Studies on the agricultural use of limed-sewage sludge: Changes in forms and behavior of some heavy metals as affected by soil pH with long-term application of the sludge

Sewage sludge contains not only nutrient elements but also toxic elements like heavy metals, and causes heavy metal accumulation in soil after long-term application to agricultural land. To establish the guidelines of sewage sludge application to agricultural land in terms of heavy metal content, forms and behavior in soil, the mobility and availability of heavy metals were analyzed by observing the chemical forms of heavy metals in soil depending on the pH changed with long-term application of sewage sludge. In this study, sequential extraction procedure was performed to fractionate the soil heavy metals, Ni, Cu, Zn and Cd into five fractions: Fraction 1. Exchangeable (1 M NH4Cl extract-able) Fraction 2. Carbonates (1 M pH5 NH4OAc extract-able) Fraction 3. Fe-Mn Oxides(0.04 M NH2OH-HCl extract-able) Fraction 4. Organically bounded (30% H2O2 digestible) Fraction 5. Residual (HF-HClO4 digestible) During a 7-year period, plots of upland field (Grey Lowland Soil, CoSL, initial pH: 6.3) were treated with various rates (0, 0.5, 1.0 and 2.0 kg/square m) of de-watered sewage sludge, twice each year, spring and autumn just before planting of spinach and cabbage, respectively (a total of 14 applications). The sludge was conditioned using calcium hydroxide as a coagulant aid. Once each year after the cabbage crop, soil samples were analyzed to determine the chemical forms and contents of the heavy metals. 1) Carbonates of Ni, Zn and Cd, and exchangeable Cu were the predominant forms of these heavy metals analyzed in the sludge before application. 2) Sludge application resulted in pronounced increases in contents of perchloric acid digestible Zn and Cd and slight increases in Ni and Cu, as analyzed in the top soil(0-15 cm), where most of the heavy metals carried by the sludge were found. In plots with the highest rate of sludge, the Zn content in top soil increased from 76.5 mg/kg before application to 208 mg/kg after the 14th application. The Cd content increased from 0.24 mg/kg to 0.47 mg/kg. At this time Zn and Cd contents in top soil were approximately two times higher than those in the subsoil(15-30 cm). In contrast, total Zn and Cd taken up by the crops during the experimental period decreased from 250 and 1.8 mg/sqaure m in the control plot to 200 and 0.6 mg/square m with the highest rate of sludge, respectively. 3) The Fe-Mn oxides of Ni, Zn and Cu, and carbonates of Cd were the most abundantly increased forms of these heavy metals in the soil due to the sludge applications. In contrast, contents of exchangeable Zn and Cd decreased by sludge application, thereby concomitantly restricting the mobility and plant availability of the metals. 4) The increase rate of Fe-Mn oxides fraction of Zn and Ni, and carbonate fraction of Cd started to plateau after the 4th application when soil pH also plateau. This phenomenon suggests that an excessive amount of calcium over cation exchange capacity of soil might be a factor suppressing sorption of heavy metals to soil as those forms, and the heavy metals were leached from the top-soil to the subsoil as soluble organic ligands. Changes in form, mobility and availability of the heavy metals ( Ni, Cu, Zn and Cd ) were observed after soil amendment with limed sewage sludge was terminated. Soil plots which had received annual rates of 0, 1,2 and 4 kg/square mof sewage sludge between 1979 and 1985 (total of 0, 7, 14 and 28 kg/square m sludge for 7 years) were then planted annually with spinach (spring) and cabbage (autumn) between 1986 and 1989 (total of 8 crops). The acid fertilizer, ammonium sulfate, was incorporated into the soils at the rate of 94 g/square m before the start of each crop. 1) During the years of sludge applications, increasing soil pH, due to the large amount of calcium hydroxide present in the sludge, decreased the mobility and plant availability of the heavy metals. The chemical forms of these metals were present mostly in the carbonate and Fe-Mn oxides fractions. 2) During the years of ammonium sulfate applications, the pH in the soil plots decreased and the metals changed to easily exchangeable forms, hereby accelerating their mobilization and plant uptake. 3) On the basis of these results, it appears that the optimum amount of sewage sludge application should be less than 1 kg/square m a year. This rate is similar to that used for organic matter application in Japan, and should maintain soil pH in the range 6.3 to 6.8. The background level of zinc was determined in the farm soils across Kanagawa prefecture to assist in developing regional regulations for governing the recycling of sewage sludge. Zinc was extracted by the perchloric acid digestion method for total content. Zinc was also fractionated into five forms mentioned above for chemical speculation. The contents were categorized according to farm-land use (orchard, paddy field, and upland field), soil taxonomy, and by locations with total zinc content above 120 mg/kg, the national upper limit allowed in sewage sludge amended soil. 1) Within land use categories, the mean zinc contents were 171, 123, and 98 mg/kg for orchard, paddy field, and upland field soils, respectively. The higher zinc levels in orchard and paddy soils were due primarily to successive applications of zinc-containing agro-chemicals and to repeated use of zinc-containing irrigation water, respectively. 2) In upland field soils, characterized by the presence of High-humid Aerosol, Humid Aerosol and Light-colored Aerosol groups, (a) the High-humid Aerosol contained more zinc (114mg/kg) than the Light-colored Aerosol (86 kg/kg); (b)the zinc content was positively correlated with the cation exchange capacity (CEC, r=0.52**, n=274); and (c) among the five fractions, the content of exchangeable zinc was the least (less than 2% of total zinc) while that of the Fe-Mn oxides tended to be the highest (40% of total zinc). The Fe-Mn oxide form of zinc also increased in content with increasing total zinc content, and thus was the predominant form of zinc in soils with high total zinc content. Upland soils containing more than 120 mg/kg of zinc were found mainly in the eastern part of Kanagawa where high CEC, fine-textured, volcanic ash soils were presented. It is noteworthy that despite successive vegetable crop cultivation in upland soils (70 times during the past 23 years), the zinc content has remained virtually unchanged. 3) In paddy field soils, characterized by the presence of Wet Aerosol, Gleyed Aerosol, Gray Lowland soil, Brown Lowland soil and Muck soil groups, the Gleyed and Wet Aerosol groups contained more zinc (169 and 147 mg/kg) than other groups of paddy soils (120mg/kg in average). Paddy soils with total zinc content more than 120mg/kg were found all across the prefecture. 4) The results of this investigation showed that (a) prolonged agricultural use in upland soils has had little or no effect on the soils' zinc content; and (b) upland soils in Kanagawa prefecture with zinc content higher than the upper threshold standard limit for Japan is due primarily to the characteristics of the natural parent material, which consists mainly of volcanic ash from Mt. Fuji. The results indicate a need to revise our regulation concerning soil zinc content in Kanagawa. Such new regulation will allow us to use and manage our existing agricultural soils more effectively. To use sewage sludge as a soil amendment for long time, we attempted heavy metal removal from sewage sludge using bacterial leaching method. Sulfur-oxidizing bacteria ( Thiobacillus thiooxidans) and iron-oxidizing bacteria (T. ferrooxidans) were inoculated into anaerobically digested sewage sludge with 0.4% of sulfur powder as a substrate or sulfuric acid as a pH adjuster (initial pH was pH4). 1) After 10-day aeration of anaerobically digested sewage sludge inoculated sulfur-oxidizing bacteria and iron-oxidizing bacteria with sulfur powder, Ni, Cu, Zn and Cd were removed 40, 40, 80 and 70 % from the sludge respectively. Final pH of the sludge was pH2. 2) After 10-day aeration of anaerobically digested sewage sludge inoculated sulfur-oxidizing bacteria and iron-oxidizing bacteria with sulfuric acid. Ni, Cu, Zn and Cd were removed 35, 0, 50 and 50 % from the sludge respectively. Final pH of the sludge was pH4. More study about this technology is necessary for long term application of sewage sludge as a soil amendment, because of the potential efficiency and feasibility of the bacterial leaching method to remove heavy metals from sludge.