

Bentonite characterization

The bentonite used in this work originates from a S&B Industrial Minerals S.A. bentonite deposit quarry in Milos island, in Greece. The chemical composition is shown in Table 1. The presence of the clay mineral montmorillonite as well as quartz and calcite impurities as detected by XRD analysis is shown in Fig. 1.

Types of sludge

Two types of sludge were used for the experiments. The first type of sludge was a primary sludge from Psyttalia wastewater treatment plant, which serves the major Athens area and which

Compressive strength results

The prepared samples were cured for 28 days at 25 °C and their compressive strength was determined, according to the methods of the European Standard EN196-1 [14]. The results are shown in Table 4, from where it can be noticed that four (4) out of the eight (8) mixtures, displayed compressive strengths which exceeded the minimum limit of 350 kPa at 28 days [11]. These are the mixtures containing cement/sludge and cement/sludge/bentonite with wet sludge from Metamorphosis and Psyttalia.

Bentonite’s

Conclusions

Mortars with 50% wet sludge (from Psyttalia or from Metamorphosis), 30% cement and 20% bentonite produced by S/S techniques showed higher compressive strength than the minimum limit of the 350 kPa at 28 days.

XRD analysis clearly showed that hydration products were well formed.

Thermal analysis verified the above result.

SEM clearly identified typical hydration products, such as ettringite (needle-like crystals), calcium silicate hydrate (gel-like flocks) and calcium hydroxide (fibrous-like

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