Conner (1990) reviewed the options available for fluoride, cyanide and other inorganic compounds. Very soluble salts such as fluoride may be difficult to s/s due to high leaching. Chlorides are problematic and can accelerate cement setting. The sulphides of Hg or As may be stable at high pH, whereas nitrate salts are highly soluble and the choice of binder is very important for successful s/s. The importance of pH on Hg availability is highlighted by the relative stability of cinnabar at pH 5. At higher and lower pH this compound becomes increasingly soluble.
Cyanides may readily combine with some metals and form simple compounds of sodium, potassium, calcium, zinc and copper. Cyanide complexes involve both organic and inorganic molecules, but, because of the toxicity of cyanide compounds, they can destroyed by oxidation, prior to s/s.
Metal’s including Zn, Cd and Pb in ash and foundry dusts have been effectively treated by binders containing GGBS or PFA. Stabilising reagents such as soluble phosphates, soluble silicates or kiln dusts may also be added to binders to improve performance.
Technology Limitations The success of s/s to treat specific waste or soil containing organic or inorganic compounds may depend on both the use of pre-treatment and the choice of binder. However some inorganic materials are difficult to treat because of the complex nature of chemical and physical interference effects (Bates, 2010). Simple problems such as inadequate mixing of binder and soil/waste, too high moisture content or aggregation into clumps can also be an issue. Factors to be considered include:
• the physical characteristics of the soil/waste being treated; sands and silts are easiest while clays may produce mixing issues
• the environment into which the treated material is to be placed (ex-situ treatment) or that it exists in (in-situ treatment)
• logistical/economic/time constraints.
A number of inorganic compounds, including copper, lead, zinc, borates, phosphates and iodides, are known to
strongly interfere with s/s through sorption, complexation and precipitation reactions. Adsorption onto C-S-H to form an impervious coating can stifle hydration reactions.
The interference of boron was improved by the use of activated PFA during s/s to form a precipitate of NaB(OH)4 . The precipitate was then physically encapsulated in the treated product and leaching of B reduced by 100x (Palomo and de la Fuente, 2003).
The effect of anions is also important as, for example, sulphate can react with cement-hydrates to cause expansive ettringite formation and matrix expansion. In practice, mixtures of inorganic and organic contaminants are common and are routinely treated. Bates et al. (1999) reported a range of proprietary s/s systems used at a number of wood preserving sites and showed that despite the mixture of metals and organics present (including As, Cr, Cu, Pb, Zn), s/s was effective. One site, American Creosote in Tennessee, contained mixed metals and organics and was treated with cement, fly ash and activated carbon. Figure 2 shows the pugmill treatment during operation.
The appropriate use of bench trials will enable the interference effects to be identified and eliminated. The bench-scale testing of a number of wood preserving sites is described by Bates et al. (2000). The full-scale remedial operation at the American Creosote Site is also discussed (Bates et al., 2002).
Summary
The immobilisation of contaminants in cement-based systems may involve sorption, precipitation and incorporation into crystalline (and non crystalline) phases. A number of contaminants are known to interfere with cement-based processes, however it is possible to eliminate these with the s/s treatment options available. Most inorganic contaminants can be successfully solidified using cement/lime-pozzolan-based binder systems, but a thorough understanding of the behaviour of the contaminants is fundamental to meeting remedial performance targets.
ENVIRONMENT INDUSTRY MAGAZINE |111|
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192