黑土對(duì)凍融有機(jī)質(zhì)的吸附作用

郭 平,明 戀,王觀竹,李悅銘,李琳慧,于濟(jì)通

(吉林大學(xué) 環(huán)境與資源學(xué)院,地下水資源與環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,長春 130012)

近年來,對(duì)溶解性有機(jī)質(zhì)(dissolved organic matter,DOM)環(huán)境行為的研究已逐漸成為土壤學(xué)、 環(huán)境科學(xué)和生態(tài)學(xué)的研究熱點(diǎn)之一.DOM具有比固相有機(jī)質(zhì)更多的活性點(diǎn)位,可參與土壤中的多種反應(yīng)[1],通過靜電吸附、 配位體交換、 配合作用、 疏水作用、 熵值效應(yīng)、 氫鍵作用和陽離子鍵橋作用等方式與土壤膠體結(jié)合[2-3].

目前,對(duì)天然溶解性有機(jī)質(zhì)[4]、 土壤溶液中的有機(jī)質(zhì)[5]、 垃圾滲濾液[6]和畜禽糞便可溶性有機(jī)質(zhì)[7]的吸附研究較多,對(duì)經(jīng)過凍融處理的DOM吸附研究較少.凍融是我國北方地區(qū)的一個(gè)重要?dú)夂蛱卣?作為一種非生物應(yīng)力,對(duì)土壤中各種物質(zhì)的環(huán)境行為具有重要影響.本文以農(nóng)業(yè)生產(chǎn)中常用的秸稈和污泥為研究對(duì)象,考察東北黑土對(duì)凍融和未凍融處理秸稈和污泥DOM的吸附作用,并分析了pH值、 離子強(qiáng)度和離子類型與黑土吸附凍融處理秸稈和污泥DOM間的關(guān)系.

1 材料與方法

1.1 材料采集與處理

黑土采集于吉林省長春市生態(tài)園農(nóng)田0～20 cm表層土壤,在室溫下自然風(fēng)干,剔除植物殘?bào)w和砂礫等雜物,按四分法獲取足量樣品研磨,過1 mm篩,保存于聚乙烯塑料封口袋中備用.黑土的基本理化性質(zhì)列于表1.

表1 供試黑土的理化性質(zhì)Table 1 Physical and chemical properties of experimental black soil

秸稈和污泥分別取自長春市生態(tài)園和長春市大成玉米廠.玉米秸稈經(jīng)蒸餾水洗凈后,風(fēng)干,剪成約1 cm的碎片后,粉碎備用.污泥樣品經(jīng)自然風(fēng)干后,粉碎過0.5 mm篩備用.將污泥和秸桿碎屑的含水率調(diào)節(jié)為飽和含水率的90%后,進(jìn)行凍融處理(-25 ℃凍結(jié)24 h,5 ℃融化24 h,48 h為一個(gè)凍融周期).秸稈和污泥經(jīng)6個(gè)凍融周期處理后,分別加入0.01 mol/L的NaCl溶液在振蕩器中振蕩浸提,用0.45 μm濾膜過濾浸提液,所得濾液即為DOM.將所得DOM保存于4 ℃冰箱備用,其基本理化性質(zhì)列于表2.

表2 DOM的基本理化性質(zhì)Table 2 Physical and chemical properties of DOM

1.2 吸附實(shí)驗(yàn)

將浸提的DOM用0.01 mol/L的NaCl溶液稀釋5倍后,用移液管分別向5個(gè)100 mL玻璃瓶中移取2,4,6,8,10 mL的DOM溶液,并用0.01 mol/L的NaCl溶液分別定容至25 mL.所得未凍融秸稈和污泥DOM的初始質(zhì)量濃度分別為13.63～111.75 mg/L和17.73～98.35 mg/L,凍融秸稈和污泥DOM的初始質(zhì)量濃度分別為43.05～146.12 mg/L和23.57～116.75 mg/L.用棉塞封瓶后,進(jìn)行高壓鍋滅菌,抑制微生物活性.冷卻后,在超凈臺(tái)內(nèi)稱取已滅菌的黑土0.5 g分別置于上述5個(gè)玻璃瓶中,于25 ℃,170 r/min下振蕩2 h達(dá)到吸附平衡后,先用0.45 μm濾膜過濾,再用紫外分光光度法測定吸附前后DOM的質(zhì)量濃度[8].

2 結(jié)果與討論

2.1 黑土對(duì)凍融與未凍融DOM的吸附作用比較

圖1為黑土對(duì)凍融與未凍融秸稈和污泥DOM的吸附作用比較.由圖1可見,無論秸稈和污泥是否經(jīng)過凍融處理,黑土對(duì)秸稈和污泥DOM的吸附量均為負(fù)值,表明秸稈和污泥DOM未被黑土吸附,反而促進(jìn)黑土有機(jī)質(zhì)的解吸釋放.在實(shí)驗(yàn)平衡質(zhì)量濃度范圍內(nèi),DOM的解吸釋放作用隨DOM平衡質(zhì)量濃度的增加而下降.主要原因?yàn)椋?/p>

1) 本文實(shí)驗(yàn)黑土有機(jī)質(zhì)的含量較高.土壤中有機(jī)質(zhì)含量越高,對(duì)DOM的吸附越弱[9].

2) 秸稈和污泥DOM親水組分和小分子組分含量較高.親水組分和小分子組分不易被土壤吸附,并促進(jìn)土壤有機(jī)質(zhì)的解吸釋放[10-11].

3) 實(shí)驗(yàn)條件.由于添加的DOM初始質(zhì)量濃度較低,因此黑土未吸附DOM,DOM反而促進(jìn)了黑土中有機(jī)質(zhì)的解吸釋放.

圖1 黑土對(duì)凍融和未凍融秸稈(A)和污泥(B)DOM的吸附作用比較Fig.1 Comparison between adsorption of DOM from straw (A) and sludge (B) by freeze-thaw and no frozen treatments

在相同平衡質(zhì)量濃度下,與未凍融秸稈和污泥的DOM比較,凍融作用增強(qiáng)了秸稈和污泥DOM對(duì)黑土有機(jī)質(zhì)的解吸作用,降低了黑土對(duì)DOM的吸附作用.這是由于未凍融秸稈和污泥DOM初始質(zhì)量濃度分別小于凍融秸稈和污泥DOM的初始質(zhì)量濃度所致,或與凍融作用改變秸稈和污泥DOM各組分含量有關(guān).由表2可見,凍融秸稈和污泥DOM中的小分子組分和親水組分所占比例分別大于其大分子組分和疏水組分所占比例,與文獻(xiàn)[12]結(jié)果相符.由圖1可見,秸稈DOM對(duì)黑土有機(jī)質(zhì)的解吸釋放呈曲線關(guān)系； 污泥DOM對(duì)黑土有機(jī)質(zhì)的解吸釋放呈線性關(guān)系,這與DOM性質(zhì)和組成有關(guān)[13].

2.2 不同影響因素對(duì)凍融DOM吸附作用的影響

圖2 pH值對(duì)黑土吸附秸稈(A)和污泥(B)DOM的影響Fig.2 Influence of pH value on the adsorption of the DOM from straw (A) and sludge (B) on black soil

2.2.1 pH值 圖2為不同pH值對(duì)黑土吸附秸稈和污泥DOM的影響.由圖2可見,黑土對(duì)秸稈和污泥DOM的吸附值均為負(fù)值,表明在不同pH條件下,秸稈和污泥DOM未被黑土吸附,反而促進(jìn)了黑土有機(jī)質(zhì)的解吸釋放.秸稈和污泥DOM對(duì)黑土的解吸釋放量隨秸稈和污泥DOM平衡質(zhì)量濃度的增加而降低.在相同秸稈和污泥DOM平衡質(zhì)量濃度下,不同pH值對(duì)黑土吸附秸稈和污泥DOM的解吸釋放量絕對(duì)值均為pH=8.5>pH=7>pH=5.5,與文獻(xiàn)[14]結(jié)果相符.pH值對(duì)DOM吸附的影響是通過改變?nèi)芤弘姾擅芏纫约癉OM組成及其吸附特征實(shí)現(xiàn)的[15],此外,土壤溶液中的H+與較多的化學(xué)反應(yīng)有關(guān),pH值改變會(huì)引起其他成分的性質(zhì)發(fā)生變化,進(jìn)而影響土壤對(duì)DOM的吸附作用.

2.2.2 離子強(qiáng)度 圖3為不同離子強(qiáng)度對(duì)黑土吸附秸稈和污泥DOM的影響.由圖3可見: 黑土對(duì)污泥DOM的吸附量均為負(fù)值;除了當(dāng)離子強(qiáng)度為0.1 mol/L、 秸稈DOM平衡質(zhì)量濃度較高時(shí)的吸附量為正值外,黑土對(duì)秸稈DOM的吸附量均為負(fù)值.表明在3種離子強(qiáng)度下,污泥DOM未被黑土吸附,反而促進(jìn)了黑土有機(jī)質(zhì)的解吸釋放； 對(duì)于秸稈DOM,當(dāng)離子強(qiáng)度和DOM平衡質(zhì)量濃度較低時(shí),DOM促進(jìn)黑土有機(jī)質(zhì)的解吸釋放,隨著DOM平衡質(zhì)量濃度的增加,DOM對(duì)黑土有機(jī)質(zhì)的解吸作用逐漸降低.在相同DOM平衡質(zhì)量濃度、 不同離子強(qiáng)度下DOM對(duì)黑土有機(jī)質(zhì)的解吸量絕對(duì)值為c(NaCl)=0.001 mol/L>c(NaCl)=0.01 mol/L>c(NaCl)=0.1 mol/L,表明黑土對(duì)秸稈和污泥DOM的吸附量隨吸附體系離子強(qiáng)度的增大而增大,與文獻(xiàn)[16]結(jié)果相符.這是由于當(dāng)離子強(qiáng)度較大時(shí),黑土和DOM表面的負(fù)電荷均被屏蔽,因此,在黑土吸附DOM時(shí)會(huì)產(chǎn)生較小的排斥力,導(dǎo)致吸附量較大.同時(shí),黑土與DOM間的排斥力減小使其相互吸附并纏繞而形成無規(guī)則團(tuán)狀,表面積增加,導(dǎo)致更多的DOM被吸附.隨著離子強(qiáng)度的減小,DOM和黑土表面的負(fù)電荷屏蔽減小,黑土和DOM間的排斥力增加,使黑土與DOM間的纏繞逐漸展開成線狀,因此吸附下降.

圖3 離子強(qiáng)度對(duì)黑土吸附秸稈(A)和污泥(B)DOM的影響Fig.3 Influence of ion strength on the adsorption of the DOM from straw (A) and sludge (B) on black soil

圖4 離子類型對(duì)黑土吸附秸稈(A)和污泥(B)DOM的影響Fig.4 Influence of ion type on the adsorption of the DOM from straw (A) and sludge (B) on black soil

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