Influence of Salinity and Acidity on Bioavailability of Sludge-Borne Heavy Metals. A Case Study of Bangkok Municipal Sludge

Abstract

This study was conducted to assess the potential of Bangkoksewage sludge for amendment of saline and acidic soil withrespect to heavy metal mobilization. Experiments were carriedout at six levels of salinity (with electrical conductivityvalues of 2, 4, 8, 19, 31 and 42 dS m-1) and two levels ofacidity (with pH values of 5 and 6). A pure sludge sample was used as a control system. Subsamples were collected andanalyzed for total and bioavailable heavy metals. Three sets ofsoil-sludge mixture were prepared using different weight ratio(1:1, 4:1 and 10:1) to simulate the conditions in the field withsludge-amended soil. Samples of soil-sludge mixture were incubated for 4 weeks to stabilize organic substances. Themobility and concentration of bioavailable heavy metal invarying soil-sludge mixture ratio with different electricalconductivity (EC) values were evaluated with the use of DTPA(diethylenetriamine pentaacetric acid) extraction techniques.Pure saline soil sample from Sakhon Nakhon (North-eastern partof Thailand) was used of salt-heavy metal interactions on thegermination percentage in soil mixed with sludge and to screencrops or varieties for their tolerances to soil salinity. Seventypes of seeds – mung bean, corn, soybean, tomato, stringbean, sunflower and marigol – were planted in each of the threesoil-sludge mixtures (1:1, 4:1 and 10:1) with different salinitylevels. Seedlings were monitored by counting the number of seeds thatare germinated and seed germination percentage can be used as anindicator for the level of salinity unsuitable for plant growth.