Influence of salinity and acidity on bioavailability of sludge-borne heavy metals. A case study of Bangkok municipal sludge

This study was conducted to assess the potential of Bangkok sewage sludge for amendment of saline and acidic soil with respect to heavy metal mobilization. Experiments were carried out at six levels of salinity (with electrical conductivity values of 2, 4, 8, 19, 31 and 42 dS m(-1)) and two levels of acidity (with pH values of 5 and 6). A pure sludge sample was used as a control system. Subsamples were collected and analyzed for total and bioavailable heavy metals. Three sets of soil-sludge mixture were prepared using different weight ratio (1:1, 4:1 and 10:1) to simulate the conditions in the field with sludge-amended soil. Samples of soil-sludge mixture were incubated for 4 weeks to stabilize organic substances. The mobility and concentration of bioavailable heavy metal in varying soil-sludge mixture ratio with different electrical conductivity (EC) values were evaluated with the use of DTPA (diethylenetriamine pentaacetric acid) extraction techniques. Pure saline soil sample from Sakhon Nakhon (North-eastern part of Thailand) was used of salt-heavy metal interactions on the germination percentage in soil mixed with sludge and to screen crops or varieties for their tolerances to soil salinity. Seven types of seeds - mung bean, corn, soybean, tomato, stringbean, sunflower and marigol - were planted in each of the three soil-sludge mixtures (1:1, 4:1 and 10:1) with different salinity levels. Seedlings were monitored by counting the number of seeds that are germinated and seed germination percentage can be used as an indicator for the level of salinity unsuitable for plant growth.