Reply to the comments on "A novel approach to peatlands as archives of total cumulative spatial pollution loads from atmospheric deposition of airborne elements complementary to EMEP data: Priority pollutants (Pb, Cd, Hg)" by V. De Vleeschouwer et al.

In the comments by De Vleeschouwer et al. (2020) on a recent paper by Miszczak et al. (2020), two major issues were critically discussed: (1) the behavior of lead in ombrotrophic peatlands, with particular regard to the possible lead vertical mobility/immobility; (2) lead data use to accurately reconstruct historical contamination. The authors stated that "some of the conclusions reached by Miszczak et al. (2020) are based on misinterpretation or incorrect sampling and data analyses approaches". A reply to comments emphasises that these topics are not an issue of the paper. Its major idea was to use the unique natural systems (that are ombrotrophic peat bogs) as complete and reliable inventories for the assessment of cumulative loads of airborne element deposition independently upon its chronology. The results of a study conducted on ten ombrotrophic peat bogs in Norway and Poland showed a striking quantitative precision of such assessment. This has led to the idea of including ombrotrophic peat bogs into the EMEP network as tools for the completion of spatial distribution data on the fugitive element deposition. It would be helpful if a bigger number of experienced and widely recognized researchers take part in such project. Simultaneously, the analysis of source data, own results and case studies makes clear that the information regarding ombrotrophic peat properties is still insufficient to reconstruct precisely the chronology of metal contamination records, despite the development of high resolution sampling and analytical techniques and interpretational approaches. The clarification of some seemingly biased records would help to elucidate unexplained or unusual lead behavior in some outstanding cases. These cases demonstrate also that despite over 40 years of studies there are still the substantial gaps in our knowledge that need to be filled up.

The role of humic substances in chromium sorption onto natural organic matter (peat).

To elucidate mechanisms of Cr3+ sorption onto the unaltered solid natural organic matter, the comparative studies of this ion binding from a solution at pH 4.0 onto three selected particle size fractions: 2000-1000 microm, 630-200 microm and 63-20 microm of markedly different HS content and structure, separated by a wet sieving from an overall sample of peat (Brushwood Peat Humus) were carried out. Comparable patterns of COOH groups and CEC(t) confirmed that for cation exchange capacity were responsible mainly cations connected with COO- functional groups. It was though found that aliphatic acids in the solid state did not take part in Cr3+ binding, thus the finest studied fraction 63-20 microm of the highest contents of functional groups showed the lowest sorption capacity for Cr3+, while similar patterns of sorbed Cr3+, soluble HS content and base CEC(0) indicated that these parameters were directly interrelated. The base ion exchange processes determined by CEC(0) (with Ca2+ as a predominant exchangeable cation) appeared to be not the major mechanisms responsible for Cr3+ sorption. For this metal, strong binding to insoluble large molecular weight organic pool two- to threefold prevailed over the ion exchange processes. Very low acid desorption indicated generally low mobility of Cr3+ -organic compounds.

Binding and chemical fractionation of heavy metals in typical peat matter.

Laboratory batch studies were conducted to evaluate the binding capacity and the mobility of metal species bound to typical humus peat matter. The identification of phase composition of mineral fractions and functional groups in the organic matter was assessed. The results showed generally high, but different retention capacity and binding strength, suggesting distinct diversity in binding mechanisms, phases and chemical nature of binding sites, depending on the metal species and their input concentrations. In general, the binding capacity of peat for the metal ions studied follows the order: Cr(3+) > Cu(2+) > Zn(2+) > Cd(2+) and results in the decrease of pH in the same order, due to displacement of H(3)O(+) from the peat by metal ions. The highest metal enrichment occurs in fractions F1(EXC), F2(CARB), F4(MRO) and F5(OM) of different binding strength adequate to exchangeable, carbonatic, moderately reducible amorphous Fe-oxide and organic/ sulphidic fractions in soils and sediments. In relation to species distribution in peats, the prevailing part of Cr(3+) is strongly bound in oxidizable organic substrate, while Cu(2+) is highly enriched in the moderately reducible F4(MRO) and the most labile F2(EXC) fractions. Cd(2+) and Zn(2+) are predominantly bound in the labile F1(EXC) and F2(CARB) fractions. Diversity of the predominant binding phases for the studied metals suggests rather weak competition for binding sites between chromium and copper ions; the strongest competition between the sorbed metal ions is anticipated for F1(EXC) and F2(CARB) fractions.