Research On Preparation And Characterization Of Heavy Metal Chromium Leaching Of Sewage Sludge-based Adsorbents

In recent years, the generation of sewage sludge is growing increasingly. Sometraditionally accepted disposal routes have already can’t meet the requirement of thelegislations, and can’t reach the goles of energy saving and environmental protection.It’s an inevitable trend to abtain more cost effective and environmentally benignalternatives, one potion that exhibits especial promise, due to its potential to valorise thesludge and recycle the resources, is the conversion of the sludge into adsorbents.In this study, residual sewage sludge has been taken as raw material, and convertedinto adsorbents, which is porous, through pyrolysis with added ferric salt as activatingagent. Single factor experiment were used to determine the effect of the five selectedfactors on physical characteristics of adsorbents, and the five control factors includepyrolysis temperature, pyrolysis dwell time, heating rate, the ratio of ferric salt anddried sludge, impregnate time. It can be concluded from the results that the optimumconditions for produce adsorbents are as follows: pyrolysis temperature is700°C,pyrolysis dwell time is60min, heating rate is20°C/min, the ratio of ferric salt and driedsludge is0.8and the impregnate time is15h. The iodine number/index and methyleneblue adsorption of the sewage sludge–based adsorbent generated under this conditionwere328.25mg/g and24.5mg/g respectively.In order to investigate the stability of heavy metals in the adsorbent, chromiumleaching tests were conducted to determine the effects of pH, pyrolysis temperature andthe initial contents of chromium in the dried sludge. By adding Potassium Dichromateto dried sludge artificially to get dried sludge that contains different amounts ofchromium. Thus, there were four kinds of dried sludge, and chromium contents were35mg/kg,100mg/kg,500mg/kg and1000mg/kg respectively. The pyrolysis temperaturewere200°C–850°C, pH were1–12.Leaching test results indicate that Cr6+is stabilized in adsorbents and can’t beeasily released, while the leaching amount of total chromium increase as the chromiumcontents of the dried sludge gradually increases, and gradually decrease as the pyrolysistemperature increases except for700°C,750°C,800°C and850°C. There are two turningpoints at700°C and800°C, and it’s slightly increases at750°C and850°C due to theorganic volatile or pore structure destruction. Thus, the leaching amount of totalchromium reaches the minimum at700°C, it is range from0.07to1.14mg/L as a resultof the pH changes, and at800°C, the leaching amounts of total chromium is range from0.07to1.3mg/L. It’s not difficult to find that the leaching amounts of total chromium ofadsorbents produced at700°C and800°C both are smaller than1.5mg/L, which is themaximum allowable concentration of total chromium of Integrated wastewater discharge Standard GB8978–1996. As mentioned previously, there are approximately40percent ferric salt in the mixture that consists of dried sludge, activating agent anddifferent amounts of Potassium Dichromate. A world of Fe2+was leached along with thechromium undoubtly. The leaching amounts of total Fe of adsorbents reaches theminimum at700°C also, with the results of0–10mg/L.It can be concluded from the results and discussion above that the adsorbentsproduced by sewage sludge that with initial contents of chromium less than1000mg/kgand pyrolysis at700°C is safe for applied to sewage treatment progress, as long as thepH of the sewage within1–12.