偕胺肟水熱碳對U(VI)-CO3/Ca-U(VI)-CO3的吸附性能研究

張志賓, 周潤澤, 董志敏, 曹小紅, 劉云海

偕胺肟水熱碳對U(VI)-CO3/Ca-U(VI)-CO3的吸附性能研究

張志賓, 周潤澤, 董志敏, 曹小紅, 劉云海

(東華理工大學(xué) 核資源與環(huán)境國家重點(diǎn)實(shí)驗(yàn)室, 南昌 330013)

水熱碳吸附材料具有制備工藝簡單、合成條件溫和、表面易改性等優(yōu)點(diǎn)。本研究以可溶性淀粉為碳源, 在硝酸鈰銨催化作用下, 將丙烯腈開環(huán)接枝到淀粉分子上, 通過水熱反應(yīng)和鹽酸羥胺還原制備偕胺肟化水熱碳(AO-HTC)。結(jié)合靜態(tài)和動態(tài)吸附實(shí)驗(yàn), 重點(diǎn)研究了溶液pH、碳酸根和鈣離子濃度對AO-HTC吸附鈾性能的影響, 通過Thomas和Yoon-Nelson模型探究AO-HTC吸附鈾的動態(tài)過程。結(jié)果表明: 隨著pH、碳酸根濃度和鈣濃度的增加, AO-HTC吸附鈾的容量逐漸降低; 摻雜5wt%AO-HTC土壤柱的穿透點(diǎn)和飽和點(diǎn)體積也隨之減小。與純土壤柱相比, 摻雜5wt%AO-HTC土壤柱的最大吸附容量(o)和吸附質(zhì)穿透50%所需的時(shí)間()增大了數(shù)倍。由此可見, AO-HTC是一種性能優(yōu)異的可滲透性反應(yīng)墻(PRB)介質(zhì), 有望用于修復(fù)鈾污染土壤和地下水。

偕胺肟基; 水熱碳; U(VI)-CO3/Ca-U(VI)-CO3; 吸附

鈾礦采冶過程中產(chǎn)生的大量含鈾廢水不僅會污染地表水, 而且會通過滲透污染地下水。鈾及226Ra、222Rn和210Po等衰變子體具有化學(xué)毒性和放射性毒性, 如果在環(huán)境中大量累積, 將會對人類的生存和發(fā)展構(gòu)成潛在的威脅[1-3]。因此, 鈾污染土壤和地下水的治理是有核國家所面臨的最具挑戰(zhàn)性的問題之一[4-6]。

目前, 土壤和地下水修復(fù)技術(shù)主要有植物修復(fù)[7]、物理修復(fù)[8]、化學(xué)修復(fù)[9]、微生物修復(fù)[10]和可滲透性反應(yīng)墻(PRB)技術(shù)[11]。其中, PRB技術(shù)具有高效、運(yùn)行簡單和管理費(fèi)用低等優(yōu)點(diǎn), 彌補(bǔ)了傳統(tǒng)修復(fù)手段的不足, 且適用于大范圍污染場區(qū)土壤及地下水的修復(fù)[12-13]。PRB是由充滿介質(zhì)的可滲透隔膜組成, 可以垂直攔截受污染的地下水流。當(dāng)水通過自然流動過程流經(jīng)PRB時(shí), 污染物會被介質(zhì)捕獲或降解, 通過物理、化學(xué)、生物或混合過程對含水層進(jìn)行修復(fù)[14-15]。常用的介質(zhì)有水熱碳、活性炭和零價(jià)鐵等[15-17]。其中, 水熱碳材料具有制備方法簡單、原料易得、合成條件溫和及耗時(shí)短等優(yōu)點(diǎn)。但水熱碳材料表面官能團(tuán)含量低且單一, 對鈾的吸附容量小[18], 不能直接用作PRB的介質(zhì)。

鈾在污染土壤和地下水中以U(VI)-CO3/ Ca-U(VI)-CO3的形態(tài)存在, 與海水中鈾的賦存形態(tài)相似[19-20]。偕胺肟基團(tuán)能夠提高吸附材料表面的活性位點(diǎn), 降低吸附材料與海水中鈾酰絡(luò)合陰離子之間的靜電斥力, 通過偕胺肟基團(tuán)中N原子和O原子與鈾酰離子絡(luò)合, 增強(qiáng)偕胺肟基團(tuán)與鈾酰離子的親和性, 從而提高吸附材料的吸附容量[21]?；诖? 偕胺肟改性材料有望作為PRB介質(zhì)治理鈾污染土壤或地下水。本課題組以可溶性淀粉為原料, 在硝酸鈰銨催化作用下, 將丙烯腈開環(huán)接枝到淀粉分子上, 通過水熱反應(yīng)和鹽酸羥胺還原成功制備偕胺肟化水熱碳球(AO-HTC), 在pH=5時(shí), AO-HTC吸附鈾的飽和容量724.6 mg·g–1, pH=1~5的范圍內(nèi), 吸附鈾的選擇性高于60%[22]。

本工作采用靜態(tài)吸附和動態(tài)吸附實(shí)驗(yàn)相結(jié)合, 重點(diǎn)研究溶液pH、碳酸根和鈣離子濃度對AO-HTC吸附鈾性能的影響, 利用Yoon-Nelson和Thomas模型研究AO-HTC對鈾的動態(tài)吸附過程, 進(jìn)一步探討AO-HTC吸附土壤和地下水環(huán)境中鈾的可行性。

1 實(shí)驗(yàn)方法

1.1 AO-HTC的制備

AO-HTC材料的制備過程如圖1所示[22]。在氮?dú)夥諊? 將10 g可溶性淀粉和3 g硝酸鈰銨加到裝有100 mL蒸餾水的三口燒瓶中, 加熱至35 ℃反應(yīng)30 min后, 加入4 mL丙烯腈, 溫度升至80 ℃反應(yīng)2 h, 冷卻至室溫后離心, 依次用無水乙醇和蒸餾水洗滌各三次、烘干, 得到中間產(chǎn)物A。取6.67 g樣品A置于高壓反應(yīng)釜內(nèi), 加入50 mL蒸餾水, 在180 ℃下反應(yīng)16 h, 用無水乙醇及去離子水洗滌產(chǎn)物數(shù)次, 烘干, 得到碳微球B。再取3 g碳微球B加到100 mL 40g·L–1鹽酸羥胺溶液中, 加入一定量的碳酸鈉固體, 并在78 ℃下攪拌反應(yīng)3 h, 產(chǎn)物冷卻到室溫后過濾洗滌至中性, 烘干, 得到偕胺肟功能化水熱碳微球(AO-HTC)。

根據(jù)本課題組前期研究, AO-HTC呈表面光滑的球狀, 粒徑為200~400 nm; 比表面積和孔體積分別為2.75 m2·g–1和0.02 cm3·g–1; FT-IR和XPS表征結(jié)果表明AO-HTC表面含有豐富的C=N、C–N、N–O等基團(tuán); 元素分析結(jié)果顯示AO-HTC的N元素含量為9.45%, 即偕胺肟基團(tuán)含量約為6.56 mmol·g–1, 大量的偕胺肟基團(tuán)為吸附鈾提供大量的位點(diǎn)[22]。

圖1 AO-HTC的制備路線圖

1.2 吸附實(shí)驗(yàn)

1.2.1 靜態(tài)吸附實(shí)驗(yàn)

鈾溶液的配制方法如下: 移取10 mL鈾標(biāo)準(zhǔn)溶液(10 mg·mL–1)至2 L燒杯中, 加入1.0 L蒸餾水, 攪拌均勻, 加入碳酸氫鈉溶液(1 mmol·L–1), 靜置20 min, 再加入硝酸鈣溶液(0.5 mmol·L–1), 攪拌均勻, 用蒸餾水定容至2 L, 即得50 mg·L–1鈾溶液[23], 使用硝酸溶液調(diào)節(jié)鈾溶液至既定pH。

準(zhǔn)確稱取0.005 g AO-HTC, 加入到50 mL鈾溶液中, 恒溫震蕩一定時(shí)間后, 取上層溶液離心, 用偶氮胂Ⅲ法[24]測定上層清液的鈾濃度。鈾吸附容量e/(mg·g–1)的計(jì)算如式(1)所示。

式中,0為溶液初始鈾濃度(mg·L–1),e為溶液平衡鈾濃度(mg·L–1),為溶液體積(L),為吸附劑質(zhì)量(g)。

1.2.2 動態(tài)吸附實(shí)驗(yàn)

動態(tài)吸附實(shí)驗(yàn)如圖2所示。柱A中裝填1 g土壤(簡稱: 純土壤柱)作為參照; 柱B中裝填A(yù)O-HTC和土壤(簡稱: AO-HTC柱)模擬PRB, 其中AO-HTC和土壤的質(zhì)量比為1:19(即0.050 g AO-HTC與0.950 g 土壤)。通過蠕動泵將含鈾溶液輸送至柱A和B上端, 控制泵的流速為1 mL·min–1, 收集流出液, 用偶氮胂Ⅲ法[24]測定鈾濃度, 直至流出液的鈾濃度與上柱液相同。

圖2 動態(tài)吸附裝置圖

2 結(jié)果與討論

2.1 靜態(tài)吸附

2.1.1 溶液pH的影響

溶液pH對AO-HTC吸附鈾性能的影響如圖3(a)所示。pH為6.0時(shí), AO-HTC材料對鈾的吸附效果最好, 吸附容量達(dá)到216 mg·g–1。pH對吸附劑表面電荷以及溶液中鈾形態(tài)有很大影響。隨著溶液pH的增大, 偕胺肟基團(tuán)中氨基的質(zhì)子化效應(yīng)減弱[25], 羥基易失去質(zhì)子留下帶負(fù)電的氧, 氧原子存在的孤對電子極易占據(jù)鈾的最外層空的電子軌道, 增加材料表面吸附位點(diǎn)對鈾的配位能力[26-27], 進(jìn)而提高吸附容量。

2.1.2 CO32–和Ca2+濃度的影響

CO32–和Ca2+濃度對AO-HTC吸附鈾性能的影響如圖3(b, c)所示。隨著CO32–和Ca2+濃度的逐漸增大, AO-HTC對鈾的吸附容量逐漸降低。固定Ca2+的濃度(0.5 mmol·L–1), CO32–濃度從1.0 mmol·L–1增大至4.0 mmol·L–1, 吸附容量從原來的213 mg·g–1降至103 mg·g–1。固定CO32–的濃度(1.0 mmol·L–1), Ca2+濃度由0.5 mmol·L–1增加至2.0 mmol·L–1, 吸附容量從213 mg·g–1降至107 mg·g–1。

圖3 pH(a)、CO32–濃度(b)、Ca2+濃度(c)對AO-HTC吸附鈾容量的影響, pH(d)、CO32–濃度(e)、Ca2+濃度(f)對土壤柱和 AO-HTC柱穿透曲線的影響

(a) 1.0 mmol/L CO32–, 0.5 mmol/L Ca2+; (b) pH=6.0, 0.5 mmol/L Ca2+; (c) pH=6.0, 1.0 mmol/L CO32–; (d) 4.0 mmol/L CO32–, 2.0 mmol/L Ca2+; (e) pH=8.0, 2.0 mmol/L Ca2+; (f) pH=8.0, 4.0 mmol/L CO32–

從鈾的形態(tài)分布圖4可知, 當(dāng)溶液pH為6.0時(shí), 鈾主要賦存形態(tài)包括(UO2)2CO3(OH)3–、UO2(CO3)34–、UO2(CO3)22–、(UO2)2CO3(OH)3–、CaUO2(CO3)32–、CaUO2(CO3)32–和Ca2UO2(CO3)3(aq)等, 即在土壤和地下水中鈾主要以鈣-鈾(VI)-碳酸絡(luò)合形態(tài)存在, 會造成可被吸附的鈾形態(tài)含量減少, 不利于吸附的進(jìn)行[28]。當(dāng)CO32–濃度>1 mmol·L–1、Ca2+濃度>0.5 mmol·L–1時(shí), CaUO2(CO3)32–和Ca2UO2(CO3)3(aq)形態(tài)占主導(dǎo), 可被吸附的鈾含量降低, 使得AO-HTC對鈾的吸附容量降低。此外, 溶液中CO32–濃度的增加, 會導(dǎo)致AO-HTC材料表面的電位值降低, 與鈣–鈾(VI)–碳酸絡(luò)陰離子之間形成靜電斥力, 抑制了AO-HTC吸附鈾, 導(dǎo)致吸附容量降低[29]。實(shí)驗(yàn)結(jié)果表明, 土壤和地下水環(huán)境中的CO32–和Ca2+是影響AO-HTC吸附鈾的重要影響因素。

圖4 CO32–(a)和Ca2+濃度(b)對溶液中鈾形態(tài)分布的影響

2.2 動態(tài)吸附

2.2.1 溶液pH對動態(tài)吸附性能的影響

為了進(jìn)一步研究AO-HTC用作PRB介質(zhì)修復(fù)鈾污染土壤和地下水的可行性, 采用動態(tài)柱實(shí)驗(yàn)研究溶液pH、碳酸根和鈣離子濃度對土壤或地下水鈾吸附性能的影響。動態(tài)柱實(shí)驗(yàn)的條件: 鈾溶液的初始濃度為20 mg·L–1, CO32–和Ca2+的濃度分別為4和2 mmol·L–1, 流速為1 mL·min–1。

采用中和沉淀法處理的鈾礦山廢水pH一般為中性或堿性[30], 導(dǎo)致鈾尾礦附近的土壤孔隙水pH接近于中性。因此, 本研究選擇溶液pH為7.0~8.0, 其對土壤柱和AO-HTC柱穿透曲線的影響如圖3(d)所示。隨著溶液pH增大, AO-HTC柱的穿透點(diǎn)和飽和點(diǎn)體積逐漸減小。這是因?yàn)槿芤褐蠧a2UO2(CO3)3(aq)和CaUO2(CO3)32–等含量隨著pH的增大而增加, 這些賦存形態(tài)不易被AO-HTC吸附[16]。與土壤柱相比, AO-HTC土壤柱的穿透點(diǎn)和飽和點(diǎn)體積顯著增加, 穿透點(diǎn)體積增加了10倍以上, 飽和點(diǎn)體積增加了5倍以上。

2.2.2 CO32–和Ca2+濃度對動態(tài)吸附性能的影響

CO32–和Ca2+濃度對土壤柱和AO-HTC柱穿透曲線的影響如圖3(e, f)所示。當(dāng)Ca2+濃度為4 mmol·L–1時(shí), 隨著CO32–濃度的增加, AO-HTC柱的穿透點(diǎn)和飽和點(diǎn)體積逐漸降低。當(dāng)CO32-濃度從1 mmol·L–1增加到4 mmol·L–1時(shí), 穿透點(diǎn)體積和飽和點(diǎn)體積分別降低了39%和48%。這是因?yàn)殡SCO32–濃度的增加, 溶液中Ca2UO2(CO3)3(aq)和CaUO2(CO3)32–等形態(tài)的含量逐漸增大。當(dāng)CO32–濃度為4 mmol·L–1時(shí), 隨Ca2+濃度增加, AO-HTC柱的穿透點(diǎn)和飽和點(diǎn)體積逐漸降低。這是因?yàn)殡S著Ca2+濃度的增加, 溶液中鈾的賦存形態(tài)以Ca2UO2(CO3)22–和Ca2UO2(CO3)3(aq)為主, 難以被AO-HTC吸附[31-32]。因此, CO32–和Ca2+易與鈾酰離子形成Ca-U(VI)-CO3絡(luò)合物, 對AO-HTC的吸附有抑制作用, 且pH、CO32–和Ca2+濃度越大, 該抑制作用越明顯[33]。

2.2.3 Thomas吸附動力學(xué)

1944年, Thomas提出用于分析柱狀床吸附的動力學(xué)模型—Thomas模型?？赏ㄟ^該模型計(jì)算出理論的最大吸附容量, 用于預(yù)測吸附材料的平衡吸附容量, Thomas模型的數(shù)學(xué)表達(dá)式如式(2, 3)所示[34]。

線性表達(dá)式如下:

其中:C為時(shí)刻柱出口的鈾濃度(mg·L–1),0為鈾初始濃度(mg·L–1),為溶液流速(mL·min–1),Th為Thomas動力學(xué)常數(shù)(mL·min–1·mg–1),0為最大吸附容量(mg·g–1),為溶液體積(mL),為吸附質(zhì)的質(zhì)量(g)。

以ln(0/C–1)-作圖, 采用Thomas線性方程對實(shí)驗(yàn)數(shù)據(jù)進(jìn)行擬合, 結(jié)果如圖5所示。Thomas模型能很好地?cái)M合吸附數(shù)據(jù), 表明吸附劑的外擴(kuò)散和內(nèi)擴(kuò)散是反應(yīng)速率的限制步驟[35]。根據(jù)擬合直線的截距和斜率計(jì)算得到的Thomas擬合參數(shù)列于表1中。隨著pH、CO32–和Ca2+濃度的增加, 土壤柱和AO-HTC柱的o逐漸降低; 與土壤柱相比, AO-HTC柱的o增大了數(shù)倍。

2.2.4 Yoon-nelson吸附動力學(xué)

Yoon-nelson模型的數(shù)學(xué)表達(dá)式是一個(gè)半經(jīng)驗(yàn)公式, 其線性形式如下[36]:

其中,C為流出液的鈾濃度(mg·L–1);0為初始溶液中鈾濃度(mg·L–1);YN為速率常數(shù)(min–1);為吸附質(zhì)穿透50%所需的時(shí)間(min)。

以ln(C/(C0-C))-作圖, 線性擬合結(jié)果如圖6所示, 其線性相關(guān)系數(shù)2>0.80, 即符合Yoon-nelson吸附動力學(xué)模型。根據(jù)擬合曲線的斜率和截距計(jì)算得到的Yoon-nelson擬合參數(shù)列于表1中。理論計(jì)算的值與實(shí)驗(yàn)值相符合, 因此可通過該模型預(yù)測柱吸附質(zhì)穿透50%所需要的時(shí)間, 對確定PRB的使用壽命以及填充材料更換周期具有重要意義[37]。隨著pH、CO32–和Ca2+濃度的增加, AO-HTC柱的值逐漸降低; 在相同實(shí)驗(yàn)條件下, AO-HTC柱的值一直大于土壤柱, 表明 AO-HTC柱具有長期且高效的鈾污染土壤修復(fù)效果。純土壤柱和AO-HTC 柱動態(tài)實(shí)驗(yàn)數(shù)據(jù)Yoon-Nelson模型線性相關(guān)度2均較高, 說明每個(gè)吸附質(zhì)分子被吸附的概率與吸附質(zhì)吸附的概率和吸附質(zhì)在吸附劑上穿透的概率成正比[38]。

圖5 純土柱(a, c, e)和 AO-HTC柱(b, d, f)在不同條件下的Thomas擬合直線

(a, b) Effect of pH; (c, d) Effect of carbonate; (e, f) Effect of calcium (a, b) 4.0 mmol/L CO32–, 2.0 mmol/L Ca2+; (c, d) pH=8.0, 2.0 mmol/L Ca2+; (e, f) pH=8.0, 4.0 mmol/L CO32–

圖6 純土柱(a, c, e)和AO-HTC柱(b, d, f)在不同條件下的Yoon-Nelson擬合直線

(a,b) Effect of pH; (c, d) Effect of calcium; (e, f) Effect of carbonate (a, b) 4.0 mmol/L CO32–, 2.0 mmol/L Ca2+; (c, d) pH=8.0, 2.0 mmol/L Ca2+; (e, f) pH=8.0, 4.0 mmol/L CO32–

表1 Thomas和Yoon-Nelson模型的擬合參數(shù)

3 結(jié)論

研究以可溶性淀粉為碳源, 在硝酸鈰銨催化作用下, 將丙烯腈開環(huán)接枝到淀粉分子上, 通過水熱反應(yīng)和鹽酸羥胺還原制備偕胺肟化水熱碳(AO-HTC), 并研究AO-HTC對 U(VI)-CO3/Ca-U(VI)-CO3的吸附性能。靜態(tài)吸附實(shí)驗(yàn)結(jié)果表明: 隨著pH、CO32–和Ca2+濃度的增加, AO-HTC的吸附量逐漸降低。動態(tài)吸附實(shí)驗(yàn)表明: 隨著pH、碳酸根濃度和鈣濃度的增加, AO-HTC柱的穿透點(diǎn)和飽和點(diǎn)體積均逐漸減小; Yoon-Nelson和Thomas模型能很好地?cái)M合吸附過程, 隨著pH、碳酸根濃度和鈣濃度的增加, 最大吸附容量o和吸附質(zhì)穿透50%所需的時(shí)間逐漸減小。溶液中CO32–和Ca2+濃度對AO-HTC吸附鈾有抑制作用, 且濃度越大, 抑制作用越明顯。與純土壤柱相比, AO-HTC柱的o和值增大了數(shù)倍。由此可見, AO-HTC是一種性能優(yōu)異的PRB介質(zhì), 有望用于修復(fù)鈾污染土壤和地下水。

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Adsorption of U(VI)-CO3/Ca-U(VI)-CO3by Amidoxime-functionalized Hydrothermal Carbon

ZHANG Zhibin, ZHOU Runze, DONG Zhimin, CAO Xiaohong, LIU Yunhai

(State Key Laboratory of Nuclear Resources and Environment, East China Institute of Technology, Nanchang 330013, China)

Hydrothermal carbon adsorption materials possess the advantages of simple preparation process, mild synthesis conditions and easy surface modification. In this UO22+speciation as a function of CO32–concentration, soluble starch used as carbon source, acrylonitrile was grafted onto starch molecule through ring opening under the catalysis of cerium ammonium nitrate. Subsequently, amidoxime hydrothermal carbon spheres (AO-HTC) were successfully synthesized by hydrothermal reaction and hydroxylamine hydrochloride reduction. Meanwhile, static and dynamic adsorption experiments were performed to investigate the effects of solution pH, carbonate and calcium ion concentration on the adsorption performance of AO-HTC for uranium. And the dynamic adsorption process of AO-HTC for uranium was also studied by Yoon-Nelson and Thomas models. The results show that the adsorption capacity of AO-HTC, the volume of penetration point as well as saturation point in the penetration curve also decreases gradually with the increase of pH, carbonate concentration and calcium concentration. The maximum adsorption capacity (o)and the required time () of adsorbate through 50% of 5% AO-HTC column are several times higher than that of pure soil column. Therefore, the research highlights that AO-HTC would act as an excellentpermeable-reactive barriers (PRB) medium and expected to remediate uranium-contaminated soil and groundwater.

aminoxime; hydrothermal carbon; U(VI)-CO3/Ca-U(VI)-CO3; adsorption

TQ174

A

1000-324X(2020)03-0352-07

10.15541/jim20190397

2019-08-03;

2019-09-22

國家自然科學(xué)基金重點(diǎn)項(xiàng)目(21561002, 21866004, 21866003)

National Natural Science Foundation of China (21561002, 21866004, 21866003)

張志賓(1981–), 男, 副教授. E-mail: zhbzhang@ecut.edu.cn

ZHANG Zhibin(1981–), male, associate professor. E-mail: zhbzhang@ecut.edu.cn

劉云海, 教授. E-mail: yhliu@ecit.edu.cn

LIU Yunhai, professor. E-mail: yhliu@ecit.edu.cn