Encapsulation and stabilisation of copper-chromium arsenic (CCA) sludge from timber treatment plants.

Milan Melicher

Cu-Cr-As sludges were subjected to cement-based stabilisation/solidification (s/s) treatments and acetic acid leaching (TCLP test). Stabilised sludges which contained sludge manufactured in the laboratory or from the Narangba treatment plant showed little loss of metals after leaching. All of s/s samples containing As in the concentration range 1800 to 41200mg/kg, (chromium and copper were in the range of 910 to 25800 mg/kg and 390 to 5500 mg/kg respectively) had a concentration of As in the leachate of below 1mgL. Arsenic, chromium and copper in the form of chromium and copper aresentates when stabilised by cement may not be considered hazardous to the environment. It was found that the leaching of metals from cement-sand mixes was dependent on the concentration of water-soluble chromium and arsenic present in the sludges. The sludges from commercial treatment plants contained water-soluble arsenic and chromium in the range of 47-904 mg/kg and 35 1294mg/kg respectively. In order to reduce the concentration of water-soluble metals and convert them into insoluble chromium and copper arsenates, some of the sludges were pretreated by the addition of sawdust as a reducing agent. The addition of 10% hardwood sawdust to sludge from the Mogo treatment plant reduced the concentration of water-soluble arsenic 7-fold and chromium 117 fold. The s/s treated sludges from the Mogo treatment plant without addition of sawdust did not pass the TCLP test. However, all of the sludges which were pretreated by sawdust met the EPA standards for disposal by landfill. Although the loadings of As and Cr in the s/s processed waste were up to 60130 mg/kg and 25000mg/kg respectively, the concentration of As and Cr in the leachates was under 5mg/L. The concentration of Cr in the leachates from s/s sludge was proportional to the water-soluble chromium (VI) in the sledges. The amount of chromium in the leachates of Cr(III)-bearing cement-sand matrices was 550 times less than the amount of Cr(III) which was originally added to the mix. Cement-sand matrices were able to reduce Cr (VI) leachability less than 2-fold: Cr(VI) is probably retained in the pores of the concrete particles. Thus, Cr(VI) was not chemically stabilised but physically encapsulated. A calculation formula for predicting the concentration of Cr in the leachates has been suggested. It was found that the leaching results fitted satisfactorily with those calculated using the formula. The addition of chromium did not significantly affect the setting and hydration of cement. Copper concentration in the leachates was 3000 times less than the theoretical leaching based on the addition of copper sulphate to cement-sand mixes. Chemical stabilisation rather than physical encapsulation was probably responsible for this reduction of copper mobility. Water-soluble copper might be stabilised by absorption on cement or by formation of copper hydroxide. The concentration of Cu in the most stabilised sludges was less than 1 mg/l.Cu in the form of CuSO4.5H20 significantly retards the hydration of cement and reduces the unconfined strength of cement-sand matrices. Arsenic trioxide (III) and As2O5 were stabilised by cement-sand mixes consisting of 1 part of cement and 2 parts of sand. These mixes released only around 1% of As present in the s/s form and the concentration of arsenic in all of the leachates was less than 1mg/L. Water-soluble As2O5 did not show a similar leaching pattern to that observed for Cr (VI). Arsenic (V) in the form of As2O5 in pure cement inhibited hydration. The contaminated cement-sand mix hardened after two days of curing, but had substanially reduced compressive strength. The unconfined compressive strength of cement-sand matrices containing 2 parts of As(V) in the form of As2O5 in pure cement inhibited hydration. The contaminated cement-sand mix hardened after two days of curing, but had substantially reduced compressive strength. The unconfirmed compressive strength of cement-sand matrices containing 2 parts of As(V) in the form of As2)5 per 100 parts of cement and 200 parts of sand was 60 N/cm2. Although As(III) affected the morphology of cement, the unconfined compressive strength of As(III)-bearing cement-sand matrix was relatively high (990 N/cm2).