Suitability of anthropogeomorphic materials as soil amendmentsa biogeochemical study

Resumo

Abstract In recent decades, the amount of waste produced by humans has increased greatly. Society demands increasingly high standards of production and quality, leading to the production of greater volumes of waste and consequent increases in the associated risks of contamination. There is international concern regarding the management of all types of wastes and the consequent environmental degradation and public health risks. Land application of wastes, such as animal excrement or vegetable residues, has been carried out since long time ago. However, many environmental problems have now appeared primary because the final goal of the addition of wastes to soils is often the elimination of wastes rather than the enhancement of soil fertility or improvement of soil physical properties. The objective of this thesis was to evaluate the suitability of anthropogeomorphic wastes as soil amendment or fertilizer. For this, different studies have been carried out and described in six chapters: four of them describe the study of Technosols and the other two describe the geochemical accelerated weathering study of biochar and biomass fly ash. Chapters 2 and 3 correspond to a column study of Technosols. The Technosols were formulated from the mixtures of organic and inorganic wastes at a ratio of 56:44 (w/w). The organic components used were an anaerobic (AN) and an aerobic (AE) sewage sludge. The inorganic wastes used were Linz-Donawitz slag (LD) and green foundry sand (FS) and were termed conditioners. A mixture of the two conditioners at a ratio of 50:50 (w/w) was made to provide a third type of conditioner (FS + LD). Controls consisted of columns filled with organic waste only (AN or AE). There were eight treatments in total: AE, AE + FS, AE + LD, AE + FS + LD, AN, AN + FS, AN + LD, and AN + FS + LD. The objectives were to evaluate the influence of the acid buffering capacity of the ingredients on the chemistry of the leachates of different Technosols and to assess the mobility of the heavy metals. The results demonstrated that the main processes determining the pH of the systems were nitrification and leaching of alkaline cations, but organic matter decomposition and carbonation may also have had an effect, although to a lesser degree. The lowest final pH values of the leachates were found in the AE sludge control (5.0) and the treatment AE + FS (5.4), whereas the highest was 11.1 in the treatment AN + LD slag; the final pH of the other leachates ranged between 7.0 and 8.0. Nitrification was strongly retarded in the treatment of Resumen / Abstract anaerobic sludge attributed to both the low initial Eh and the probable absence of nitrifiers. Nitrification was also impeded in those mixtures in which LD slag was used as a component (due to the liming effect). The results of the heavy metal concentrations in the same leachates indicated that the concentrations of Cu, Ni and Pb in the leachates of anaerobic sludge treatments were generally higher than those of aerobic sludge treatments. The concentration of Cu was significantly higher in the leachates of the Technosols containing LD slag than the others. The concentrations of Zn in the leachates of the aerobic sludge control and the aerobic sludge + foundry sand Technosols were significantly higher at pH < 6.0 due to more Zn bound to Fe-Mn oxide was dissolved. The concentrations of Ni in 80% of the leachates exceeded the EU drinking water limiting for Ni (0.02 mg L-1). The concentrations of Pb were lower in the leachates of Technosols containing foundry sand attributed to the retention of Pb by the smectite present in the foundry sand. The concentrations of Cd in the leachates of all the Technosols were relatively higher than the controls, while the concentrations of Cr were higher in the controls. These results showed that the elaboration of Technosols has significantly altered the behaviour of the elements present in the starting waste material. Chapters 4 and 5 correspond to a greenhouse experiment of Technosols in which the feasibility of different Technosols as substrates for ryegrass was evaluated. The Technosols were made from mixtures of sewage sludges and conditioners at two ratios: 60:40 and 50:50 (w/w, DW). Three types of sludges were used: aerobic sludge (AE), anaerobic sludge (AN) and CaO-treated aerobic sludge (AL). Four kinds of conditioners ¿ 5% of green foundry sand (FS), 10% of Linz-Donawitz slag (LD), 2% of barley straw (BS) and 23 or 33% of fly ash from pine bark combustion (FA) ¿ were used as amendments. Three Technosols containing 60% of sludge and 23% of FA were referred to as M1-AE, M1-AN and M1-AL. Another three with 50% of sludge and 33% of FA were identified as M2-AE, M2-AN and M2-AL. Controls were set for each type of sludge only (AE, AN, and AL sludges). The analyses of soils and soil solutions (sampled in situ) showed that the Technosols elaborated from anaerobic sludge contained the highest contents of N and P, and organic carbon (OC), while the Technosols derived from aerobic sludge had the lowest level of OC and macro nutrients. Aerobic sludge treated with CaO had the lowest availability of P, due to precipitation of large amount of phosphates with Ca. Resumen / Abstract The analyses of plants showed that the total above-ground biomass yields was in the order of M2-AN (21.1 g) ~ M1-AL (20.2 g) ~ M2-AL (19.9 g) ~ AN (19.8 g) ~ M1-AN (19.6 g) > AE (11.2 g) ~ M1-AE (11.0 g) > AL (9.8 g) ~ M2-AE (9.4 g) (¿~¿ denotes the lack of significant differences at P < 0.05). The addition of conditioners to the AN sludge did not improve the plant yield significantly; the addition of conditioners to the AL sludge significantly increased the plant yield; the addition of conditioners to the AE sludge significantly reduced the plant yield. The concentration of Cu in the ryegrass of the unamended AN sludge was above the Cu toxicity level for plants (20 mg kg-1). In general, the addition of conditioners efficiently decreased the bioavailability of heavy metals. Chapters 6 and 7 corresponded to the simulated geochemical weathering studies of biochar produced from biosolids and fly ash produced from pine bark combustion, respectively. The experiments were realized in a modified Soxhlet reactor. The weathering process took place at 30 °C, with a flow rate of water of 5 mL min-1 (equivalent to the rainfall of 273 mm h-1). The study of biochar weathering had two treatments, with and without the addition of humic acid (treatment BC-HA and BC-B, respectively) and the weathering time lasted for 300 hours in total. The study of fly ash geochemical weathering was conducted with different weathering periods: 1, 24, 120, 300, 800, 1304 and 1808 hours (h). The results of the geochemical weathering study of biochar revealed that substantial amounts of K (8.5¿10.2%) and S (20.2¿28.3%) were recovered in the weathering solutions. Noticeable Ca (17.9¿20.7%) and P (15.4%) in the solid were released but only a few were recovered in the weathering solutions because of the precipitation. The presence of humic acids increased this dissolution of the mineral ash fraction of the biochar and thus the availability of K, S, Ca, Mg and P. On the contrary, the humic acid induced N immobilisation. Nitrogen availability in the biochar was already very low (< 1.0% of the total N) due to the probable recalcitrant heterocyclic N structure. The pH of the biochar dropped from 8.4 to 7.5; this was mainly attributed to loss of base cations through leaching and probable carbonation of the alkaline solutions exposed to the air. The results of the geochemical accelerated weathering of fly ash showed that the weathering process is mainly through alkalinolisis. At the shortest time (1 h) there is already dissolution and mobilisation of alkaline cations and sulphates; at increasing weathering time, there is the progressive carbonation of the alkaline solution. The main processes that occur during the weathering of these materials are: (i) the neoformation of some secondary Resumen / Abstract products, mainly phosphates, and lower quantity of amorphous silica and low order grade 2:1 mineral during the carbonation process, (ii) carbonation, and (iii) the leaching of alkaline cations and sulphates. These occur with different intensity depending on the weathering time. Initially, the dominant cation in solution was K+ followed by Na+, and SO42- being the main neutralizing anion and with major mobility at short weathering times (< 300 h). However, at long weathering times, Na had the highest mobility, followed by SO42- and K+. These three ions had the highest concentrations and geochemical mobility in all the time sequence. The following important elements, based in their concentrations, are Si, Ca2+ and Mg2+, whose behaviours were very different from the former three ions. After reaching peak concentrations ¿ which were interpreted as the concentrations of oversaturation under the alkaline conditions of the experiment ¿, their concentrations and activities decreased considerably due to neoformation and/or adsorption, related to the thermochemical equilibrium of the system under the final conditions. This suggested the formation secondary minerals, most of which were amorphous or of low order grade that could not be detected by the XRD analysis. The presence of ion pair and complexes of sulphates of alkaline earth elements and bases indicated that sulphate minerals should be an important phase under evaporitic conditions. However, under the conditions of the present study none of the oversaturation and thermodynamic stability of sulphates has been found. The ions F-, Cl- and NO3- also followed exponential reactions, although at different concentrations and time: F- and Cl- reached equilibrium from 300 h on and NO3- after 800 h. Concentrations of heavy metals in the weathering solutions were negligible, due to their low activity at the alkaline pH of the final solutions. The phosphates were the minerals most oversaturated. In the case of silica, the formation of 2:1 clay and tectosilicates (feldespatoides and/or zeolites) associated with the secondary form of quartz seems plausible. The formation of the former could be inhibited or limited due to the low concentrations of Al and Mg in the solution at long weathering time and due to kinetic reasons. Under evaporitic conditions, carbonates could be formed, in addition to sulphates and, in some cases, hydroxides, especially at long weathering time. In both cases, the geochemical weathering study with Soxhlet reactor, the weathering concepts and mechanisms proposed by Pédro have demonstrated its importance in the interpretation of the weathering processes of biochar and fly ash. Keywords: Sewage sludge; Fly ash from biomass combustion; Linz-Donawitz slag; Foundry sand; Technosols; Biochar; Carbon sequestration.