| Abstract |
A study entitled “Sewchar for carbon sequestration, heavy metal stabilization and ornamental crop production” was conducted in the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani during 2020-2023. The objective of the study was to prepare and characterize biochar from sewage sludge (sewchar) generated at Muttathara sewage treatment plant, Thiruvananthapuram through pyrolysis and to study its potential for carbon sequestration, heavy metal stabilization and for ornamental crop production using marigold (Tageteserecta) as test crop. The study was carried out in five parts; viz pyrolysis of sewage sludge and sewchar characterization, leaching study, sorption and desorption experiment, incubation study and field experiment. Sewchar was produced from sewage sludge through the process of slow pyrolysis. The temperature and residence time for preparation was standardised and sewage sludge pyrolyzed at a temperature of 400℃ for 2 hours showed the highest sewchar yield (56.64 %). The scanning electron micrographs of sewchar showed a complex morphology with several channels, pores and exhibited structure resembling to fish scale bundles with different particle dispersal of inorganic components. The prepared sewchar had a specific surface area of 8.69 m2 g-1, pore volume of 0.014 cm2 g-1, water holding capacity of 58.75 %, bulk density of 0.41 Mg m-3 and moisture content of 0.68 %. The results revealed that it had a pH of 6.20, EC of 2.25 dSm-1 with CEC and AEC of 45.41 cmol(+) kg-1 and 8.43 cmol(-) kg-1 respectively. The elemental composition analysis revealed a total carbon content of 5.28 %, total nitrogen 0.92 %, hydrogen 0.41 % and sulphur 0.91 %. The organic matter fractionation showed that it contains 92.01 g kg-1 humin, 4.55 g kg-1 fulvic acid and 0.78 g kg-1 humic acid. With respect to nitrogen fractions, sewchar contained 22.40 mg kg-1 NO3-N and 184.80 mg kg-1 NH4-N. It also contained 7.8 % P, 1.6 % K, 0.69 % Ca, 0.51 % Mg, 0.91 % S, 1275 mg kg-1 Fe, 1083 mg kg-1 Mn, 264 mg kg-1 Zn, 292.2 mg kg-1 Cu and 9.70 mg kg-1 B. The total heavy metal analysis in sewchar showed highest content of Cr (79.9 mg kg-1) which was followed by Pb (67.5 mg kg- 1), Ni (46.9 mg kg-1) and Cd (4.32 mg kg-1). A soil column experiment was conducted for 180 days to assess the leaching and mobility of nutrients and heavy metals from sewage sludge (SS), sewage sludge compost (SSC) and sewchar (SC). The leachable content of heavy metals (Pb, Cd, Ni and Cr) in sewchar was significantly lower than those in SSC and SS. Total N, K and Ca content in the leachate was highest in SS treatments followed by SSC and SC. Total P, Mg and S was highest in SSC followed by SS and SC. The highest value of micro nutrient content in leachate was observed followed by SS and SC. The highest value of micro nutrient content in leachate was observed in SS followed by SSC and the least in SC. Soil analysis during the initial and final stages of the experiment showed that the total heavy metal (Pb, Cd, Ni and Cr) was the highest in treatment receiving 10 % SC and the least in 5 % SSC, except for Pb where the least value was observed in 5 % SS. Regarding the available heavy metal the highest value was observed in the treatment which received 10 % SS and the least in 5 % SC. Heavy metal fractionation studies of the unleached (30th day) and leached soil (180th day) showed that the fractions such as exchangeable, carbonate, reducible and residual fractions were high in SS, followed by SSC and the lowest was observed in SC. All these fractions were found to decrease from 30th day to 180th day. The sorption and desorption behaviour of sewchar, sewage sludge, sewage sludge compost and soil for nutrients showed that sorption behaviour of substances followed the order SC > SSC > soil > SS and desorption in the order SS > soil > SSC > SC. As the nutrient concentration increased the sorption was found to decrease. An incubation study was conducted to assess the effect of sewchar on carbon sequestration, nitrogen dynamics, heavy metal fractions and available nutrients. The active pool of carbon, total organic carbon, water soluble carbon and microbial biomass carbon was highest in FYM treatments during the initial days while sewchar treatments showed the highest value for slow carbon, recalcitrant carbon and particulate organic carbon during the entire period of incubation. The maximum carbon mineralisation was observed on the 30th day of incubation with highest value in treatment receiving 50 g SSC followed by FYM treatment. The aggregate associated carbon in the soil followed the order macro aggregate associated carbon > micro aggregate associated carbon > silt + clay associated carbon. Sewchar applied treatments had the highest macro aggregate associated carbon while FYM applied treatment have the highest micro and silt + clay associated carbon during the initial days and later it was high in 50 g SC applied treatment. A gradual reduction in the carbon content was observed in all the fractions from 0 to 180th day of incubation irrespective of the treatments. Organic matter fraction studies showed that humic acid, fulvic acid and humin content were highest in SSC, FYM, SC treatments respectively. During the initial days of incubation, the highest value for NH4-N was observed in the treatment with 50g SSC and NO3-N for 50g FYM while at later stages it was highest with 50g SC. The organic nitrogen content in soil followed the order SS > SSC > FYM > SC > control during the entire period of incubation. The total nitrogen content in the soil also followed the similar trend.The total heavy metal content was highest in the sewchar treatments and available heavy metal was highest in SS treatment. Heavy metal fractionation studies revealed that among the different fractions of cadmium the highest value was found in residual fraction followed by oxidisable, exchangeable and reducible fractions. For Ni it followed the order residual fraction >oxidisable fraction > reducible fraction > exchangeable fraction while for chromium, the residual fraction accounts the highest followed by oxidisable and reducible fraction and for lead it follows the order residual fraction >oxidisable fraction > reducible fraction. For all the heavy metals a slight increase in residual fraction was observed from 0 to 180 days of incubation. Sewchar treated soil showed the highest pH during the entire period of incubation and the highest value for EC was observed in FYM treatment during the initial days and in later stages it was highest in SC followed by SSC and SS. In initial stages the highest mean value for available major nutrients (P, K, Ca, Mg, S and B) was recorded in treatment which received FYM while treatment which received SS had the highest micro nutrient content. In the later stages of incubation available nutrient content was highest in the sewchar treatment. A field experiment conducted to evaluate the effect of SS, SSC, SC and FYM showed that sewchar application @ 5 t ha-1 along with 10 t ha-1 FYM improved the plant growth parameters, floral parameters, biochemical parameters and yield parameters of marigold. It also improved the physical, chemical and biological properties of soil. Bulk density, porosity, water holding capacity, water stable aggregates, electrochemical properties of soil and nutrient availability were better with sewchar applied treatments. The highest plant height (138 cm), number of primary branches (23.05), number of secondary branches (47.12), flower weight (9.21 g), number of flowers (98) and least days to first flowering (31.82) was observed in treatment receiving 5 t ha-1 SC + 10 t ha-1 FYM. The highest flower yield of 15.12 t ha-1 was obtained from the treatment applied with SC @ 5 t ha-1 + 10 t ha-1 FYM. The treatment receiving sewage sludge @ 20 t ha-1 recorded the highest content of available heavy metals where as treatment receiving sewchar @ 5 t ha-1 + 10 t ha-1 FYM recorded the lowest heavy metal content. The residual effect of sewchar on yield was studied by raising a residual marigold crop in the same field after the harvest of first crop and the highest flower yield of 7.32 t ha-1 was observed in treatment receiving SC @ 5 t ha-1 + 10 t ha-1 FYM. From the present study it can be concluded that sewage sludge can be converted to sewchar through the process of slow pyrolysis at 400℃ for 2 hours. Pyrolysis process enhanced the surface area, pore volume, provides better pore structure and also causes the enrichment of nutrients as well as the heavy metals present in them, but reduces the availability of heavy metals. In the leaching experiment application of SC to soil decreased the leaching losses of nutrients and heavy metals. Porous nature of sewchar helped the maximum sorption and minimum desorption of nutrients compared to SSC, SS and soil. Application of sewchar improved the carbon pools and nitrogen fractions of soil and there was a gradual and sustained release of nutrients from sewchar. Sewchar application increased the total heavy metal content in the soil but decreased the available heavy metal due to the transformation of heavy metals from bioavailable exchangeable and carbonate (unstable) forms to stable forms (residual). Heavy metal fractionation study revealed that as the incubation days progressed from 0 to 180 days there was a slight decline in exchangeable Cd, reducible Cd and reducible lead and a slight increase in oxidisable fractions of Cd, Cr, Pb, residual fractions of Pb, Cr and reducible fraction of Cr. Field application of SC @ 5 t ha-1 + 10 t ha-1 FYM along with NPK as per POP was found to be superior in marigold crop production. There was also a reduction in the availability of heavy metals and improvement in physical, chemical and biological properties of soil. Thus, the soil application of sewchar to marigold @ 5 t ha-1 along with 10 t ha-1 FYM and recommended dose of fertilizers can be recommended to improve soil fertility through the gradual release of available nutrients, heavy metal immobilisation, carbon sequestration and increased flower yield. |
