Every year, municipalities collect vast amounts of leaf litter, which have to be disposed of. Studies assessing the total amount of leaf litter in European cities are missing; however, the number of city trees has been a focus of recent studies. For instance, the total number of shrubs and trees in urban Sheffield, United Kingdom, increased from the 1900s to 2010 by 50.5% [1
], indicating the high value of green areas in cities. A survey from England revealed that town size had no effect on tree density, and mean density was 58.4 trees and shrubs ha−1
]. Leaf litter originates from private gardens, parks and other green areas dedicated to citizens’ relaxation and sport activities. While leaf litter from parks is often contaminated with soil, it may be less affected by pollutants [3
], whereas leaf litter collected from the road surface may contain both soil contamination and organic, as well as inorganic pollutants.
Contamination with pollutants is a potential problem for public acceptance of biomass conversion plants fed with urban leaf litter. Up until now, there is little known about the concentration of heavy metals and polycyclic aromatic hydrocarbons (PAHs) in leaf litter from urban roads. Heavy metals pose a serious threat to human health [4
]. Past literature has reported heavy metal concentrations in soil [5
] and the distribution of concentrations in soil within cities, with higher values for Cd and Pb for roadside areas compared to urban park areas [7
]. Plant availability and uptake of heavy metals, including the possibility of phytoremediation of soils (e.g., [8
]), as well as concentrations of heavy metals in urban water (e.g., [9
]) and in plants grown in urban areas (e.g., [10
]) have also been determined. The general behavior of heavy metals in combustion depends on the volatility of the element in question [11
]. The more volatile heavy metals (e.g., Pb, Zn, Hg) have a tendency to vaporize in combustion and condense on fine particles and, therefore, are found in cyclone or filter fly ash or escape with the flue dust into the atmosphere. However, if the combustion unit is fitted with an efficient particulate removal system, the amount of heavy metals escaping the combustion system with the exhaust is minimal [12
]. Less volatile heavy metals on the other hand, such as As, Cr, Cu and Ni remain in the bottom ash of the furnace to a large extent [13
]. Concentrations in the various ash fractions must be considered, as they may restrict the potential uses of the ashes and may require disposal if the concentration in the ash fraction is higher than is legally allowed for application as a fertilizer and liming agent on agricultural fields or in forests [12
]. To our knowledge, there is no scientific investigation on the degree of heavy metal contamination in street leaf litter designated for combustion. Potential sources of contaminants are motor vehicle emissions, tire wear and asphalt road surfaces, as well as domestic fire emissions, gasworks and commercial incinerators [14
]. Research on the concentration of inorganic, as well as organic contaminants in urban biomass has focused on detecting suitable plants as bio-indicators for pollution (e.g., metals in moss [15
], grass [16
], Quercus ilex
] as well as in Taraxacum officinale
, Pinus sylvestris
, and in Tilia cordata
]) and ensuring healthy food production in urban environments (e.g., [19
]). Two British practical trials investigating the contamination of street leaf sweepings found that the pollution with PAHs was too high for composting purposes [20
Energy recovery from urban leaf litter may provide the opportunity to exploit a new renewable energy resource. However, for common energy recovery technologies (e.g., anaerobic digestion or direct combustion), urban leaf litter may be a problematic feedstock. Due to the high fiber and ash content of leaves, the methane yield from anaerobic digestion is low, as was shown by Kosse et al. [22
] for urban chestnut leaf litter (137 lN
VS) and Liew et al. [23
] for an urban tree leaf litter mixture consisting mainly of maple leaves (55.4 lN
VS). Concerning combustion, it is well known that high mineral concentrations in non-woody biomass may lead to problems regarding emissions of nitrogen oxides (NOx
) polychlorinated di-benzo dioxins and furans (PCDD, PCDF), sulphur oxides (SOx
), carbon-monoxide (CO) and particulate matter (PM), as well as corrosion, ash slagging and fouling due to ash melting at low temperatures [11
]. Recent studies have shown that the chemical composition of urban leaf litter is not as suitable for combustion as woody biomass [3
]. In particular, the concentration of N, Mg, S, K and total ash are considerably higher. Piepenschneider et al. [3
] found a mean total ash concentration of 15.92% dry matter (DM) in leaf litter. Compared to the range of ash concentrations in woody biomass (0.3% to 5.0% DM, [11
]), the ash concentration was 3- to 53-fold higher in leaf litter than in woody biomass.
An integrated technique has been suggested (i.e., integrated generation of solid fuel and biogas from biomass, IFBB) to produce solid fuel from non-woody biomass with decreased mineral, N and total ash concentrations by means of a washing and dehydration treatment [27
]. The IFBB process has been shown to reduce the mineral concentrations especially of water soluble elements, such as K and Cl, for semi-natural grassland [28
], park leaf litter [3
] and urban green cut biomass [29
]. However, little is known about the possible contamination of the urban input materials with heavy metals and PAHs in the IFBB system. Piepenschneider et al. [30
] investigated the concentration of 16 elements (Ca, K, Mg, N, Na, P, S, Al, Cd, Cl, Cr, Cu, Mn, Pb, Si and Zn) in urban grass clippings from roadsides and found that the city grass clippings did not contain elevated heavy metal concentrations in comparison to agricultural or landscape conservation grass material. The study by Piepenschneider et al. [30
] also investigated the effect of the IFBB system and found a reduction in the concentration of Ca, K, Mg, N, Na, P, S, Cl, Mn, Si, Zn and total ash, but an increase for Cr and Cu. Therefore, there is a knowledge gap regarding the organic and inorganic pollution contained in street leaf litter and the effect of the IFBB system on this potentially contaminated material.
Thus, the aims of this study were:
To investigate the concentrations of PAHs and heavy metals in urban leaf litter from street sweepings;
To determine the effect of provenience and collection technique on the concentration of PAHs and heavy metals in urban leaf litter from street sweepings;
To investigate the effect of washing and dehydration through the IFBB system on the heavy metal concentration;
To develop a linear regression model to predict the heavy metal concentration in urban leaf litter.
2. Materials and Methods
2.1. Collection of Leaf Litter
Leaf litter was collected from the city of Kassel, which is located in central Germany and has about 200,000 inhabitants within 107 km2. Sampling was conducted in collaboration with the local cleansing department, who provided leaf litter in accordance with their routine work. Therefore, we were able to distinguish among various proveniences ((i) main road (MR); (ii) residential area (RA) and (iii) city center (CC)) and between different collection techniques ((i) vacuum technique (VT) and (ii) sweeping technique (ST)). The three proveniences had different cleaning cycles: main roads were cleaned weekly, residential areas biweekly and the city center was cleaned every second day. The vacuum technique is appropriate for sites which cannot be swept (e.g., parks and small greens along sideways) and where leaf litter is initially gathered with leaf blowers and subsequently picked up by a suction unit. The sweeping technique is usually applied on compacted surfaces and leaf litter is picked up with a rotating brush after moisturizing. Leaf litter collection took place after litter fall in calendar weeks 41, 43, 45 and 47 in 2014, resulting in 24 samples (4 dates × 3 proveniences × 2 collection techniques). For conservation, samples were ensiled in 60 L airtight polyethylene barrels for a minimum of 12 weeks.
2.2. Washing Facility and Washing of Leaf Litter
The washing facility is 100 × 100 × 60 cm (width × depth × height) with a sink in the lower part as a sedimentation zone for dirt and waste, which were removed after washing. Approximately 550 L of water were constantly swirled by pressing air through perforated pipes (7 pipes with 6 holes, each with a 4-mm diameter) at the bottom of the tank using a side channel compressor (RICO, 1.1 kW, intake pressure 1000 mbar, rotational speed 2900/min). A screen basket (98 × 98 × 45 cm, width × depth × height) with a mesh size of 1.1 cm was inserted into the washing facility. Ascending air bubbles broke up leaf litter clots and mobilized adhering dirt particles which sank through the sieve into the sedimentation zone. The screen basket held back leafy material, which could then be evacuated from the washing area.
After evacuating the material from the plastic barrels, the biomass was mixed and impurities (i.e., bottles, plastics, stones, wood, etc.) were removed. Depending on water content approximately 3 to 10 kg of leaf litter were used. Machine washing was conducted for 5 min at a water temperature of 10 to 12 °C. The washed material was further processed by mechanical separation with a screw press (type AV, Anhydro Ltd., Kassel, Germany). The conical screw had a pitch of 1:6 and a rotational speed of 6 r·min−1. The cylindrical screen encapsulating the screw had a perforation of 1.5 mm.
2.3. Chemical Analysis
Subsamples of each sample were taken at each treatment stage: (i) unwashed; (ii) washed; and (iii) mechanically dehydrated. From these subsamples, dry matter was determined by drying the material at 105 °C. For elemental analysis, material was dried at 60 °C. Subsequently, material was ground with a cutting mill (SM 1, Retsch, Haan, Germany) to pass a 5-mm sieve and subsequently with a sample mill (1093 Cyclotec, Foss, Hamburg, Germany) to 1 mm. C, H and N concentrations in samples were determined with an elemental analyser (Vario Max CHN Elementar Analysesysteme, Hanau, Germany). Concentrations of Fe and Mn were determined with X-ray fluorescence spectroscopy. For determination of additional element concentrations, material was processed with microwave pressure digestion. Concentrations of Cr, Cu, Ni, as well as Zn were measured with inductively coupled plasma optical emission spectrometry, concentrations of As, Cd, Pb, as well as Tl were measured with inductively coupled plasma mass spectrometry, and Hg was measured with cold-vapor atomic spectrometry. In this study, the term “heavy metals” refers to the elements As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Tl and Zn.
PAHs were determined in unwashed material shortly after opening the barrels. Therefore, samples were taken and stored at 4 °C prior to extraction and determination of PAHs with the gas chromatographic method with mass spectrometric detection. As there is a broad range of PAHs, preselection of substances was necessary. 16 PAHs, which are commonly used and recommended by the US-Environmental Protection Agency to assess the occurrence of this group of substances in the environment, were measured, namely Naphthalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, Benz(a)anthracene, Chrysene, Benzo(b)fluoranthene, Benzo(k)fluoranthene, Benzo(a)pyrene, Indeno(123-cd)pyrene, Dibenz(ah)anthracene, and Benzo(ghi)perylene.
2.4. Statistical Analysis
and Figure 2
were generated with SigmaPlot, Version 12.3 (Systat Software, San Jose, CA, USA) and Figure 3
was generated with R, Version 3.0.2 (R Foundation for Statistical Computing, Vienna, Austria). R software was also used to detect differences in element concentrations between or among proveniences, collection techniques and degree of materials processing by applying either Wilcoxon’s rank-sum test for differences between two independent groups or the Kruskal-Wallis test for differences among more than two independent groups with the post-hoc function “kruscalmc”. In the case of degree of materials processing, groups were not independent, so significant results should be interpreted against this background. The dendrogram was based on Euclidean distances, which were hierarchically clustered with Ward’s method. Regression analysis was conducted with R software by applying the “lm” function. Residuals were normally distributed except for Ni, Fe and Cu. Homoscedasticity was verified visually, which revealed Cu to be the only heteroscedastic element.
The investigated urban leaf samples were partly contaminated with heavy metals and PAHs. Concentrations of heavy metals were considerably higher than those found in wood, but still below the limiting values of the German standard for non-woody materials. Concerning the investigated organic pollutants, the concentrations of PAHs in the material were lower than expected. There was a clear effect of provenience for both heavy metal and PAH contamination, with the samples from the city center being the least contaminated, whereas samples from main roads and residential areas had higher concentrations of contaminants. A possible explanation for this finding might be the different collection cycles, with the shortest collection cycle occurring in the city center. It is logical that fast collection after leaf fall will reduce contamination with road dust and soil. Hence, if leaves are used for energetic purposes, fast collection is advisable. The effect of collection technique was not always a significant factor, but for As, Pb and Fe, collection with the sweeping technique resulted in significantly higher contamination than the suction technique. Thus, it is advisable to use the suction technique where possible to produce a cleaner material. The washing and subsequent dewatering with the IFBB process proved to be successful in reducing heavy metal contamination in the biomass, especially for As, Cr, Cu, Fe, Mn and Ni. However, these elements will be contained in the wash water or the press liquid, so further research is necessary to investigate the safe disposal of both. A linear regression model was developed with total ash concentration as a useful indicator for heavy metal contamination. Further research is necessary regarding the evaluation and development of more complex models to predict heavy metal and other contaminants more precisely. The urban leaf litter collected in the present study had heavy metal contaminations that would allow for direct combustion. Furthermore, these levels could even be reduced by a washing and dewatering step, which would ensure that heavy metal contamination is well below critical values.